Two new rare-earth oxyborates Ba₄BiTbO(BO₃)₄ and Ba_1.54Sr_2.46BiTbO(BO₃)₄ and luminescence properties of the Ba₄BiTb_1−xEu_xO(BO₃)₄ phosphors

Abstract

Single crystals of two new terbium oxyborates Ba₄BiTbO(BO₃)₄ and Ba_1.54Sr_2.46BiTbO(BO₃)₄ were obtained by the high-temperature solution method. They crystallize in the hexagonal P6₃/mmc group (Z = 2) with lattice parameters of a = 5.41865(9) Å, c = 26.3535(5) Å, V = 670.12(3) ų and a = 5.36534(19) Å, c = 26.0661(10) Å, V = 649.83(5) ų, respectively. Their crystal structures feature two kinds of layers: [Tb(BO₃)₂]_n³ⁿ⁻ formed by corner-sharing TbO₆ octahedra and BO₃ triangles, as well as [Bi(BO₃)₂O]_n⁵ⁿ⁻ consisting of Bi₂O₁₃ dimers and BO₃ groups, with alkali-earth cations sitting inside and between the layers. In addition, solid solutions of Ba₄BiTb_1−xEu_xO(BO₃)₄ (0 ≤ x ≤ 0.2) were prepared via the solid-state reaction method. The obtained products were characterized by powder XRD, SEM, IR/Raman, XPS, DRS, and luminescence spectroscopy. It was found that as the Eu³⁺ doped content varies from x = 0 to 0.2, the emission color of the Ba₄BiTb_1−xEu_xO(BO₃)₄ phosphors can be adjusted from cyan to near-white and then to orange-red or from green to orange and then to red under the excitation of 349 and 377 nm, respectively. Furthermore, the emission intensities and chromaticity coordinates were found to be sensitive to the temperature for the phosphor Ba₄BiTb_0.999Eu_0.001O(BO₃)₄ upon 377 nm excitation. The above results demonstrate that Ba₄BiTb_1−xEu_xO(BO₃)₄ phosphors have potential as multifunctional materials for solid-state lighting and temperature sensing applications.

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1. Introduction

Among solid-state lighting technology, white light-emitting diodes (w-LEDs) are excellent candidates to replace conventional incandescent lamps for their merits of high brightness, low energy consumption, long lifetime, small size, environmental-friendly nature, and so on.^1,2 In the past years, cadmium chalcogenide quantum dots (QDs) have been proposed to be the promising alternatives to the phosphors in w-LEDs due to their tunable and narrow-band emission, high photoluminescence quantum yield (PLQY), and good stability.³ However, the cadmium chalcogenide QDs have some issues hindering their practical applications. For example, to achieve good stability and high PLQY, high reaction temperatures and delicate surface passivation (shelling) are often required. Lately, fully inorganic perovskite QDs, CsPbX₃ (X = Cl, Br, I), have received rapidly growing attention as another option for further advancing w-LEDs.⁴ Different from chalcogenide QDs, perovskite QDs can be synthesized at much lower temperatures, and can easily achieve high PLQY without the need to grow passivation shells. Although different types of nanomaterials can be used for LEDs,^5–7 to date, the most efficient and popular commercial w-LEDs are based on the combination of a blue-emitting InGaN or GaN chip with a yellow-emitting phosphor, i.e. Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺).⁸ However, the lack of red component in YAG-based w-LEDs results in a low color-rendering index (typically ≤80) and a high correlated color temperature (typically ≥4500 K), which in turn restricts their potential applicability in many fields, such as the general illumination or medical lighting.⁹ To solve this problem and obtain an ideal warm w-LEDs, an alternative strategy was proposed which is to employ red, green, and blue phosphors excited by ultraviolet (UV) or near-ultraviolet (n-UV) LED chips.¹⁰ This scheme is often prone to problems of complex mixing ratios and overlapping of energy levels among various phosphors.¹¹ Besides, different attenuation situations of each phosphor will result in an unstable white light.¹² Therefore, it has also been proposed to manufacture w-LEDs by coating single-phase white-light-emitting phosphors on UV/n-UV LED chips, which has always been a research hotspot in solid-state lighting.

Among the trivalent rare-earth ions, Tb³⁺ and Eu³⁺ are the most frequently used activators in luminescent materials that can produce green or red light under n-UV excitation due to the characteristic transitions of Tb³⁺:⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) or Eu³⁺:⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4).^13–16 When Tb³⁺ and Eu³⁺ ions are co-doped into one host matrix, multi-color emission may occur owing to the efficient energy transfer from Tb³⁺ to Eu³⁺, the representative examples including KBaY(MoO₄)₃:Ln³⁺ (Ln³⁺ = Tb³⁺, Eu³⁺, Sm³⁺, Tb³⁺/Eu³⁺, Tb³⁺/Sm³⁺), Sr₃Y(BO₃)₃:Tb³⁺, Eu³⁺, Sr₈ZnY(PO₄)₇:Tb³⁺, Eu³⁺ and K₂Tb_1−xEu_xHf(PO₄)₃.^17–20 Once Tb³⁺ and Eu³⁺ ions are further combined with blue-emitting activators, such as Ce³⁺ or Eu²⁺, white light emission can be obtained, like the cases of BaY₂Si₃O₁₀:Ce³⁺, Tb³⁺, Eu³⁺, MgY₄Si₃O₁₃:Ce³⁺, Tb³⁺, Eu³⁺, Sr_1.5Ca_0.5SiO₄:Eu³⁺, Tb³⁺, Eu²⁺ and Gd₂O₂S:Eu³⁺, Tb³⁺, Eu²⁺.^21–24

The most common blue phosphors are usually produced by doping Ce³⁺ or Eu²⁺ into a host material.^25–27 The syntheses of these phosphors require a reducing atmosphere, as the rare-earth ions Ce³⁺ and Eu²⁺ are in their lower valence states. As everyone knows, Bi³⁺ ion is a representative non-rare-earth activator with 6s² electronic configuration, which can exhibit blue emission generated by the parity-allowed 6s²–6s6p inter-configurational transitions.²⁸ Bi³⁺ has low toxicity and is stable in the air, and its synthesis condition is relatively mild compared to Ce³⁺ and Eu²⁺. Consequently, Bi³⁺ can also be used as a blue-emitting center to combine with Tb³⁺ and Eu³⁺ ions to obtain single-composition white phosphors, and some reported examples include β-GdB₃O₆:Bi³⁺, Tb³⁺, Eu³⁺, Ca₃ZrSi₂O₉:Bi³⁺, Tb³⁺, Eu³⁺, Gd₂MoB₂O₉:Bi³⁺, Tb³⁺, Eu³⁺ and NaYGeO₄:Bi³⁺/Tb³⁺/Eu³⁺.^29–32 However, doping with three ions such as Bi³⁺, Tb³⁺, and Eu³⁺ can lead to complicated energy transfer, so tuning to accurate white color remains still a challenge.

