A novel bright cyan emitting phosphor of Eu²⁺ activated Ba₆BO₃Cl₉ with robust thermal stability for full-spectrum WLED applications

Abstract

The currently available commercial white light-emitting diodes (WLEDs) suffer from a poor spectral continuity due to the absence of cyan light. Satisfactory bright cyan phosphors are urgently needed to promote the performance of WLEDs. Herein, we report a bright cyan-emitting phosphor Ba₆BO₃Cl₉:Eu²⁺. The internal quantum efficiency and external quantum efficiency of the Ba₆BO₃Cl₉:Eu²⁺ phosphor were measured to be 86.2% and 41.3%, respectively. Also, this phosphor exhibits strong endurance to luminescence thermal quenching, with the luminescence intensity at 150 °C retaining 93.9% of the value at room temperature. When applied in full-spectrum phosphor-converted WLEDs, the Ba₆BO₃Cl₉:Eu²⁺ phosphor may efficiently compensate for the lack of cyan light emission of WLED devices prepared using blue/yellow/red phosphor strategies and also contribute to the improvement of R_a (color rendering index) of WLED devices based on RGB (red/green/blue) phosphor strategies. In addition, the Ba₆BO₃Cl₉:Eu²⁺ phosphor can be applied as an independent efficient blue-cyan component to replace the blue light component in common WLEDs and reduces the proportion of high-energy blue light that is harmful to vision protection. These results demonstrate that Ba₆BO₃Cl₉:Eu²⁺ will have a wide range of potential applications in indoor healthy lighting as a cyan phosphor.

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Introduction

With the growing public attention to the protection of eyesight and prevention of myopia, it is highly required to search for full-spectrum lighting sources that can simulate sunlight for indoor illumination applications.^1–3 In the context of indoor illumination applications, warm white light-emitting diodes (WLEDs) are regarded as a more competitive upcoming lighting source compared with incandescent and compact fluorescent lamps, which exhibit many unrivaled advantages, such as high efficiency, energy saving, a long service life, environmental friendliness, and reliability.^4–6 In the current market of indoor illumination, most WLEDs are excited by blue light chips on account of their low cost, which leads to serious spectral loss and a high proportion of high-energy blue light that is unfriendly to the eyes.^3,7–9 For example, the best-selling WLEDs on the market are built using a blue light-emitting diode (LED) chip in conjunction with Y₃Al₅O₁₂:Ce³⁺. However, due to the absence of red and cyan light, this variety of WLEDs has a poor R_a (color rendering index) and a high CCT (correlated color temperature), making them unsuitable for indoor lighting applications.^5,9–12 A simple and efficient approach has been put forward to optimize R_a and CCT by adding additional red-emitting phosphors such as (Ca,Sr)AlSiN₃:Eu²⁺, Sr₂Si₅N₈:Eu²⁺, SrLiAl₃N₄:Eu²⁺, K₂SiF₆:Mn⁴⁺, and so on, but full spectrum WLEDs are still far from being achieved due to the existence of the cyan gap between the blue light and the green light.^13,14 In recent years, an alternative preparation method has been proposed to achieve WLEDs by coating tricolor RGB (red, green, and blue) phosphors on ultraviolet (UV) or near-ultraviolet (n-UV) emitting LED chips, which can achieve a higher R_a and lower CCT compared with blue light excited LEDs.^6,15–17 However, tricolor phosphor converted systems still cannot fully meet the needs of people for healthy lighting. The reasons why common LEDs are insufficient for healthy lighting can be attributed to the following: First, the blue light content of ordinary LEDs is too high, far above the solar spectrum. Long-term exposure to ordinary LED lighting with high blue light content will affect the eyes somewhat, and even cause the risk of disease.^3,9,18 Second, common LEDs have a gap near 480 nm between blue light and green light in the spectrum, which leads to a poor spectral continuity.^11,15,16 Long-term exposure to this kind of light can create inertia in our visual perception, which somewhat affects our judgment of color.⁸ Therefore, it is meaningful to obtain a novel WLED spectrum for visual health and human cognitive habit of color, which can weaken the blue light component and fill the missing light component near the 480 nm cyan-gap to make the spectrum smoother and of better continuity.^11,19,20 A feasible method is to design and prepare a bright cyan-emitting phosphor that can be efficiently excited by n-UV LED chips to compensate for the lack of cyan emission or replace the blue light component of the traditional tricolor phosphor converted WLED (pc-WLED) systems.

The selection of a suitable luminescent center ion and host should be addressed as a priority when designing a novel phosphor.^21–23 Among the numerous luminescent ions, Eu²⁺ is one kind of excellent activator, which has attracted the wide attention of researchers.^24–27 Generally, Eu²⁺-based phosphors usually exhibit a broad band spectrum and high luminous efficiency due to its 4f–5d allowed transition.^28–30 As a result, some Eu²⁺-based phosphors have been successfully commercialized, such as the commercial blue emitting BaMgAl₁₀O₁₇:Eu²⁺ and Sr₃MgSi₂O₈:Eu²⁺ phosphors, commercial (Ba,Sr)₂SiO₄:Eu²⁺ green phosphors and commercial (Ca,Y)-α-SiAlON:Eu²⁺ red phosphors.^29,31,32 The ability of Eu²⁺-based phosphors to emit a variety of colors can be attributed to its 5d electrons located in outer orbitals, which are easily affected by their coordination surroundings.^33–35 Hence, it is possible to obtain a highly effective cyan phosphor based on an Eu²⁺ activated phosphor by designing a suitable crystal field environment. However, the thermal quenching (TQ) effect still restricts the practical application of Eu²⁺-based phosphors during the operation of an LED to a large degree.^34,36,37 Several methods have been employed to attenuate the thermal quenching behavior, such as glass-ceramic phosphors, constructing solid solution phosphors, and ceramic coating (SiO₂, TiO₂) over phosphors.^38–40 Nevertheless, these strategies are generally at the cost of luminescence intensity and efficiency.⁴¹ As is known to all, in essence, the structural rigidity of host compounds and lattice has a great influence on the TQ behavior.^20,22,31 Some researchers have reported that host compounds with a highly condensed and rigid structure framework generally have good thermal stability.^6,15,17 But it remains a challenge to search for host compounds which possess rigid frame structures and provide suitable crystal field environments for Eu²⁺ to obtain a highly efficient cyan Eu²⁺-based phosphor with good thermal stability. In recent years, halogen-containing-oxysalt crystalline materials have attracted extensive attention due to their rich structure and unique advantages in terms of optical properties.^1,5,9 The introduction of halogen anionic ligands for oxidation can efficiently influence the crystal field due to the system of mixed-anionic-ligands with different electronegativity and ion radius.^42,43 Nevertheless, in the synthesis procedure of some fluoride phosphors, corrosive hydrofluoric acid may be used, which is harmful to the human body if applied improperly.³² Some halogen-containing-oxysalts are sensitive to moisture and need to be kept in a dry environment. Among the halogen-containing-oxysalt phosphors, halogenated borate phosphors have been reported as good candidates for WLEDs due to their affluent crystal structure types, abundant resources, low synthetic temperature, good chemical and thermal stability, and excellent luminescence properties.⁴² Lian et al. reported the development of unique, extremely effective NaBa₄(AlB₄O₉)₂X₃ (X = Cl, Br):Eu²⁺ phosphors for n-UV driven WLEDs.⁴⁴ Li et al. prepared Eu²⁺-doped Ba₂B₅O₉X (X = Cl, Br) that generate blue violet light with an ultra-narrow band and great efficiency.⁴² In our previous work, we reported a kind of orange-yellow phosphor of Ba_1.31Sr_3.69(BO₃)₃Cl:Eu²⁺ with good thermal stability.⁴³

Herein, we utilized the previously reported chlorine-rich borate Ba₆BO₃Cl₉(BBC) with a highly condensed structure as a host to design a novel bright cyan Eu²⁺-based phosphor for full-spectrum WLED applications.⁴⁵ The as-prepared phosphors exhibited an intense broad excitation band spanning from 250 to 450 nm, and nicely matched the output of n-UV LED chips. Under an excitation of 355 nm, BBC:0.05Eu²⁺ showed a bright cyan emission peak at 470 nm. The phosphor features a decent cyan emission efficiency and robust endurance for luminescence thermal quenching. Ba₆BO₃Cl₉ phosphors were found to have a high internal quantum efficiency (IQE) and an external quantum efficiency (EQE) of 86.2% and 41.3%, respectively. The luminescence intensity of the sample at 150 °C maintains 93.9% of the value at ambient temperature. Moreover, BBC:Eu²⁺ exhibits almost same CIE chromaticity coordinates at different temperatures and good cycling stability in hot and cold cycles. The BBC:Eu²⁺ phosphor was incorporated into WLEDs prepared using RGB or blue/yellow/red phosphors on n-UV chips to assess the potential for its application in WLEDs. By comparing the performance metrics of WLEDs where cyan BBC:Eu²⁺ phosphors are employed and not employed, we found that the Ba₆BO₃Cl₉:Eu²⁺ phosphor may efficiently compensate for the absence of cyan light in LED devices utilizing blue/yellow/red phosphor strategies and also contribute to the improvement of R_a of LED devices based on RGB phosphor strategies. In addition, the Ba₆BO₃Cl₉:Eu²⁺ phosphor can be applied as an independent efficient blue-cyan component to replace the blue light component in common WLEDs. The prepared WLEDs exhibit a fine warm white light and a lower proportion of high-energy blue light, which is conducive to vision protection and promotes the realization of healthy lighting. According to our findings, Eu²⁺-doped BBC phosphors are attractive options for full-spectrum WLED devices that have decent luminescence efficiency and robust thermal stability.

Experimental section

Materials and synthesis

Using the conventional high-temperature solid-state reaction technique, a number of Ba₆BO₃Cl₉:xEu²⁺ samples were created. In accordance with the molar amount of the components, various raw materials including BaCO₃ (AR, Aladdin), H₃BO₃ (AR, Aladdin), BaCl₂·2H₂O (AR, Aladdin), and Eu₂O₃ (AR, Aladdin) were weighed and then thoroughly mixed and ground for 0.5 h in an agate mortar. The final combination was put inside an alumina crucible and pre-fired for 10 h at 500 °C in a KSL-1100X Kejing box-type furnace. The resulting abstracts were reground and secondary fired at 750 °C for 24 h under a 95%N₂ + 5%H₂ reducing atmosphere in a tube furnace. The samples were collected, gently cooled to ambient temperature, and then pulverized into fine powders in an agate mortar for later analysis. During prefiring and calcination both heating up and cooling down were conducted at a rate of 5 °C min⁻¹.

Characterization studies

X-ray powder diffraction (XRD) patterns in a range of 10°–70° were obtained using a Bruker D8 focus diffractometer (Cu Kα, 40 kV, 40 mA, Germany) at a scanning speed of 10° min⁻¹ with a sampling interval of 0.02°. The General Structure Analysis System (GSAS) software was used to perform Rietveld refinements. Field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi) was used to acquire the morphology and elemental mapping. Diffuse reflectance spectra were obtained using a 3600 UV-vis-NIR spectrophotometer (Shimadzu, Japan) with white BaSO₄ powder serving as the standard reference. A fluorescence spectrophotometer (Edinburgh Instruments FLSP-920) fitted with a 450 W xenon lamp as the excitation source was used to acquire photoluminescence excitation (PLE) and photoluminescence (PL) spectra. The resistance to thermal quenching of the samples was determined using the same equipment coupled with a temperature controller, and fluorescence decay curves and time-resolved PL (TRPL) spectra were also acquired using the same equipment and a tunable laser (pulse width = 4 ns; gate = 50 ns) as the excitation source. A photonic multi-channel analyzer C10027 (Hamamatsu, Japan) was used to measure the quantum efficiency. Using HAAS 2000 photoelectric measurement equipment (350–1100 nm, EVERFINE, China), the electroluminescence (EL) spectral features of the manufactured WLED devices were evaluated.

The LED device preparation

The WLED was constructed using the methods of RGB and blue/yellow/red phosphors paired with 395 nm-emitting n-UV chips in order to assess the application possibility of the BBC:Eu²⁺ phosphor in WLEDs. The components of LED devices are listed as following: LED₁ [BaMgAl₁₀O₁₇:Eu²⁺(BAM:Eu²⁺), Y₃Al₅O₁₂:Ce³⁺(YAG:Ce³⁺) and CaAlSiN₃:Eu²⁺ phosphor], LED₂ [BAM:Eu²⁺, BBC:Eu²⁺, YAG:Ce³⁺ and CaAlSiN₃:Eu²⁺ phosphor], LED₃ [BBC:Eu²⁺, YAG:Ce³⁺ and CaAlSiN₃:Eu²⁺ phosphor], LED₄ [BAM:Eu²⁺, (Ba,Sr)₂SiO₄:Eu²⁺ and CaAlSiN₃:Eu²⁺ phosphor], LED₅ [BAM:Eu²⁺, BBC:Eu²⁺, (Ba,Sr)₂SiO₄:Eu²⁺ and CaAlSiN₃:Eu²⁺ phosphor], LED₆ [BBC:Eu²⁺, (Ba,Sr)₂SiO₄:Eu²⁺ and CaAlSiN₃:Eu²⁺ phosphor], and LED₇ [BBC:Eu²⁺ phosphor]. To produce a composite solution, the phosphor mixtures were completely blended with epoxy resins (A : B = 1 : 1) for 30 minutes. A 395 nm n-UV chip was coated with the composite solution to construct pc-WLED, which was then dried at 353 K for electroluminescence quality tests performed at a current of 20 mA.

The computational methodology

According to Wu et al., the structural information of Ba₆BO₃Cl₉ helped in the reconstruction of the monoclinic structure of BBC.⁴⁵ Density functional theory (DFT) calculations, performed using the Vienna ab initio simulation package (VASP), served as the basis for the first-principles computations. The optimized structures were developed using ab initio calculations. A 2 × 2 × 1 supercell with 76 atoms was developed with the help of the crystallographic data of BBC, which has four different types of Ba with various coordination environments. The electrons of Ba(5s²5p⁶6s²), B(2s²2p¹), O(2s²2p⁴), Cl(3s²3p⁵), and Eu (4d¹⁰4f⁷5s²5p⁶6s²) were treated as valence electrons.

Results and discussion

Crystal structure and morphology

The analysis and visualization of the XRD patterns of Ba₆BO₃Cl₉:xEu²⁺(0 ≤ x ≤ 0.1) are shown in Fig. 1a. All the XRD patterns are identical to the reference card of Ba₆BO₃Cl₉ (first reported by Hongping Wu and Hongwei Yu et al.),⁴⁵ demonstrating that Eu²⁺ ions are efficiently accommodated into the host matrix without producing any discernible impurity phase. As shown in the enlarged XRD patterns at the 2θ value of 22.4–23°, diffraction peaks ascribed to (−114), (114), and (210) crystal faces obviously switch to larger angles as smaller Eu²⁺ (r = 1.25 Å, CN = 8; r = 1.30 Å, CN = 9, CN: coordination number) progressively substitutes Ba²⁺ (r = 1.42 Å, CN = 8; r = 1.47 Å, CN = 9) due to lattice contraction.⁴⁶ The Rietveld refinement of XRD patterns of BBC and BBC: 0.05Eu²⁺ were conducted using XRD data (2θ of 10–120°) with Ba₆BO₃Cl₉ as the fundamental framework in order to better understand the site occupation of the Eu²⁺ ions and the structural evolution. The refinement outcomes for BBC and BBC:0.05Eu²⁺ were plotted as shown in Fig. 1b and c. The Rietveld refinement reveals that the crystal system of all the samples is monoclinic based on the P2₁/c space group. The detailed cell parameters of BBC and BBC:0.05Eu²⁺, are presented in Table S1.† The cell parameters of BBC:0.05Eu²⁺ are smaller than undoped BBC, which supports that diffraction peaks move to a bigger angle and provides more evidence that Eu²⁺ ions can substitute Ba²⁺ positions in the BBC host matrix. According to the Rietveld refinement, the occupancy ratios of Eu²⁺ at the sites of Eu_Ba1²⁺, Eu_Ba2²⁺, Eu_Ba3²⁺, Eu_Ba4²⁺, Eu_Ba5²⁺ and Eu_Ba6²⁺ are 1.56%, 1.71%, 4.95%, 5.25%, 6.60% and 9.90%, respectively. The refinement results are relatively credible based on the low residual factors for BBC and BBC:0.05Eu²⁺. The pertinent structural information and other detailed refinement outputs are included in Tables S1, S2, and S3.† In Table S4† more information is provided on the bond characteristics of the BBC and BBC:0.05Eu²⁺ samples.