In the previous study, we systematically investigated the BaO–Bi₂O₃–PbO–Eu₂O₃–B₂O₃ system and discovered a new borate Ba₃BiPbEuO(BO₃)₄.³³ On this basis, we synthesized a series of Ba₃BiPbY_1−xEu_xO(BO₃)₄ (0 ≤ x ≤ 1) solid solutions and studied their luminescence properties. However, there have been no reports of doping Tb³⁺ activators in this host to obtain white or color-adjustable phosphors, and of course, temperature sensing properties of this type of phosphor have also not been investigated so far. Because Pb belongs to a toxic element that limits the practical application of this phosphor in w-LEDs. Our attempt to replace Pb²⁺ with iso-valent Ba²⁺ and Sr²⁺ ions led to the characterization of two new rare-earth oxyborates, Ba₄BiTbO(BO₃)₄ and Ba_1.54Sr_2.46BiTbO(BO₃)₄. In this work, we first introduced the syntheses and crystal structures of these two new phases, and then, described the luminescence performance, energy transfer process, and temperature sensing behavior of Ba₄BiTb_1−xEu_xO(BO₃)₄ (0 ≤ x ≤ 0.2) fluorescent powders. It was found that the syntheses of this series of new borates can be carried out at a relatively low temperature without the need for a reducing atmosphere. Besides, the white light and color-tunable emission from green to red can be realized in Ba₄BiTb_1−xEu_xO(BO₃)₄ by simply changing the concentrations ratio of Tb³⁺/Eu³⁺ and selecting appropriate excitation irradiation. Additionally, a strong temperature sensitization phenomenon was found for this phosphor under 377 nm excitation, suggesting that Ba₄BiTb_1−xEu_xO(BO₃)₄ is a kind of luminescent material with potential application value in the field of w-LEDs and temperature sensors.

Note that Ba₄BiTbO(BO₃)₄ and Ba_1.54Sr_2.46BiTbO(BO₃)₄ are the first quaternary (quinary) borate found in the BaO (and SrO)–Bi₂O₃–Tb₂O₃–B₂O₃ system, and also, the luminescence properties of Ba₄BiTb_1−xEu_xO(BO₃)₄ (0 ≤ x ≤ 0.2) are presented here for the first time. This study also reveals that Ba₄BiLnO(BO₃)₄ (Ln = trivalent rare-earth ions) forms a large family of compounds, in which Ba²⁺ can be partially replaced by other divalent cations, such as Pb²⁺ and Sr²⁺. We believe that this work will enrich the structural chemistry of borates and help the development of new emission-tunable phosphors for w-LED applications and highly efficient luminescent thermometers for temperature-sensing applications.

2. Experimental section

2.1. Materials and methods

All chemicals for the synthesis were purchased from Sinopharm Chemical Reagent Co. Ltd, including BaCO₃ (A.R.), SrCO₃ (A.R.), Bi₂O₃ (A.R.), Y₂O₃ (99.99%), Tb₄O₇ (99.99%), Eu₂O₃ (99.99%) and H₃BO₃ (A.R.). Powder X-ray diffraction (XRD) measurements were conducted on a Bruker D8 ADVANCE diffractometer, which operated at 40 kV and 40 mA with Cu K_α radiation (λ = 1.5406 Å). Scanning electron microscope (SEM) images were taken using a Hitachi SU8020 instrument (accelerating voltage 20 kV) equipped with an energy-dispersive X-ray detector (EDX). Infrared (IR) spectra were obtained with a VERTEX 70 Fourier Transform Infrared Spectrometer (Bruker). Raman spectroscopy data were recorded on the powder sample by a Renishaw InVia Raman spectrometer, which has a spectral resolution of 1 cm⁻¹ and is equipped with a confocal DM 2500 Leica optical microscope (with a 50× objective) and a 785 nm diode laser. Diffuse Reflectance Spectra (DRS) were measured with a Hitachi UH4150 UV-visible/NIR spectrophotometer, scanning at 120 nm min⁻¹. The elemental valence states were analyzed via X-ray Photoelectron Spectroscopy (XPS) on a Thermo Scientific ESCALAB 250Xi spectrometer with monochromatized Al K_α radiation (hν = 1486.6 eV). Photoluminescence excitation (PLE) and emission (PL) spectra as well as luminescence decay curves were collected via the Edinburgh FLS 1000 fluorescence spectrometer. A 150 W Xe lamp and a 60 W μF flash lamp were used as the excitation source for the steady-state spectrum and decay curve measurements, respectively. Quantum yield (QY) and variable temperature PL spectra were measured by employing a barium sulfate-coated integrating sphere (150 mm in diameter) and a temperature controller (Oxford, OptistatDN2) attached to the FLS 1000 system, respectively.

2.2. Synthetic procedures

Single crystals of the title compounds were grown via the high-temperature solution method. Typically, a mixture of 0.2894 g BaCO₃, 0.9112 g Bi₂O₃, 0.1828 g Tb₄O₇ and 0.0907 g H₃BO₃ (the molar ratio of 6 : 8 : 1 : 6) for Ba₄BiTbO(BO₃)₄, and a mixture of 0.3879 g BaCO₃, 0.2031 g SrCO₃, 0.6411 g Bi₂O₃, 0.0735 g Tb₄O₇ and 0.1945 g H₃BO₃ (the molar ratio of 20 : 14 : 14 : 1 : 32) for Ba_1.54Sr_2.46BiTbO(BO₃)₄ were transferred to Pt crucibles after thorough grinding, respectively. In the furnace, these mixtures were gradually heated to 950 °C, where they were kept for 6 h, then cooled to 700 °C at a rate of 1.5 °C h⁻¹, and further to 400 °C at 5 °C h⁻¹, followed by cooling to room temperature at 20 °C h⁻¹. The colorless transparent crystals of the title compounds were observed, which showed a hexagonal plate-like habit with various sizes ranging from 0.1 to 1.0 mm in diameter and several tenths of a millimeter in thickness. They were mechanically isolated from the solidified melt and further examined through SEM-EDX analyses, which confirmed the existence of heavy elements of Ba, Bi, Tb, and O (Ba, Sr, Bi, Tb, and O) (B is too light to be detected, see Fig. S1 and S2†).

Polycrystalline samples of Ba₄BiYO(BO₃)₄ and Ba₄BiTb_1−xEu_xO(BO₃)₄ (x = 0, 0.001, 0.002, 0.005, 0.01, 0.05, 0.1, 0.2, and 1) were prepared by the traditional solid-state reaction method. The required metal carbonates or oxides and boric acid were weighted in stoichiometric ratio and thoroughly mixed and ground in an agate mortar for 30 min. Subsequently, they were transferred into alumina crucibles and preheated at 500 °C for 12 h to remove CO₂ and H₂O. Then further sintering at 830 °C for 96 h with multiple intermittent grinding was implemented to ensure the complete chemical reaction. Finally, the obtained products were naturally cooled to room temperature and ground again for further characterization.

2.3. Single-crystal X-ray diffraction

Single-crystal XRD data of two crystals were collected at room temperature on a SuperNova (Mo) X-ray source (λ = 0.71073 Å). Data reduction, cell refinements, and analytical absorption corrections were accomplished using CrysAlisPro software.³⁴ The structures were solved by direct methods and refined with anisotropic displacement parameters for all atoms by full-matrix least-squares fitting on F² using SHELX-2019.³⁵

For Ba₄BiTbO(BO₃)₄, the site-occupancy refinements indicated that there is an atomic site (Wyckoff 4e position) that was co-occupied by Ba and Bi atoms. In the initial refinement, Ba²⁺ and Bi³⁺ cations were placed at the same 4e site, with their atomic coordinates and anisotropic displacement parameters being constrained to be identical. After convergence of the refinement, rather large displacement parameters were obtained for this (Ba/Bi) site, suggesting additional positional disorder. Eventually, a significant improvement in the refinement was achieved after splitting the (Ba/Bi) into two close positions. A similar situation was found in Ba_1.54Sr_2.46BiTbO(BO₃)₄, except that (Ba/Bi) was replaced by (Sr/Bi). The structures were checked with the program MISSYM, and no obvious additional crystallographic symmetry was detected.³⁶ General crystallographic information is presented in Table 1. Atomic coordinates, site occupancies, and equivalent isotropic displacement parameters, as well as selected bond lengths and angles, are provided in Tables S1–S3.†

Table 1 Crystallographic data for Ba₄BiTbO(BO₃)₄ and Ba_1.54Sr_2.46BiTbO(BO₃)₄^a

a R1 = Σ||F0| − |Fc||/Σ|F0| and wR2 = [Σw(F0^2 − Fc^2)^2/ΣwF0^4]^1/2 for F0^2 > 2σ(F0^2).
Formula	Ba4BiTbO(BO3)4	Ba1.54(7)Sr2.46(7)BiTbO(BO3)4
Formula weight	1168.50	1046.19
Crystal size (mm^3)	0.25 × 0.20 × 0.05	0.25 × 0.25 × 0.15
Space group	P63/mmc (no. 194)	P63/mmc (no. 194)
a (Å)	5.41865(9)	5.36534(19)
c (Å)	26.3535(5)	26.0661(10)
V (Å^3)	670.12(3)	649.83(5)
Z	2	2
dcalc (g cm^−3)	5.791	5.347
μ (mm^−1)	29.920	33.542
2θmax (°)	70	66
Unique reflections	636	536
Observed [I ≥ 2σ(I)]	576	491
No. of variables	35	37
GOF on F0^2	1.128	1.066
R1/wR2 [I ≥ 2σ(I)]	0.0417/0.0964	0.0604/0.1077
R1/wR2 (all data)	0.0464/0.0988	0.0645/0.1094

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3. Results and discussion

3.1. Crystal structure description

Since Ba₄BiTbO(BO₃)₄ and Ba_1.54Sr_2.46BiTbO(BO₃)₄ are isostructural, we chose Ba₄BiTbO(BO₃)₄ to describe the crystal structure. The fundamental building blocks in this structure are TbO₆ octahedra, BO₃ triangles, and Bi₂O₁₃ dimers composed of two corner-sharing BiO₇ hexagonal pyramids, as shown in Fig. 1(a). Within the ab plane, TbO₆ octahedra are arranged hexagonally and further bridged by B1-centered BO₃ groups via common O atoms to form a 2D infinite sheet of [Tb(BO₃)₂]_n³ⁿ⁻ [layer A, see Fig. 1(b)]. There is another type of 2D sheet that has a composition [Bi(BO₃)₂O]_n⁵ⁿ⁻ [layer B, see Fig. 1(c)]. This layer is constructed by corner-sharing Bi₂O₁₃ dimers, with a pair of boron atoms (two B2) accommodated within one-half of the three-membered rings of the layer. Four sheets, ABA′B′, are required to complete a translation period of the c-axis direction, where the sheets A′ and B′ are related to A and B by a 6₃-screw axis. Some alkali-earth cations (Ba1 and Ba3) fill the space between the layers, while others (Ba2) reside in the channels that pass through the layers B and B′ to provide charge compensation.

Fig. 1 The unit cell of Ba₄BiTbO(BO₃)₄ projected approximately along [100] (a), the sheets A and B viewed along [001] (b and c), and the local environment of each metal cation site (d). Ba1, Ba2, and Ba3: black, blue, and orange balls, respectively; BiO₇: magenta hexagonal pyramids; TbO₆: deep sky blue octahedra; B1O₃ and B2O₃: cyan and green triangles, respectively. In plot (c), when the structure is viewed along [001], two B2O₃ triangles overlap, while two BiO₇ hexagonal pyramids share a corner to form a Bi₂O₁₃ dimer.

As shown in Table S1,† there are nine unique atomic sites in the asymmetric unit of Ba₄BiTbO(BO₃)₄, including 2Ba, 1(Ba/Bi), 1Tb, 2B, and 3O, all located in crystallographically special positions. Two distinct Ba atoms are both nine-coordinated, but adopt different coordination configurations: Ba1 in an irregular polyhedron and Ba2 in a tri-capped trigonal prism [Fig. 1(d)]. The Ba–O distances are normal, as shown in Table S2.† Concerning the disordered (Ba/Bi) site, it is found that Ba and Bi atoms are statistically distributed over two very close positions [Ba3–Bi1 = 0.320(3) Å]. Each is strongly bonded to one oxygen and also weakly bonded to six other O atoms to form a hexagonal pyramid, as shown in Fig. 1(d), which reflects the fact that the 6s² non-bonded electron pairs of Bi³⁺ are stereochemically active. The Tb atom occupies the Wyckoff 2a position with local D_3d symmetry. It is surrounded by six oxygen atoms in an octahedral environment with six equal Tb–O bond distances of 2.255(6) Å, which are very close to those reported in K₃TbB₆O₁₂ [2.229(4) Å–2.348(5)].³⁷ Two B atoms are each connected to three oxygen atoms, forming a BO₃ planar triangle with bond lengths in the range of 1.372(6)–1.379(7) Å, and the bond angles deviated slightly from 120°. These are common values as found in other known borates.^33,38 The calculated total bond valences for Ba1, Ba2, Tb, B1, and B2 are 2.049, 2.052, 3.282, 2.991, and 2.937, respectively, consistent with their expected oxidation states.³⁹