Fig. 1 Crystal structure characterization of BBC:Eu²⁺ phosphors. (a) XRD patterns of BBC:xEu²⁺ samples in the 2θ range of 10–70° and enlarged XRD patterns in the 2θ range of 22.4–23.0°. (b) and (c) are the Rietveld refinements of BBC and BBC:0.05Eu²⁺ samples. (d) The local crystal environment of six kinds of different Ba²⁺. (e) The coordination geometry of the unit cell of BBC. (f) Cell configuration of BBC samples.

Fig. 1f depicts the cell configuration of Ba₆BO₃Cl₉. In the unit cell of Ba₆BO₃Cl₉, there exists six types of Ba²⁺ cations with different coordination environments, which are Ba(1, 2)OCl₈, Ba(3, 4, 6)O₂Cl₇ and Ba(5)OCl₇, respectively. According to Fig. 1e and Table S4,† the coordination atoms and numbers of Ba₁ and Ba₂ are the same despite the slight differences in the bond distances and angles between them. The Ba(1, 2)OCl₈ polyhedron connected with Ba(2, 1)OCl₈ and Ba(5)OCl₇ polyhedra by edge-sharing, and Ba(3, 4)O₂Cl₇ and Ba(6)O₂Cl₇ by face-sharing respectively, which means that the coordination environments of Ba(1) and Ba(2) are basically identical. A similar result was also obtained with Ba(3) and Ba(4). The Ba(3, 4)O₂Cl₇ polyhedron connected with Ba(4, 3)O₂Cl₇, Ba(5)OCl₇, and Ba(6)O₂Cl₇ polyhedra by edge-sharing, respectively and with Ba(1, 2)OCl₈ by face sharing. Oxygen atoms are connected with the boron atom to produce the BO₃ functional group, which is surrounded by Ba-centered polyhedra. The Ba-centered polyhedra can be divided into two parts: Ba(1, 2)OCl₈ and Ba(5)OCl₇ below the BO₃³⁻ functional group plane, and Ba(3, 4, 6)O₂Cl₇ is above the BO₃³⁻ functional group plane.

SEM images (Fig. 2a–h) were used to identify the morphological traits and elemental components of the typical BBC:0.05Eu²⁺. SEM images (Fig. 2a, b) of BBC:0.05Eu²⁺ show that the resulting phosphor is mostly composed of granules with sizes between 2 and 5 μm and the morphology features erratic forms. The homogenous distribution of the elements Ba, B, O, Cl, and Eu within the phosphor particle is confirmed by the elemental mapping of a chosen granule, which also reveals the effectiveness of Eu²⁺ doping without forming any impurity phase.

Fig. 2 (a) and (b) SEM images and (c)–(h) elemental mapping of a representative particle of BBC:0.05Eu²⁺.

Luminescence properties

Fig. 3a shows the diffuse reflection spectra (DRS) of BBC without doping and BBC:0.05Eu²⁺. The BBC host exhibits almost 100% reflectance in the wavelength range of 450–800 nm. The electronic transition between valence and conduction bands of the BBC host may account for its mild absorption at 250–450 nm. It is generally found that there is a sizable energy difference (4.2 eV) between the 5d and 4f electron orbitals of Eu²⁺ ions.^47,48 According to ESI Note 1,† the calculated optical bandgap for BBC is 5.98 eV based on the Kubelka–Munk absorption spectrum (Fig. 3b), which is sufficient to create the emitting levels of Eu²⁺. Moreover, the 4f → 5d transition of Eu²⁺ results in a broad adsorption band of 250–450 nm, which clearly becomes visible as the amount of Eu dopant rises.⁴⁹ PLE and PL emission spectra of BBC:0.05Eu²⁺ at room temperature were recorded and are presented in Fig. 3c. A broad excitation band spanning from 250 to 450 nm is visible in the PLE spectrum of BBC:Eu²⁺, when the emission peak at 470 nm is monitored, and this correlates nicely with the output of n-UV-LED chips (360–400 nm).⁵⁰ The PL spectrum of BBC:Eu²⁺ show an asymmetric PL emission band peak at 470 nm and a lengthy tail appearing from 480 to 650 nm due to the permitted 4f⁶5d¹ → 4f⁷ (⁸S_7/2) transitions of Eu²⁺ when excited at 355 nm.^51,52 The type of the 4f → 5d transitions examined in the absorption process is connected to the energy distribution and band shape of the emission spectrum.²⁰ The wide emission band should be related to the multi-site occupation of Eu²⁺ in the BBC host with a complex coordination environment. As can be seen from Fig. 3c, the excitation and emission spectra have an overlapped part, presenting a small Stokes shift. Phosphors with small Stokes shift usually have the benefit of high luminescence efficiency and thermal stability, but easily suffer from reabsorption losses and aggregation-induced quenching.^53,54

Fig. 3 Luminescence properties of BBC:Eu²⁺ phosphors. (a) Diffuse reflection spectra of BBC and BBC:Eu²⁺ samples. (b) The plot of (αhν)²vs. the photon energy hν for BBC. (c) PL and PLE spectrum of BBC:0.05Eu²⁺ samples. (d) Luminescence spectra of BBC with different Eu²⁺ doping concentrations (0.01 ≤ x ≤ 0.10). (e) The relationship between the integral PL intensity and Eu²⁺ doping concentrations for BBC:xEu²⁺. (f) Fluorescence decay curves of BBC:xEu²⁺ (0.01 ≤ x ≤ 0.10).

Fig. 3d shows the luminescence spectra of BBC with different Eu²⁺ doping concentrations (0.01 ≤ x ≤ 0.10). It has been discovered that the ideal doping ratio of Eu²⁺ ions in the BBC host is 0.05, as shown in Fig. 3e. The concentration quenching occurs when the doping concentration exceeds 0.05, which is caused by a higher likelihood of non-radiative energy transfer between the adjacent Eu²⁺ ions in the BBC host.^25,31,33 In order to better understand the mechanism of concentration quenching for Eu²⁺-doped BBC, the critical distance R_c and electric multipolar–multipolar interactions are explored. (ESI Note 2†) The dipole–quadrupole interaction is the main cause of the concentration quenching behavior for BBC:xEu²⁺ phosphors, as seen in the inset of Fig. 3e.

Fluorescence lifetime fluctuations may be another reflection of the concentration quenching effect. Fig. 3f depicts the decay curves of the series of samples of BBC:xEu²⁺ (0.01 ≤ x ≤ 0.10) phosphors monitored at 470 nm under 375 nm laser radiation. A double exponential equation can accurately represent each of the BBC:xEu²⁺ decay curves. According to ESI Note 3,† the average lifetimes of BBC:xEu²⁺ (0.01 ≤ x ≤ 0.10) were calculated to be 648.69, 643.40, 636.55, 627.64, 625.99, and 620.77 ns for x values of 0.01, 0.03, 0.05, 0.07, 0.09, and 0.10, respectively. The results of the detailed calculations are given in Table S5.† The average lifetimes decrease quickly when the Eu²⁺-doping concentration increases due to an increase in the non-radiative energy transfer among Eu²⁺ activators. The distance between Eu²⁺ ions gradually decreases with an increasing Eu²⁺ concentration, and this increases the energy transfer rate and improves the possibility of the excited state energy to be captured by the luminescence quenching center. As a result, the luminescence lifetime becomes shorter.

Analysis of Eu²⁺ site occupations

The asymmetric emission spectrum (λ_ex = 355 nm) presented in Fig. 3c shows that Eu²⁺ has more than one emission center in BBC host lattices. It is difficult to accurately distinguish the location of emission peaks caused by different luminescence centers at room temperature due to thermal vibration.^24,55 The PL spectrum of Eu²⁺ doped BBC was recorded under an excitation of 355 nm at liquid helium temperature and is shown in Fig. 4a. The luminescence spectrum at 8 K consists of an asymmetric broad emission band (400–650 nm) peak at 470 nm. In addition, a small shoulder at 425 nm can be observed, which provides strong evidence that Eu²⁺ occupies different lattice sites in the BBC host. According to the structural characteristics research, Eu²⁺ ions have a possibility of entering four kinds of Ba cationic sites with different coordination environments in BBC lattices, which makes it reasonable to deconvolute the emission spectrum into four Gaussian sub-bands. As shown in Fig. 4a, the wide asymmetric PL band can be neatly split into four Gaussian sub-bands, labeled Eu(I), Eu(II), Eu(III), and Eu(IV), centering at 22 936 cm⁻¹ (457 nm), 21 459 cm⁻¹ (470 nm), 20 921 cm⁻¹ (496 nm) and 19 608 cm⁻¹ (522 nm), respectively. The integral intensity ratios of the four fitting peaks are listed in Table S6† (10.88, 34.09, 33.07, and 21.95), reflecting the contribution of different Eu²⁺ sites to the emission spectrum. To verify the multi-site occupations of Eu²⁺, the excitation spectra monitored at distinct emission positions of 457, 470, 496, and 522 nm are recorded. As shown in Fig. 4b, Eu(I) and Eu(IV) have considerably different normalized PLE spectra shapes compared to Eu(II) and Eu(III). Given that the crystal field environment of Eu²⁺ determines the form and location of excitation peaks, we speculate that the coordinate environment of Eu(I) and Eu(IV) should be discriminative from others. Considering the crystal structure of BBC, the emission sub-peak of Eu(I) and Eu(IV) may originate from Ba(5)OCl₇ or Ba(1,2)OCl₈. In addition, the identical spectral profile of Eu(II) and Eu(III) may be attributed to analogous Ba(3,4)O₂Cl₇ and Ba(6)O₂Cl₇. These findings support the earlier notion regarding the multi-site occupancy of Eu²⁺ by demonstrating the presence of at least four distinct Eu²⁺ luminescent centers in the BBC host.

Fig. 4 Analysis of Eu²⁺ site occupations. (a) The PL spectrum and Gaussian fitting of BBC:0.05Eu²⁺ monitored at liquid helium temperature. (b) Normalized PLE spectra of BBC:0.05Eu²⁺ samples measured at 457 nm, 470 nm, 496 nm, and 522 nm emissions at liquid helium temperature. (c) The normalized two-dimensional time-resolved emission spectra (TRES) of BBC:0.05Eu²⁺ under 375 nm excitation at room temperature. (d) The normalized spectrum slices of TRES at 0.2, 0.5, 1.0, and 1.5 μs.

To corroborate the existence of multiple luminescent sites in Eu²⁺-doped BBC, the time-resolved emission spectra (TRES) of BBC:Eu²⁺ were recorded under a 375 nm laser irradiation at an ambient temperature and recorded at a 2 nm interval between 420 and 680 nm. For Eu²⁺-doped BBC, the emission time map and the normalized spectrum slices of TRES at 0.2, 0.5, 1.0, and 1.5 μs were exhibited as shown in Fig. 4c and d. Fig. 4c clearly shows a considerable red-shift, suggesting the presence of numerous luminescent centers. No noticeable peak shift was observed for BBC:Eu²⁺ at the decay time spanning from 0 to 2.5 μs, while the spectral terminal wavelength progressively moves from 530 to 565 nm with the decay time increasing from 0.2 to 1.5 μs. The precise number of luminescent centers was verified using four typical normalized TRES slices at particular time intervals of 0.2, 0.5, 1.0, and 1.5 μs, as shown in Fig. 4d. Peak a was initially noticed at 0.2 μs; peak b appeared when the decay time was extended to 0.5 μs; and peak c and peak d progressively became visible with the decay time being extended to 1.5 μs. Therefore, these results demonstrate that there are at least four luminescent centers in Eu²⁺-activated BBC, each of which relates to distinct Ba lattice sites.

In order to validate our above hypothesis about the site occupations of Eu²⁺, we can qualitatively judge which crystallographic site is substituted by the divalent europium ion in BBC based on ESI Note 4.† Because the coordination number (CN) and ionic radius of Ba (5) are smaller than others, the Gaussian sub-bands peak at 522 nm (Eu (4) emission) may originate from the Ba (5) sites occupied by Eu²⁺. It is a pity that we cannot distinguish the site occupations of Eu (1), Eu (2) and Eu (3) because other sites possess identical CN and radius. Luckily, the emission peak position is impacted not just by the CN and cationic radius, but also influenced by the charge of the anion and the bond length, etc. According to ESI Note 5,† the crystal field splitting (ε_cfs) is a useful index for evaluating the site occupations of activator ions, which is closely related to the coordination number, polyhedron shape, and size, while being irrelevant of the anion types.^56,57 As shown in Fig. 1d, the shape of Ba (1, 2) sites and Ba (3, 4) sites is a single cap square antiprism while the shape of Ba (6) sites is a triplecap triangular prism in the Ba₆BO₃Cl₉ host. Although the shape of Ba (6) sites is different from Ba (1, 2) sites and Ba (3, 4) sites, it is difficult to compare the difference of β^Q_poly, which is a constant depending on the type of the coordination polyhedron. It can be seen that the short bond length indicates a bigger degree of crystal field splitting and a longer emission wavelength. The bond length of Ba–O and Ba–Cl bonds for BBC:Eu²⁺ are provided in Table S4† in order to compare the ε_cfs of Eu²⁺ 5d orbits at various locations. The average bond length R[Ba(6)–O,Cl] (3.1479 Å) < R[Ba(3, 4)–O,Cl] (3.1606 Å) < R[Ba(1,2)–O,Cl] (3.2234 Å). Therefore, we speculate that the highest energy emission peak at 457 nm Eu (1) should be assigned to Eu²⁺ occupying the Ba (1,2) site in the weakest crystal field. The Eu (2) peak at 470 nm probably originates from the occupation of Ba (3,4) located in a relatively weak crystal field. The Eu (3) peak at 496 nm may be ascribed to Eu²⁺ in Ba (6) sites. These inferences are in agreement with the above analyses of lattice occupancy.

In order to better understand the multi-site occupancies of Eu²⁺ in BBC host lattices, the band structure and state densities for BBC were studied using the DFT calculation approach. As shown in Fig. 5a, we consider BBC to be a direct band gap material of 4.37 eV because the lowest point of the conduction band and the highest point of the valence band are situated at the G-point. Given the considerable underestimation by DFT-PBE, the computed bandgap is lower than that (5.98 eV) inferred by DRS. As shown in Fig. 5c, the band structure and composition of valence and conduction bands were obtained by calculating the total and partial density of states. It is possible to roughly split the total and partial densities of BBC into three regions. The first region is made up of the Ba-5p and Cl-3s states, both of which have energy levels distant from the Fermi surface (−15 to −10 eV). These states do not offer much to luminescence since the electrons in these states are almost impossible to be excited by n-UV light. The B-2s2p, O-2p, and Cl-3p states can be found in the second area, which is near to the Fermi level (−5 eV to 0 eV). Concentrating on the area around zero energy, the valence band minimum is found to be mostly made up of the O 2p orbital due to the strong electronegativity of O ions, which absorb the majority of electrons. The conduction band is located in the final region, which is dominated by the Ba 3d orbital compared with other elements due to Ba-centered polyhedra connecting together and forming the 3D [Ba₆Cl₉]_N framework of the host lattice. As a result, the O 2p → Ba 3d transition is the primary band gap transition form of the BBC host. As shown in Fig. 5b and d, the BBC:Eu_Ba model is developed to investigate the effect of Eu²⁺ doping on the bandgap structure. The 4f orbital of Eu²⁺ lies near the Fermi level. As is known to all, the Eu²⁺ 4f ground state cannot optically transition to the 5p6s excited state. Therefore, the transition of Eu²⁺ 4f to 5d states is the major energy transition form.