Although the title compounds are isostructural with our previously reported Pb²⁺-analog, Ba₃BiPbEuO(BO₃)₄, their crystal structures show a certain difference.³³ For example, in Ba₃BiPbEuO(BO₃)₄, both Pb²⁺ and Bi³⁺ cations are located at the same 4e site with identical positional and thermal parameters, while in Ba₄BiTbO(BO₃)₄ [Ba_1.54Sr_2.46BiTbO(BO₃)₄], this site is split into two very close positions, occupied by Ba²⁺ and Bi³⁺ (Sr²⁺ and Bi³⁺), respectively. This is understandable because both Pb²⁺ and Bi³⁺ have a 6s² electron configuration and, of course, a similar coordination environment, while the ionic radii and coordination geometries around Ba²⁺ and Bi³⁺ as well as Sr²⁺ and Bi³⁺ are very different. Inspection of structural details (Tables S1–S3†) reveals that when Sr²⁺ partially replaces Ba²⁺ in Ba₄BiTbO(BO₃)₄ to form Ba_1.54Sr_2.46BiTbO(BO₃)₄, the Ba1 and Ba2 sites are mixed with some Sr atoms, with the refined compositions Ba_0.58(2)Sr_0.42(2) and Ba_0.38(3)Sr_0.62(3), respectively. The Ba3/Bi1 site is still statistically distributed, but Ba3 is completely replaced by Sr3. Given identical coordinated numbers, (Ba/Sr)–O distances are significantly smaller than the corresponding Ba–O ones as expected. In addition, there is also a remarkable reduction in cell axis lengths and volume due to the smaller size of Sr²⁺ compared with Ba²⁺ (Table 1). What's more, this work also indicates that Ba₄BiLnO(BO₃)₄ (Ln = trivalent rare-earth ions) forms a large class of compounds, where Ba²⁺ can be partially substituted by other divalent ions, like Pb²⁺ and Sr²⁺.

3.2. Phase purity and morphology

A series of Ba₄BiTb_1−xEu_xO(BO₃)₄ (0 ≤ x ≤ 1) samples were prepared and their crystallinity and phase purity were assessed by powder XRD, as shown in Fig. 2(a). For comparison, the simulated pattern of Ba₄BiTbO(BO₃)₄ based on its single-crystal structure is provided at the bottom of the figure. All observed XRD patterns are similar to each other and also consistent with the simulated pattern of the fully Tb³⁺ sample, showing that the introduction of Eu³⁺ into this host still maintains single-phase and does not cause significant changes in the crystal structure. Another noteworthy feature is that the enlarged diffraction peaks from 2θ = 27.5°–28.0° of the Ba₄BiTb_1−xEu_xO(BO₃)₄ samples, belonging to the representative (016) crystal face, slightly shift towards lower angles relative to the position of Ba₄BiTbO(BO₃)₄ with increasing x. Since the effective ionic radius of Eu³⁺ (r = 0.947 Å, CN = 6) is larger than that of Tb³⁺ (r = 0.923 Å, CN = 6), but smaller than that of Bi³⁺ (r = 1.07 Å, CN = 7),^40,41 such a shift implies that Eu³⁺ ions prefer to substitute Tb³⁺ instead of Bi³⁺ sites in the Ba₄BiTbO(BO₃)₄ host. This is understandable because the coordination configuration around Eu³⁺ and Tb³⁺ is very similar and relatively regular, while that around Bi³⁺ is severely distorted due to the steric effect of 6s² lone pair electrons [see Fig. 1(d)].

Fig. 2 (a) XRD patterns of the Ba₄BiTb_1−xEu_xO(BO₃)₄ (0 ≤ x ≤ 1) phosphors. (b and c) Rietveld refinements of the XRD files for Ba₄BiTb_1−xEu_xO(BO₃)₄ (x = 0 and 1). (d) The cell parameters (a, c, and V) obtained from Rietveld fitting against the Eu³⁺ content (x).

To further recognize the effect of doping Eu³⁺ ions on the crystal structures of Ba₄BiTb_1−xEu_xO(BO₃)₄, Rietveld refinements were conducted on the powder XRD profiles of Ba₄BiTb_1−xEu_xO(BO₃)₄ (x = 0, 0.001, 0.002, 0.005, 0.01, 0.05, 0.1, 0.2, and 1) using TOPAS Academic software.⁴² The initial model used for the Rietveld refinements was the single-crystal structure of Ba₄BiTbO(BO₃)₄ (Tables 1 and S1†). The constraint condition is that atomic coordinates of B and O, all atomic occupancies, and isotropic displacement parameters were fixed, while the positional parameters of heavy atomic sites [including z(Ba1), z(Ba3), and z(Bi1)] and lattice constants (a and c) were refined along with other parameters. The final refinement results are presented in detail in Table S4† as well as Fig. 2(b), (c) and S3.† As representatives, the lattice parameters of the species for 0 ≤ x ≤ 0.2 are plotted against the substitution level x in Fig. 2(d). The refinements proceeded smoothly and the residual factors R_wp are all less than 10.4%, which further confirmed that this series of solid solutions are isomorphic and crystallize in the hexagonal system with space group P6₃/mmc. In addition, the cell parameters (a, c, and V) show an obvious linear expansion along with the increase in Eu³⁺ concentration. These results conform to Vegard's rule, indicating that Eu³⁺ ions are completely dissolved in the Ba₄BiTbO(BO₃)₄ host to form perfect solid solutions of Ba₄BiTb_1−xEu_xO(BO₃)₄.

Because particle size and surface roughness are important factors influencing the luminescence performance of fluorescent powders, FE-SEM and EDX analyses were conducted to investigate the morphological characteristics of the obtained powder samples. Fig. 3 displays the FE-SEM images, EDX spectrum, and elemental mapping of the prepared Ba₄BiTb_0.999Eu_0.001O(BO₃)₄ phosphor. It can be seen that the particles are aggregated together and have good crystallinity with smooth surface and irregular morphology, and the particle size is in the range of several micrometers. This may be due to the sample being obtained through a solid-state reaction route of intermediate grinding. The EDX pattern confirms the presence of the dominant elements Ba, Bi, Tb, Eu, B, and O, all homogeneously distributed over the same selection region of the sample, as demonstrated in the elemental mapping images. This indicates that the target solid solution has been successfully synthesized (note that the boron content cannot be accurately measured due to its low atomic weight, while the gold signal comes from the gold sputtering process).

Fig. 3 FE-SEM images, typical EDX results, and elemental mapping of the Ba₄BiTb_0.999Eu_0.001O(BO₃)₄ phosphor. Note that a thin layer of Au was evaporated on the sample surface to provide conductivity before SEM inspection.

3.3. IR and Raman spectra

Although single crystal XRD analyses indicated the presence of BO₃ groups in the title compounds, IR and Raman spectroscopy were still studied to further confirm the coordination environment of B atoms. Ba₄BiLnO(BO₃)₄ (Ln = trivalent rare-earth ions) crystallizes in the centrosymmetric P6₃/mmc (D_6h⁴, No. 194) group (Z = 2). Based on group theoretical analysis,^33,43 138 Brillouin zone center modes are predicted, which have the mechanical representation: Γ_vibr = 8A_1g + 2A_1u + 2A_2g + 11A_2u + 10B_1g + 2B_1u + 2B_2g + 9B_2u + 11E_2u + 12E_2g + 13E_1u + 10E_1g, where the E mode is doubly degenerate and usually observed as one frequency. These modes can be further subdivided into 2E_2u + 2E_2g + 2E_1u + 2E_1g asymmetric stretching (ν₃), 2A_1g + 2A_2u + 2B_1g + 2B_2u symmetric stretching (ν₁), 2A_1g + 2A_2u + 2B_1g + 2B_2u out-of-plane bending (ν₂), 2E_2u + 2E_2g + 2E_1u + 2E_1g in-plane bending (ν₄), 2A_1g + 2A_2u + 2B_1g + 2B_2u + 2E_2u + 2E_2g + 2E_1u + 2E_1g translational and 2A_1u + 2A_2g + 2B_1u + 2B_2g + 2E_2u + 2E_2g + 2E_1u + 2E_1g librational modes of the BO₃ groups. The remaining modes correspond to translations of the Ba1 (A_1g + A_2u + B_1g + B_2u + E_2u + E_2g + E_1u + E_1g), Ba2 (A_2u + B_1g + E_2g + E_1u), (Ba3/Bi1) (A_1g + A_2u + B_1g + B_2u + E_2u + E_2g + E_1u + E_1g), Tb1 (A_2u + B_2u + E_2u + E_1u) and O3 (A_2u + B_1g + E_2g + E_1u) sites. It should be remembered that three vibrational modes (A_2u + E_1u) belong to the acoustic branch, therefore the total number of optical modes in the crystal is 135 (Γ_optic = 8A_1g + 2A_1u + 2A_2g + 10A_2u + 10B_1g + 2B_1u + 2B_2g + 9B_2u + 11E_2u + 12E_2g + 12E_1u + 10 E_1g). Since A_1u, A_2g, B_1g, B_1u, B_2g, B_2u, and E_2u modes are silent, there are a total of 86 optical modes distributed into 52 Raman-active (Γ_R = 8A_1g + 12E_2g + 10E_1g) and 34 IR-active (Γ_IR = 10A_2u + 12E_1u) modes, leading to 30 Raman and 22 IR frequencies, respectively. It is difficult to accurately assign the individual bands to specific vibrational modes, as some modes have very similar wavenumbers, but a rough assignment of groups for both IR and Raman spectra is possible.

Fig. S4† presents the polycrystalline IR and Raman spectra measured in the internal mode region for Ba₄BiLnO(BO₃)₄ (Ln = Y, Tb and Eu). Three compounds show similar spectral profiles, illustrating that they are isostructural to each other, and the introduction of different rare-earth ions does not affect the fundamental vibrational modes of the main structural units. Both IR and Raman spectra above 550 cm⁻¹ can be divided into four regions, of which the area of 1100–1400 cm⁻¹ shows the asymmetric stretching vibrations (ν₃) of the BO₃ groups. The absorption bands between 850 and 980 cm⁻¹ can be assigned as the BO₃ symmetric stretching vibrations (ν₁). The absorption caused by the out-of-plane bending (ν₂) of the trigonal group appears as bands in the range of 700–800 cm⁻¹. The remaining bands from 550 to 680 cm⁻¹ are the result of the in-plane BO₃ bending (ν₄). These assignments are consistent with those of other reported borates.^44–46 As everyone knows, for the ideal BO₃ group with D_3h symmetry, the modes ν₁(A₁′) and ν₂(A₂′′) should be IR and Raman inactive, respectively.⁴³ The appearance of vibrations ν₁(2A_2u) in the IR and ν₂(2A_1g) in the Raman spectra indicates that the BO₃ groups in Ba₄BiLnO(BO₃)₄ deviate from the ideal symmetry. In fact, our structural analysis reveals that there are two crystallographically independent B atoms, both of which occupy the 4f position with local symmetry C_3v instead of D_3h (see Table S1†).

3.4. XPS spectra

To get an idea of the valence states of the constituent elements in the system, we conducted XPS studies on the phosphor Ba₄BiTb_0.999Eu_0.001O(BO₃)₄ over a wide energy range, as shown in Fig. S5(a).† The XPS survey scan shows the respective photoemission peaks caused by the constituent elements (Ba, Bi, Tb, Eu, B, and O) and the Auger peaks of Ba and O (Ba MNN & O KLL). The appearance of the C 1s signal is due to the absorption of adventitious hydrocarbons on the sample surface during the sample handling for XPS measurements. The chemical composition and bonding information were further probed utilizing the detailed, high-resolution core-level XPS scans of Ba 3d, Bi 4f, Tb 3d, Eu 3d, B 1s, and O 1s, as illustrated in Fig. S5(b)–(g).† The Ba 3d can be deconvoluted into two spin-orbital splitting components, Ba 3d_3/2 and Ba 3d_5/2, located at the binding energies (BEs) of 795.48 and 780.25 eV, respectively.⁴⁷ The XPS spectrum related to Bi 4f consists of two intense and symmetrical peaks observed at 164.47 (Bi 4f_5/2) and 159.25 (Bi 4f_7/2) eV with an energy difference of ∼5.22 eV.⁴⁸ The Tb 3d XPS peaks also exhibit doublet characteristics, which are assigned to Tb 3d_3/2 (∼1278.86 eV) and Tb 3d_5/2 (∼1244.18 eV), separated by 34.68 eV.⁴⁹ These BE values can be used as fingerprints to identify the presence of Ba²⁺, Bi³⁺, and Tb³⁺, respectively. In Fig. S5(e),† due to the very low Eu³⁺ content in the synthesized sample, the Eu 3d XPS signal is relatively weak, but it can be resolved into two components, corresponding to Eu 3d_3/2 (∼1168.40 eV) and 3d_5/2 (∼1138.02 eV), similar to previous results of other Eu³⁺-based materials.^33,50 For the B 1s and O 1s spectra, they can be fitted to symmetric single peaks with BEs of 191.32 and 531.60 eV, respectively, which are typical values related to B–O or O–cation bonding states in borates.^51–53 In addition, no other impurity peaks (including those from Eu²⁺ and Tb⁴⁺ ions) were identified from the XPS spectra. These results in combination with XRD and EDX data ensure the successful preparation of Ba₄BiTb_1−xEu_xO(BO₃)₄ phosphors, which provides a solid basis for subsequent investigations of optical absorption and PL properties.