Fig. 5 The calculated electronic structure for BBC and BBC:Eu_Ba. (a) The band structure of the BBC host. (b) The band structure of BBC:Eu_Ba. (c) Total and partial state densities of BBC. (d) Total and partial state densities of BBC:Eu_Ba.

The site choices can also be estimated by comparing the change of thermodynamic stability caused by the doping of ions through the calculation of the defect formation energy (E_f).²⁸ The E_f for Ba (1), Ba (3), Ba (5), and Ba (6) replaced with Eu²⁺ were computed. The replacement processes are as follows:

Eu + BBC = BBC:Bu_Ba + Ba_i (1)

And E_f was computed by

E_f = E(doped) − E(undoped) + μ_Ba − μ_Eu (2)

where the DFT total energies of BBC:Eu_Ba and BBC are represented by E(doped) and E(undoped). In view of the reducing environment being used throughout the material production, the chemical potential μ_Ba and μ_Eu were almost comparable to the energies of the corresponding metal atoms. Table S7† contains the computed values for the total energy and defect formation energy. The defect formation energy can be arranged in the order of BBC:Eu_Ba(6) < BBC:Eu_Ba(5) < BBC:Eu_Ba(3) < BBC:Eu_Ba(1). The more lower the formation energy, the more likely the sites are to be occupied by Eu²⁺, which can exhibit higher stability. The results indicate that Eu²⁺ substitution for Ba sites may be in the order of Ba (6), Ba (5), Ba (3, 4), Ba (1, 2) sites, which explains why there is more cyan light and less blue and yellow light in the spectrum.

Temperature dependence of the Eu²⁺ luminescence

The luminescence properties of phosphor materials are significantly impacted by the working temperature of LED devices, which can reach 100 °C or higher.^17,19,33 Hence, thermal stability is a significant index for the evaluation of phosphors applied in LED devices. This metric calls for phosphors to have good emission intensities as well as stable peak shapes and positions at elevated temperatures.²⁹ The temperature-dependent emission spectra of BBC:0.05Eu²⁺ at various temperatures ranging from 25 to 300 °C are shown in Fig. 6a. As shown in Fig. 6b, when the temperature rises from 25 to 300 °C, the luminescence intensity of BBC:0.05Eu²⁺ drops while the full-width at half-maximum (FWHM) rises. At 150 °C, the integral intensity of BBC:0.05Eu²⁺ can maintain about 93.9% of the initial value at ambient temperature, which is relatively high compared to other reported blue and cyan phosphors (Table S9†). Further research is required to fully understand the remarkable thermal stability of these samples.

Fig. 6 Temperature-dependent luminescence characteristics of the BBC:Eu²⁺ phosphor. (a) PL spectra of BBC:0.05Eu²⁺ under an excitation of 355 nm with varying temperature. (b) Chart of the changing trend of normalized integral intensities and FWHM of BBC:0.05Eu²⁺ (λ_ex = 355 nm) with varying temperature. (c) The relationship of ln[(I₀/I_t) − 1] with 1/k_bT in the BBC:0.05Eu²⁺ phosphor. (d) Configurational coordinate diagram of BBC:Eu²⁺. (e) The contour map reflecting the emission spectrum of BBC:0.05Eu²⁺ that varies with the temperature (the inset revealed the shift of CIE chromaticity coordinates). (f) The cycling performance of BBC:0.05Eu²⁺ and BAM:Eu²⁺ evaluated between 25 and 150 °C in 10 times cycles.

We can determine that the activation energy ΔE for the BBC:0.05Eu²⁺ phosphor is 0.3612 eV based on ESI Note 6.† ΔE has been assigned to the thermal quenching caused by nonradiative relaxation through the junction. In general, thermal quenching may occur in the way of increment of non-radiative transition and thermal ionization with increasing temperature.⁴¹ As illustrated in Fig. 6d, the non-radiative transition process can be explained. When irradiated with n-UV light, the electrons in the ground state (4f⁷6s²5d⁰) leap to the excited state (4f⁶6s²5d¹) along path (1). At room temperature, the majority of electrons undergo radiative transition and emit visible light along path (2). As the temperature increases, more and more electrons violently break through the energy barrier ΔE to reach the intersection of the ground and excited state (path (3)) and afterwards return to the ground state via path (4) through non-radiative transition with no luminescence, caused by the thermal activation of the activated luminescent center due to phonon interaction. The thermal ionization process occurs along path (5), where electrons in the excited state enter the conduction band of the host at an elevated temperature. According to the report of Dorenbos, the degree of thermal ionization of a phosphor is primarily determined by the gap of the Eu²⁺ 5d energy level, and the conduction band and small energy gap are always related to the poor thermal stability of phosphors.³ Considering the band gap of BBC is equal to 5.98 eV, which is large enough, and the good cycling stability of the phosphor in hot and cold cycles which will be discussed below, we tend to believe that the non-radiative crossover transition is the main driving force leading to the thermal quenching of BBC:Eu²⁺.

It is also important to ensure that LEDs maintain chromaticity stability under high-temperature operating conditions. Fig. 6e shows the contour map reflecting the emission spectra of BBC:0.05Eu²⁺ that vary with temperature. There is no obvious change in the peak pattern of the emission band of BBC:Eu²⁺ with increasing temperature. The peak position is almost unchanged and only a slight red shift occurs which cannot be distinguished by human eyes. CIE color coordinates were calculated on the basis of the PL spectrum measured under different temperatures ranging from 25 °C to 300 °C, that are listed in Table S8.† The CIE color coordinates just slightly shift from (0.1673, 0.2393) to (0.1771, 0.2397) when the temperature increases from 25 °C to 150 °C. The CIE chromaticity diagram shown in the inset of Fig. 6e can be used to directly reflect the change of CIE color coordinates. According to the ESI Note 7,† the chromaticity consistency of cyan BBC:Eu²⁺ is excellent across a wide temperature range.

The cycling performance test was conducted 10 times across a temperature range of 25–150 °C, in which a cycle indicates that the temperature-dependent emission spectrum was recorded after heating the sample from an ambient temperature to 150 °C at 40 °C min⁻¹ and then decreasing the temperature to room temperature at the same rate. Fig. 6f shows the change in the trend of the integral emission intensity for each cycle, where the initial intensity measured at 150 °C was normalized. The comprehensive test results are shown in Fig. S1.† The findings demonstrate that the luminescence intensity of BBC:Eu²⁺ may be replicated for at least 10 subsequent heating and cooling cycles without seeing a substantial change. In contrast, only 52% of the initial luminescence intensity was maintained for BAM after measurement. BBC:Eu²⁺ demonstrates a better reversibility.

The robust thermal stability is generally related with a large bandgap and high structural rigidity of the host lattice.^12,14 As mentioned above, BBC:Eu²⁺ has a large bandgap of 5.98 eV based on the experimental result and 4.37 eV based on theoretical calculations, which is relatively large compared with other materials. In addition, the high rigidity plays important roles in luminescence efficiency and thermal stability.^31,41,42 Based on ESI Note 8,† the BBC phosphor has a high structural rigidity, which could substantially reduce the probability of non-radiative transitions by lattice vibration, contributing to less emission loss during the heating process. As reported by Zhuo and his colleagues, the Debye temperature is also a proxy for photoluminescence quantum yield.²³ BBC has a high Debye temperature, so the luminescence intensity should be high. According to Fig. S2,† the temperature-dependent XRD patterns of BBC:0.05Eu²⁺ show that BBC:Eu²⁺ still maintains good phase stability even at high temperatures. No impurity phase is formed, and the peak position as well as the peak intensity remains almost unchanged.