3.5. UV-vis diffuse reflection spectra

To investigate the optical absorption properties, the UV-vis diffuse reflection spectra of Ba₄BiLnO(BO₃)₄ (Ln = Y, Tb, and Eu) were measured, as shown in Fig. S6.† All samples show a very high reflectance between 90 and 100% relative to the white standard BaSO₄ (optical grade) in the visible region, indicating a high optical grade of the as-prepared compounds. Below 400 nm, there is an intense broad absorption band, which is due to the collective effect of the ¹S₀ → ¹P₁/³P₁ transition of Bi³⁺ and the host lattice absorption.²⁸ For the Tb and Eu compounds, the Tb³⁺:4f⁸ → 4f⁷5d¹ absorption and O²⁻ → Eu³⁺ charge transfer absorption are also located in this area, respectively. Moreover, several sharp characteristic absorption peaks at about 393, 466, 527, 535 and 592 nm are assigned to ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₂, ⁷F₀ → ⁵D₁, ⁷F₁ → ⁵D₁ and ⁷F₁ → ⁵D₀ transitions of Eu³⁺, respectively, while the peak at 486 nm is ascribed to the ⁷F₆ → ⁵D₄ transition of Tb³⁺.⁵⁴

The optical band gap of Ba₄BiLnO(BO₃)₄ can be estimated by the following formula:⁵⁵

[F(R_∞)hν]n = A(hν − E_g) (1)

where hν is the photon energy, A is a proportional constant, E_g is the optical band gap energy and the value of n is 1/2 or 2 for an indirect or a direct allowed transition, respectively. The F(R_∞) can be described by the Kubelka–Munk function:⁵⁶

F(R_∞) = (1 − R)2/2R = K/S (2)

here, R, K, and S are the reflection, absorption, and scattering coefficients, respectively. From the linear extrapolation of the [F(R_∞)hν]ⁿ–hν plot to [F(R_∞)hν]ⁿ = 0, the indirect and direct band gaps of Ba₄BiYO(BO₃)₄ are estimated to be approximately 3.36 and 3.55 eV, while they are 3.34 (3.39) and 3.53 eV (3.59) for its Tb (Eu)-analog, respectively (see Fig. S6†). These E_g values are comparable to those previously reported in Ba₃BiPbEuO(BO₃)₄ [3.02 (indirect) and 3.35 (direct) eV].³³

3.6. PLE and PL spectra

As is known to all, Bi³⁺ possesses 6s² lone pair electrons, therefore, the position of its excitation and emission bands depends largely on the nature of the host lattice. The ground state of Bi³⁺ is ¹S₀ and its excited states are composed of ³P₀, ³P₁, ³P₂, and ¹P₁ in order of increasing energy. For the electron transition of Bi³⁺, ¹S₀ → ¹P₁ is parity- and spin-allowed, but it is usually located in the deep ultraviolet region (typically ≤250 nm), so it is difficult to use in LED devices. ¹S₀ → ³P₂ is spin-forbidden and the luminous intensity is low. ¹S₀ → ³P₀ is strictly forbidden because the total angular momentum does not change (ΔJ = 0). In contrast, ¹S₀ → ³P₁ has the longest luminous wavelength and can even enter the n-UV region (around 350–420 nm). Although it is forbidden according to the spin-selection rule, it will become partially allowed by mixing with singlet and triplet states (ΔJ = 1).⁵⁷

The luminescent properties of the title compounds were characterized by the photoluminescence excitation (PLE) and emission (PL) spectra. As depicted in Fig. 4(a), the PLE spectrum of Ba₄BiYO(BO₃)₄ (λ_em = 397 nm) shows three broad excitation bands, attributing to the ¹S₀ → ¹P₁ transition of Bi³⁺ (240 nm), and ¹S₀ → ³P₁ transitions of Bi³⁺ (I) (320 nm) and Bi³⁺ (II) (349 nm), respectively,^58–61 which are related to the crystal structure where the disordered (Ba/Bi) site is split into two positions (Table S1†). Upon 240 nm excitation, the PL spectrum presents an asymmetric emission band extending from 340 to 530 nm with the maximum at about 397 nm, which is ascribed to the ³P₁ → ¹S₀ transition of Bi³⁺.^58–61 The profiles of the emission spectra under 320 and 349 nm excitation are similar to those under 240 nm excitation, except that the emission intensities are much lower.

Fig. 4 The PLE and PL spectra of Ba₄BiYO(BO₃)₄ (a), Ba₄BiTbO(BO₃)₄ (b) and Ba₄BiEuO(BO₃)₄ (c).

Fig. 4(b) shows the luminescence spectra of Ba₄BiTbO(BO₃)₄. The PLE spectrum monitored at 543 nm comprises two parts: the absorption band from 230 to 275 nm (centered at ∼236 nm) corresponding to the 4f⁸ → 4f⁷5d¹ transition of Tb³⁺ overlapped with the ¹S₀ → ¹P₁ transition of Bi³⁺, and the other from 275 to 500 nm mainly associated with the intra-configurational 4f⁸ → 4f⁸ transitions of Tb³⁺ from ⁷F₆ levels to ⁵D₀ (318 nm), ⁵G₂ (342 nm), ⁵L₈ (353 nm), ⁵D₂ (360 nm), ⁵D₃ (377 nm) and ⁵D₄ (485 nm) levels.^54,62 The PL spectrum excited at 377 nm exhibits four groups of emission peaks in the region from 450 to 650 nm, of which the strongest at 543 nm is due to the ⁵D₄ → ⁷F₅ transition, while others at 488, 584, and 621 nm belong to the ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₄, and ⁵D₄ → ⁷F₃ transitions, respectively.^54,62 The spectrum clearly shows strong ⁵D₄ emission, and no significant ⁵D₃ emission was observed due to efficient cross-relaxation processes [⁵D₃(Tb³⁺) + ⁷F₆(Tb³⁺) → ⁵D₄(Tb³⁺) + ⁷F₀(Tb³⁺)] as expected for a fully concentrated Tb³⁺ material.⁶³ In addition, the PL spectrum under 349 nm excitation (Bi³⁺ absorption) includes intense Tb³⁺ emission together with weak Bi³⁺ emission, suggesting the occurrence of Bi³⁺ to Tb³⁺ energy transfer in Ba₄BiTbO(BO₃)₄.

Fig. 4(c) presents the luminescence spectra of Ba₄BiEuO(BO₃)₄. When monitoring 612 nm emission, the PLE spectrum consists of one broad absorption band in the region of 280–357 nm (centered at ∼299 nm) and several sharp excitation peaks in the longer wavelength region. The wide band is the typical absorption of O²⁻ → Eu³⁺ charge transfer superimposed with the ¹S₀ → ³P₁ transition of Bi³⁺, while the sharp peaks at 362, 382, 394, 414, 465, and 526 nm are attributed to ⁷F₀ → ⁵D₄, ⁵L₇, ⁵L₆, ⁵D₃, ⁵D₂ and ⁵D₁ transitions of Eu³⁺, respectively.^54,62 Upon 394 nm excitation, the PL spectrum shows a series of sharp emission peaks within 570–710 nm. These peaks located at 580, 593, 612, 656, and 704 nm are readily assigned to ⁵D₀ → ⁷F_J (J = 0–4) transitions of Eu³⁺, respectively. When excited at 349 nm (Bi³⁺ absorption), besides the very small residual emission of Bi³⁺, the strong characteristic emission peaks of Eu³⁺ can also be observed, which means an efficient energy transfer from Bi³⁺ to Eu³⁺. The comparison of the PLE spectrum for Ba₄BiLnO(BO₃)₄ (Ln = Tb and Eu) and PL spectrum for Ba₄BiYO(BO₃)₄ reveals a significant spectral overlap between the emission band of Bi³⁺ and the f–f absorption of Tb³⁺ and Eu³⁺ in the wavelength range of 360–425 nm. Therefore, the effective energy transfer from Bi³⁺ to both Tb³⁺ and Eu³⁺ can be expected. It is noticeable that among the observed emission transitions, the magnetic dipole ⁵D₀ → ⁷F₁ is predominant, suggesting that the surrounding environment of Eu³⁺ is centrosymmetric. This is in line with our crystallographic research, which shows that Eu³⁺ may replace Tb³⁺ and mainly occupies the six-fold coordinated Wyckoff 2a site, which has a regular octahedral coordination configuration. Therefore, the ligand field around Eu³⁺ is centrosymmetric [see Fig. 1(d) and Table S1†].

Fig. 5(a) shows the PLE and PL spectra of the representative phosphor, Ba₄BiTb_0.998Eu_0.002O(BO₃)₄. The PLE spectra monitored with Bi³⁺ emission of 397 nm and Tb³⁺ emission of 543 nm are similar to those of Ba₄BiYO(BO₃)₄ and Ba₄BiTbO(BO₃)₄, respectively, which means that the energy transfer of Bi³⁺ → Tb³⁺ is still apparent in this Bi³⁺/Tb³⁺/Eu³⁺ coexisting phosphor. On the contrary, the PLE spectrum of Ba₄BiTb_0.998Eu_0.002O(BO₃)₄ by monitoring Eu³⁺ emission at 612 nm differs slightly from those of Ba₄BiTbO(BO₃)₄ and Ba₄BiEuO(BO₃)₄. It is primarily composed of the characteristic excitation bands of Tb³⁺:4f⁸ → 4f⁷5d¹ and 4f⁸ → 4f⁸ transitions, with small admixtures of those of Bi³⁺:¹S₀ → ¹P₁ and Eu³⁺:4f⁶ → 4f⁶ transitions. On the other hand, under 240 nm (Bi³⁺:¹S₀ → ¹P₁) excitation, the PL spectrum of Ba₄BiTb_0.998Eu_0.002O(BO₃)₄ contains not only the Bi³⁺ blue emission band at 370–475 nm but also the Tb³⁺ green emission peak at 543 nm and the Eu³⁺ orange emission peak at 593 nm. The presence of Bi³⁺ and Tb³⁺ absorption bands when monitoring the Eu³⁺ emission and the observation of both Tb³⁺ and Eu³⁺ characteristic emissions when excited by Bi³⁺ absorption suggests the occurrence of Bi³⁺ → Eu³⁺, Tb³⁺ → Eu³⁺ and Bi³⁺ → Tb³⁺ energy transfer in the three-ion coexisting phosphor.

Fig. 5 The PLE and PL spectra of the Ba₄BiTb_0.998Eu_0.002O(BO₃)₄ phosphor (a) as well as the concentration-dependent PL spectra of the Ba₄BiTb_1−xEu_xO(BO₃)₄ (0 ≤ x ≤ 0.2) fluorescent powders [(b) λ_ex = 349 nm; (c) λ_ex = 377 nm; (d) λ_ex = 394 nm]. The integrated emission intensities of Bi³⁺ in the range of 370–475 nm, Tb³⁺ at about 543 nm (⁵D₄ → ⁷F₅), and Eu³⁺ at about 593 nm (⁵D₀ → ⁷F₁) were also calculated and shown in the insets of the figure.