Quantum efficiency and WLED applications

As an essential technological criterion to assess novel phosphors, the internal quantum efficiencies (IQE) of Ba₆BO₃Cl₉:0.05Eu²⁺ was found to be 86.2% when excited by 355 nm n-UV light based on ESI Note 9.† The external quantum efficiencies (EQE) of the selected sample was calculated to be 41.3%. Compared with other reported blue and cyan phosphors, BBC:0.05Eu²⁺ can be regarded as a bright cyan phosphor. The related spectrum is shown in Fig. 7a.

Fig. 7 Quantum efficiency and pc-WLED applications of BBC:Eu²⁺ phosphors. (a) The QE spectra of BBC:0.05Eu²⁺. (b–d) The PL spectrum of prepared full spectrum w-LEDs created with blue, yellow, and red phosphors combined with the 395 nm-emitting InGaN chip.

Fig. S3a† shows the EL spectrum of n-UV-pumped single-color pc-LED (LED₇) measured at a current of 20 mA, and the inset contains images of the associated LED devices. The n-UV LED chip and introduced cyan-emitting phosphors are responsible for the two distinct sections of the emission spectrum, which have a sharp peak at 395 nm and a wide emission band peak at about 470 nm, respectively. A bright cyan light ranging from 470 to 550 nm can be clearly generated with the CIE chromaticity coordinates of (0.162, 0.228) by excitation with LED₇, which can efficiently supplement the missing part of the common LED spectrum. This suggests that the BBC:Eu²⁺ cyan phosphor has great potential to be used in full spectrum WLEDs to promote spectral continuity.

To assess the potential applications of the reported cyan-light phosphors BBC:Eu²⁺ in a high-R_a full-spectrum WLED, the commercial BAM:Eu²⁺ blue phosphor, YAG:Ce³⁺ yellow phosphor and CaAlSiN₃:Eu²⁺ red phosphor with and without the as-prepared cyan-light phosphor BBC:Eu²⁺ were combined with 395 nm n-UV LED chips to prepare WLEDs for electroluminescence (EL) measurements. As can be seen from Fig. 7b and c, the obvious loss in the cyan-green light part in the EL spectrum of LED₁ can be well compensated and the color rending index (R_a) of WLED was obviously enhanced after the addition of the cyan-light phosphor BBC:Eu²⁺. Moreover, the BBC:Eu²⁺ phosphor can act as independent efficient blue-cyan components to replace blue light components applied for WLEDs. The prepared WLED LED₃ exhibited an exceptional warm white light [high R_a = 83, low CCT of 4005 K and coordinates of (0.399, 0.488)] and a reduced percentage of high-energy blue light, which is conducive to vision protection and promotes the realization of healthy lighting. In the total luminescence spectrum of LED₁ and LED₃, the high-energy blue light (400–450 nm) makes up 7.4% and 3.3%, respectively. In addition, the commercial BAM:Eu²⁺ blue phosphor, (Ba, Sr)₂SiO₄ green phosphor, and CaAlSiN₃:Eu²⁺ red phosphor with and without the cyan-light phosphor BBC:Eu²⁺ were also combined with 395 nm n-UV LED chips to prepare WLEDs. The EL spectrum of these LEDs are presented in Fig. S3.† The full set of 15 CRIs (color rendering index) and R_a of the as-prepared WLEDs are listed in Table S10.† Evidently, the BBC:Eu²⁺ phosphor also contributes to the improvement of the R_a of the WLEDs based on RGB phosphor strategies. The manufactured WLED LED₆ exhibited an exceptional warm white light with a decreased percentage of the blue light component. In light of this, we can say that the BBC:Eu²⁺ cyan phosphor may successfully fill the cyan gap of the WLEDs based on commercial tricolor phosphor combinations, and it may be a good choice for n-UV-activated WLED devices featuring full-spectrum luminescence and a high R_a for healthy lighting.

Conclusions

In conclusion, a series of neoteric bright cyan light phosphors of Ba₆BO₃Cl₉:Eu²⁺ were obtained utilizing a solid-state reaction approach, and their structural and luminescence characteristics were investigated. With the introduction of Eu²⁺ dopants, BBC:Eu²⁺ can emit a bright cyan light when excited by n-UV-LED chips, and the IQE and EQE were measured to be 86.2% and 41.3%, respectively. Due to the strong structural rigidity and large band gap of the host lattice, the phosphor also exhibits outstanding resistance to luminescence thermal quenching and can maintain 93.9% of the initial emission intensity at 150 °C. Besides, BBC:Eu²⁺ exhibits a negligible change of the chromaticity coordinate at high temperatures as well as an excellent cycling stability in hot and cold cycles. By comparing the performance metrics of WLEDs with and without BBC:Eu²⁺, we found that Ba₆BO₃Cl₉:Eu²⁺ phosphors effectively compensate for the lack of cyan emission in LED devices based on blue/yellow/red phosphor strategies and also contribute to the improvement of R_a of LED devices based on RGB phosphor strategies. Moreover, the Ba₆BO₃Cl₉:Eu²⁺ phosphor can act as an independent efficient blue-cyan component to replace the blue light component applied for pc-WLEDs. The prepared pc-WLEDs exhibit an outstanding warm white light with a smaller percentage of blue light, which is conducive to vision protection and promotes the realization of healthy lighting. These findings show that Ba₆BO₃Cl₉:Eu²⁺ has a tremendous potential to be used as a blue-cyan phosphor in high R_a full-spectrum WLEDs for indoor healthy lighting.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The research is financially supported by the National Key R&D program of China (Grant No. 2019YFA0709101), the National Natural Science Foundation of China (Grant No. 52072364), R & D Projects in Key Areas of Guangdong Province (Grant No. 2020B0101010001) and the Key Research Program of the Chinese Academy of Sciences, (Grant No. ZDRW-CN-2021-3-3-06). cEeshi (https://www.eceshi.com) is acknowledged for the DFT calculation analysis.

References

1. L. Fu, D. Wu, Y. Xiao, W. Zou, J. Kuang, Y. Liang, F. Du, J. Peng and X. Ye, Turning on the luminescence of Sr₅(PO₄)₃Cl: Eu²⁺ by doping of La, Gd and Lu ions for full-spectrum lighting application, J. Alloys Compd., 2021, 878, 160354.
2. M. Cai, T. Lang, T. Han, D. Valiev, S. Fang, C. Guo, S. He, L. Peng, S. Cao, B. Liu, L. Du, Y. Zhong and E. Polisadova, Novel Cyan-Green-Emitting Bi³⁺-Doped BaScO₂F, R⁺ (R = Na, K, Rb) Perovskite Used for Achieving Full-Visible-Spectrum LED Lighting, Inorg. Chem., 2021, 60, 15519–15528.
3. W. Wang, T. Tan, S. Wang, S. Zhang, R. Pang, D. Li, L. Jiang, H. Li, C. Li and H. Zhang, Low-concentration Ce³⁺-activated ScCaO(BO₃) blue-cyan phosphor with high efficiency toward full-spectrum white LED applications, Mater. Today Chem., 2022, 26, 101030.
4. D. Hou, S. Zheng, Z. Lin, Y. Zhang, W. Zhang, K. Chen, Q. Peng, H. Lin, J.-Y. Li, J. Song and R. Huang, A Mn⁴⁺ activated (Gd,La)₂(Zn,Mg)TiO₆ deep-red emission phosphor: The luminescence properties and potential application for full-spectrum pc-LEDs, J. Lumin., 2022, 247, 118895.
5. M. Tian, P. Li, Z. Wang, Z. Li, J. Cheng, Y. Sun, C. Wang, X. Teng, Z. Yang and F. Teng, Controlling multi luminescent centers via anionic polyhedron substitution to achieve single Eu²⁺ activated high-color-rendering white light/tunable emissions in single-phased Ca₂(BO₃)_1−x(PO4)_xCl phosphors for ultraviolet converted LEDs, Chem. Eng. J., 2017, 326, 667–679.
6. P. Dang, Q. Zhang, D. Liu, G. Li, H. Lian, M. Shang and J. Lin, Hetero-valent substitution strategy toward orange-red luminescence in Bi³⁺ doped layered perovskite oxide phosphors for high color rendering index white light-emitting diodes, Chem. Eng. J., 2021, 420, 127640.
7. S. Sreevalsa, P. A. Parvathy, S. K. Sahoo and S. Das, Full-color emitting crystal engineered Sr₃Al_1−xSi_xO_4+xF_1−x: Eu^2+/3+ oxyfluorides for developing bendable lighting composites, J. Alloys Compd., 2021, 880, 160483.
8. P. C. Hughes and R. M. Neer, Lighting for the Elderly: A Psychobiological Approach to Lighting, Hum. Factors, 1981, 23, 65–85.
9. R. Zhang and J.-F. Sun, An efficient perovskite-type Rb₂CaPO₄F:Eu²⁺ phosphor with high brightness towards closing the cyan gap, J. Alloys Compd., 2021, 872, 159698.
10. C. Hu, B. Peng, K. X. Song, B. Liu, D. Wang and I. M. Reaney, The cyan-green luminescent behaviour of nitrided Ba₉Y₂Si₆O₂₄: Eu²⁺ phosphors for W-LED, Ceram. Int., 2018, 44, S2–S6.
11. M. Zhao, H. Liao, M. S. Molokeev, Y. Zhou, Q. Zhang, Q. Liu and Z. Xia, Emerging ultra-narrow-band cyan-emitting phosphor for white LEDs with enhanced color rendition, Light: Sci. Appl., 2019, 8, 38.
12. J. Tang, J. Si, G. Li, T. Zhou, Z. Zhang and G. Cai, Excellent enhancement of thermal stability and quantum efficiency for Na₂BaCa(PO₄)₂:Eu²⁺ phosphor based on Sr doping into Ca, J. Alloys Compd., 2022, 911, 165092.
13. S. Huang, M. Shang, Y. Yan, Y. Wang, P. Dang and J. Lin, Ultra-broadband green-emitting phosphors without cyan gap based on double-heterovalent substitution strategy for full-spectrum WLED lighting, Laser Photonics Rev., 2022, 16, 2200473.
14. Z. Yang, G. Liu, Y. Zhao, Y. Zhou, J. Qiao, M. S. Molokeev, H. C. Swart and Z. Xia, Competitive site occupation toward improved quantum efficiency of SrLaScO₄:Eu red phosphors for warm white LEDs, Adv. Opt. Mater., 2022, 10, 2102373.
15. Q. Zhang, X. Wang and Y. Wang, Design of a broadband cyan-emitting phosphor with robust thermal stability for high-power WLED application, J. Alloys Compd., 2021, 886, 161217.
16. S. Liu, B. Deng, J. Yang, J. Liu, J. Chen, F. Zeng, H. Wang, R. Yu and G. Zhang, Multi-site occupancies and luminescence properties of cyan-emitting Ca_9−xNaGd_2/3(PO₄)₇:Eu²⁺ phosphors for white light-emitting diodes, J. Rare Earths, 2022, 40, 243–252.
17. J. Qiao, L. Ning, M. S. Molokeev, Y. C. Chuang, Q. Liu and Z. Xia, Eu²⁺ Site Preferences in the Mixed Cation K₂BaCa(PO₄)₂ and Thermally Stable Luminescence, J. Am. Chem. Soc., 2018, 140, 9730–9736.
18. P. Strobel, S. Schmiechen, M. Siegert, A. Tücks, P. J. Schmidt and W. Schnick, Narrow-Band Green Emitting Nitridolithoalumosilicate Ba[Li₂(Al₂Si₂)N₆]:Eu²⁺ with Framework Topology whj for LED/LCD-Backlighting Applications, Chem. Mater., 2015, 27, 6109–6115.
19. X. Zhang, J. Zhang, X. Wu, L. Yu, Y. Liu, X. Xu and S. Lian, Discovery of blue-emitting Eu²⁺-activated sodium aluminate phosphor with high thermal stability via phase segregation, Chem. Eng. J., 2020, 388, 124289.
20. R. Shafei, D. Maganas, P. J. Strobel, P. J. Schmidt, W. Schnick and F. Neese, Electronic and Optical Properties of Eu²⁺-Activated Narrow-Band Phosphors for Phosphor-Converted Light-Emitting Diode Applications: Insights from a Theoretical Spectroscopy Perspective, J. Am. Chem. Soc., 2022, 144, 8038–8053.
21. S. Jiao, R. Pang, S. Wang, H. Wu, T. Tan, S. Zhang, L. Jiang, D. Li, C. Li and H. Zhang, Regulating chromium ions site occupancy and enhancing near-infrared luminescence properties of Sr₂P₂O₇:Cr³⁺ phosphor through synthesizing under reduction atmosphere, Mater. Res. Bull., 2022, 149, 111710.
22. H. Liu, W. Zhang, H. Liang, Q. Zeng, J. Shi and D. Wen, Discovery of a new phosphor via aliovalent cation substitution: DFT predictions, phase transition and luminescence properties for lighting and anti-counterfeiting applications, J. Mater. Chem. C, 2021, 9, 1622–1631.
23. Y. Zhuo, A. M. Tehrani, A. O. Oliynyk, A. C. Duke and J. Brgoch, Identifying an efficient, thermally robust inorganic phosphor host via machine learning, Nat. Commun., 2018, 9, 4377.
24. J. Xiang, J. Zheng, Z. Zhou, H. Suo, X. Zhao, X. Zhou, N. Zhang, M. S. Molokeev and C. Guo, Enhancement of red emission and site analysis in Eu2+ doped new-type structure Ba₃CaK(PO₄)₃ for plant growth white LEDs, Chem. Eng. J., 2019, 356, 236–244.
25. P. Dai, Q. Wang, M. Xiang, T.-M. Chen, X. Zhang, Y.-W. Chiang, T.-S. Chan and X. Wang, Composition-driven anionic disorder-order transformations triggered single-Eu²⁺-converted high-color-rendering white-light phosphors, Chem. Eng. J., 2020, 380, 122508.
26. S. Wang, Y. Wu, Y. Fan, L. Wu and J. Yu, Crystal structure and photoluminescence properties of blue-green-emitting Ca_1−xSr_xZr₄(PO₄)₆: Eu²⁺ (0 ≤ x≤1) phosphors, Mater. Res. Bull., 2020, 125, 110781.
27. H. Yu, W. Su, L. Chen, D. Deng and S. Xu, Excellent temperature sensing characteristics of europium ions self-reduction Sr₃P₄O₁₃ phosphors for ratiometric luminescence thermometer, J. Alloys Compd., 2019, 806, 833–840.
28. Y. Fang, Y. Su, L. Dong, G. Zhang, P. Chen, Y. Liu, L. Shen, X. Yu, P. Tang, H. Chen, F. Huang and J. Hou, Tendentious multiple sites occupation towards white light emission in single-phase Ba_2(1-y/3)Ca_(1-y/3)Sr_yB₂Si₄O₁₄:Eu²⁺ phosphors, J. Solid State Chem., 2022, 309, 122963.
29. T. Yang, T. Zhang, S. Huang, T. D. Christopher, Q. Gu, Y. Sui and P. Cao, Structure tailoring and defect engineering of LED phosphors with enhanced thermal stability and superior quantum efficiency, Chem. Eng. J., 2022, 435, 133873.
30. X. Li, P. Li, C. Liu, L. Zhang, D. Dai, Z. Xing, Z. Yang and Z. Wang, Tuning the luminescence of Ca₉La(PO₄)₇:Eu²⁺ via artificially inducing potential luminescence centers, J. Mater. Chem. C, 2019, 7, 14601–14611.
31. Y. Wei, L. Cao, L. Lv, G. Li, J. Hao, J. Gao, C. Su, C. C. Lin, H. S. Jang, P. Dang and J. Lin, Highly Efficient Blue Emission and Superior Thermal Stability of BaAl₁₂O₁₉:Eu²⁺ Phosphors Based on Highly Symmetric Crystal Structure, Chem. Mater., 2018, 30, 2389–2399.
32. X. Zhang and J.-F. Sun, Intense blue emission of perovskite-type fluoride phosphor Cs₄Mg₃CaF₁₂: Eu²⁺ as a promising pc-WLEDs material, J. Alloys Compd., 2020, 835, 155225.
33. H. Liu, H. Liang, W. Zhang, Q. Zeng and D. Wen, Improving the thermal stability and luminescent efficiency of (Ba,Sr)₃SiO₅:Eu²⁺ phosphors by structure, bandgap engineering and soft chemistry synthesis method, Chem. Eng. J., 2021, 410, 128367.
34. R. Zhou, C. Liu, L. Lin, Y. Huang and H. Liang, Multi-site occupancies of Eu²⁺ in Ca₆BaP₄O₁₇ and their potential optical thermometric applications, Chem. Eng. J., 2019, 369, 376–385.
35. W. Wang, M. Tao, Y. Liu, Y. Wei, G. Xing, P. Dang, J. Lin and G. Li, Photoluminescence Control of UCr₄C₄-Type Phosphors with Superior Luminous Efficiency and High Color Purity via Controlling Site Selection of Eu²⁺ Activators, Chem. Mater., 2019, 31, 9200–9210.
36. W. Ji, S. Wang, Z. Song and Q. Liu, Luminescent thermal stability and electronic structure of narrow-band green-emitting Sr-Sialon:Eu²⁺ phosphors for LED/LCD backlights, J. Alloys Compd., 2019, 805, 1246–1253.
37. L. Wang, X. Kong, P. Li, W. Ran, X. Lan, Q. Chen and J. Shi, Narrow-band green emission of Eu²⁺ in a rigid tunnel structure: site occupations, barycenter energy calculations and luminescence properties, Inorg. Chem. Front., 2019, 6, 3604–3612.
38. Y. Zhang, Z. Zhang, X. Liu, G. Shao, L. Shen, J. Liu, W. Xiang and X. Liang, A high quantum efficiency CaAlSiN₃:Eu²⁺ phosphor-in-glass with excellent optical performance for white light-emitting diodes and blue laser diodes, Chem. Eng. J., 2020, 401, 125983.
39. C. Xie, L. Mei, Y. G. Liu, R. Mi, H. Yu, X. Bu, J. Chen, J. Yang and D. Liu, Simultaneous Spectral Tuning and Thermal Stability Adjustment in Ca₈ZnGa_(1-x)La_x(PO₄)₇:Eu²⁺ Phosphors, Inorg. Chem., 2022, 61, 3263–3273.
40. S. Zhang, Y. Zhao, J. Zhou, H. Ming, C.-H. Wang, X. Jing, S. Ye and Q. Zhang, Structural design enables highly-efficient green emission with preferable blue light excitation from zero-dimensional manganese(II) hybrids, Chem. Eng. J., 2021, 421, 129886.
41. W. Wang, H. Yang, M. Fu, X. Zhang, M. Guan, Y. Wei, C. C. Lin and G. Li, Superior thermally-stable narrow-band green emitter from Mn²⁺-doped zero thermal expansion (ZTE) material, Chem. Eng. J., 2021, 415, 128979.
42. Y. Wei, X. Y. Qu, G. G. Li, Z. Y. Cheng, M. S. Molokeev, C. C. Lin, T. S. Chan, C. K. Chang, Y. C. Chuang and J. Lin, Ultra-narrow band blue emission of Eu²⁺ in halogenated (Alumino)borate systems based on high lattice symmetry, J. Am. Ceram. Soc., 2019, 102, 2353–2369.
43. H. Y. Wu, H. M. Li, L. H. Jiang, R. Pang, S. Zhang, D. Li, G. Y. Liu, C. Y. Li, J. Feng and H. J. Zhang, Design of a mixed-anionic-ligand system for a blue-light-excited orange-yellow emission phosphor Ba_1.31Sr_3.69(BO₃)₃Cl:Eu²⁺, J. Mater. Chem. C, 2020, 8, 3040–3050.
44. Z. P. Lian and Q. F. Yan, Eu²⁺-Doped NaBa₄(AlB₄O₉)₂X₃ (X = Cl, Br) phosphors with intense two-center blue emission and high color purity for n-UV pumped white light-emitting diodes, J. Mater. Chem. C, 2016, 4, 7959–7965.
45. W. Li, H. P. Wu, H. W. Yu, Z. G. Hu, J. Y. Wang and Y. C. Wu, Ba₆BO₃Cl₉ and Pb₆BO₄Cl₇: structural insights intoortho-borates with uncondensed BO₄ tetrahedra, Chem. Commun., 2020, 56, 6086–6089.
46. R. D. Shannon, Revised effective ionic-radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1976, 32, 751–767.
47. J. Zhou, M. Chen, J. Ding, J. Zhang, J. Chen, D. Wu and Q. Wu, Site occupation engineering of activator in a green phosphor Sr₈CaLu(PO₄)₇: Eu²⁺ with high quantum yield for solid state lighting, Ceram. Int., 2021, 47, 31940–31947.
48. X. Sheng, P. Dai, Z. Sun and D. Wen, Site-selective occupation of Eu²⁺ activators toward full-visible-spectrum emission in well-designed borophosphate phosphors, Chem. Eng. J., 2020, 395, 125141.
49. M. Wu, D. Deng, F. Ruan, B. Chen and S. Xu, A spatial/temporal dual-mode optical thermometry based on double-sites dependent luminescence of Li₄SrCa(SiO₄)₂: Eu²⁺ phosphors with highly sensitive luminescent thermometer, Chem. Eng. J., 2020, 396, 125178.
50. L. Liu, L. Chen, R. Wang, Y. Xiao, F. Yang, J. Peng, F. Du and X. Ye, Tuning luminescence of Ba₃Si₆O₁₂N₂:Eu²⁺ phosphor for full-spectrum warm white LED lighting, J. Alloys Compd., 2021, 869, 159377.
51. M. Liao, F. Wu, D. Zhu, X. Zhang, H. Dong, Z. Lin, M. Wen and Z. Mu, Towards single broadband white emission in Rb_0.5K_1.5CaPO₄(F, Cl): Eu²⁺ via selective site occupancy engineering for solid-state lighting applications, Chem. Eng. J., 2022, 449, 137801.
52. T. Yang, Z. Ma, S. Huang, T. Zhang, K. Zhao, L. Yin, Q. Gu and P. Cao, Ultra-narrow-band blue-emitting K₂SrBa(PO₄)₂:Eu²⁺ phosphor with superior efficiency and thermal stability, J. Alloys Compd., 2022, 892, 162066.
53. J. Zhang, L. Zhang, F. Liu, H. Wu, G. Pan, Y. Luo, Z. Hao and J. Zhang, Smaller stokes shift induced highly efficient broadband near infrared garnet phosphor, Laser Photonics Rev., 2022 DOI:10.1002/lpor.202200586.
54. C. Cheng, H. Huang, M. J. Talite, W. Chou, J. M. Yeh and C. Yuan, A facile method to prepare “green” nano-phosphors with a large Stokes-shift and solid-state enhanced photophysical properties based on surface-modified gold nanoclusters, J. Colloid Interface Sci., 2017, 508, 105–111.
55. S. Zhang, Y. Nakai, T. Tsuboi, Y. Huang and H. J. Seo, Luminescence and Microstructural Features of Eu-Activated LiBaPO₄ Phosphor, Chem. Mater., 2011, 23, 1216–1224.
56. Z. Song and Q. Liu, Effects of neighboring polyhedron competition on the 5d level of Ce³⁺ in lanthanide garnets, J. Phys. Chem. C, 2019, 123, 8656–8662.
57. Z. Song and Q. Liu, Structural indicator to characterize the crystal-field splitting of Ce³⁺ in garnets, J. Phys. Chem. C, 2020, 124, 870–873.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi00015j

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