Since the UV-excitation at ∼240 nm does not match the output wavelength of commercial n-UV (λ_em = 350–420 nm) LED chips, and also the Ba₄BiTb_1−xEu_xO(BO₃)₄ (0 ≤ x ≤ 0.2) phosphors show a blue-to-purple emission (without passing through the white light region) under this excitation, we chose 349 nm (Bi³⁺:¹S₀ → ³P₁) as the excitation wavelength for Bi³⁺ to measure the concentration-dependent PL spectra of Ba₄BiTb_1−xEu_xO(BO₃)₄. As seen from Fig. 5(b), the spectra exhibit typical Bi³⁺:³P₁ → ¹S₀, Tb³⁺:⁵D₄ → ⁷F_J (J = 6, 5) and Eu³⁺:⁵D₀ → ⁷F_J (J = 1–4) transitions in the blue, green, and orange emission regions, respectively. With an increase in the Eu³⁺ content (x), the emission intensities of both Bi³⁺ and Tb³⁺ decline monotonously, whereas that of Eu³⁺ first increases until x = 0.05 and then appears a downfall. This is easy to understand because the occurrence of Bi³⁺ → Tb³⁺ and Bi²⁺ → Eu³⁺ energy transfer results in a reduction of Bi³⁺ emissions. The incorporation of Eu³⁺ into the Ba₄BiTbO(BO₃)₄ host would cause the substantial energy transfer from Tb³⁺ to Eu³⁺, and meanwhile, the relatively high efficiency of Bi³⁺-to-Eu³⁺ energy transfer would suppress the Bi³⁺-to-Tb³⁺ energy transfer, therefore, a decrease in Tb³⁺ emissions can be anticipated. In addition, the increase of Eu³⁺ emissions with increasing Eu³⁺ content up to x = 0.05 is also understandable because both Bi³⁺ and Tb³⁺ can transfer energy to Eu³⁺. However, once the Eu³⁺ content (x) exceeds 0.05, the concentration quenching occurs, resulting in the weakening of the Eu³⁺ emission intensity. Moreover, as shown in Fig. 5(c), when excited at 377 nm (Tb³⁺:⁷F₆ → ⁵D₃), the PL spectra exhibit both Tb³⁺ and Eu³⁺ emissions, with the latter dominating. As the Eu³⁺ concentration increases, the emission intensity of Tb³⁺ at 543 nm decreases, while that of Eu³⁺ at 593 nm shows the opposite trend, as a consequence of Tb³⁺ → Eu³⁺ energy transfer. However, if we use 394 nm (Eu³⁺:⁷F₀ → ⁵L₆) as the excitation wavelength, only the characteristic emissions of Eu³⁺:⁵D₀ → ⁷F_J (J = 0–4) are visible in Fig. 5(d), indicating that the inverse transition (Eu³⁺ → Bi³⁺ and Eu³⁺ → Tb³⁺) is forbidden. In this case, the PL intensity increases linearly with the Eu³⁺ doping content (x), and no concentration quenching occurs up to x = 0.2. These observations show that the emission profiles and color of Ba₄BiTb_1−xEu_xO(BO₃)₄ are influenced not only by the dopant concentration but also by the selected excitation wavelength. Similar phenomena were reported in CdTb_1−xSm_xGaB₂O₇, CaCO₃:Eu³⁺, Tb³⁺ and CeO₂:Eu³⁺ as well.^64–66

3.7. Fluorescence lifetime and energy transfer mechanism

To further illustrate the energy transfer processes, Fig. 6 shows the fluorescence decay curves for the Bi³⁺:³P₁ → ¹S₀ and Tb³⁺:⁵D₄ → ⁷F₅ emission bands of Ba₄BiTb_1−xEu_xO(BO₃)₄ under excitation at 349 and 377 nm, respectively. All the decay profiles can be fitted into a triple-exponential function:in this expression, I₀ and I_t represent the PL intensity at time 0 and t, respectively. B₁, B₂, and B₃ stand for the fitting constants. τ₁, τ₂, and τ₃ are lifetimes for the triple-exponential components. The average decay lifetime (τ_avg) can be calculated using the equation given below:⁶⁷

Fig. 6 Decay curves of the Ba₄BiTb_1−xEu_xO(BO₃)₄ phosphors monitoring ³P₁ → ¹S₀ transition of Bi³⁺ (a) and ⁵D₄ → ⁷F₅ transition of Tb³⁺ (b) under excitation at 349 and 377 nm, respectively.

The τ_avg values obtained for different phosphors are also provided in Fig. 6, from which it is easy to see that increasing the Eu³⁺ concentration causes a reduction in the decay lifetimes of Bi³⁺ and Tb³⁺ emissions, which strongly supports the energy transfer from Bi³⁺ to Tb³⁺/Eu³⁺ and from Tb³⁺ to Eu³⁺.

Based on the above analyses, a schematic representation of the possible energy transfer pathways in Ba₄BiTb_1−xEu_xO(BO₃)₄ is described in Fig. 7. Upon excitation at 349 nm, the electrons on Bi³⁺ ions can be effectively excited from the ground state ¹S₀ to the excited state ³P₁. Then they relax nonradiatively to the lowest vibration mode of the ³P₁ state. Afterward, some of the excited electrons return to the ground state of Bi³⁺ to generate the broadband blue emission, while the others can transfer their energy to the ⁵D₃ level of Tb³⁺ and the ⁵L₆ level of Eu³⁺ due to energy level matching, followed by non-radiative relaxation from ⁵D₃ to ⁵D₄ (Tb³⁺) and from ⁵L₆ to ⁵D₀ (Eu³⁺) states. Finally, a series of characteristic emissions of Tb³⁺:⁵D₄ → ⁷F_6,5,4,3 and Eu³⁺:⁵D₀ → ⁷F_0,1,2,3,4 occur. Apart from the energy transfer from Bi³⁺ to both Tb³⁺ and Eu³⁺, some of the excited electrons can also transfer their energy from the ⁵D₃ level of Tb³⁺ to the ⁵L₆ excited level of Eu³⁺, which enhances the Eu³⁺ emission, accompanied by a decrease in the Bi³⁺ and Tb³⁺ emissions.

Fig. 7 Schematic energy-level diagrams of Bi³⁺, Tb³⁺, and Eu³⁺ in Ba₄BiTb_1−xEu_xO(BO₃)₄, showing energy-transfer process (NR: nonradiative).

3.8. Emitting light color analysis

In general, the Commission Internationale de I'Eclairage (CIE) chromaticity coordinates are used to characterize the emission color of the phosphor material, and the Correlated Color Temperature (CCT) is used to study which type of light the phosphor can emit. The CIE coordinates of the Ba₄BiTb_1−xEu_xO(BO₃)₄ (0 ≤ x ≤ 0.2) phosphors were determined by their corresponding PL spectra, and the CCT values were calculated using McCamy's empirical formula:⁶⁸

CCT = −449n³ + 3525n² − 6823.3n + 5520.33 (5)

where n = (x − x_e)/(y − y_e) and (x_e = 0.332, y_e = 0.186). The obtained data are summarized in Table 2, and the corresponding CIE chromaticity diagrams are displayed in Fig. 8. It is evident that upon 349 nm excitation, the chromaticity point changes systematically from cyan (0.2235, 0.3106) to orange-red (0.5714, 0.3549), by adjusting the Eu³⁺ content from x = 0 to 0.2. Among these phosphors, Ba₄BiTb_0.999Eu_0.001O(BO₃)₄ can emit three colors (blue, green, and red from Bi³⁺, Tb³⁺, and Eu³⁺, respectively) simultaneously, resulting in near-white light emitting from the single-phase host. It demonstrates the CIE coordinates of (0.3365, 0.2938) and a CCT of 5242 K, which are close to the standard white-light illumination [(0.3333, 0.3333) and CCT = 5454.12 K].⁶⁹ When λ_ex = 377 nm, the chromaticity point can be tuned almost linearly from green (0.2785, 0.5058) to orange (0.4854, 0.4389) and then to red (0.6120, 0.3797) by controlling the concentration ratio of Tb³⁺/Eu³⁺. For an intuitionistic display, digital photographs of the selected phosphors under n-UV irradiation are also presented in Fig. 8, where the near-white emission for Ba₄BiTb_0.999Eu_0.001O(BO₃)₄ (λ_ex = 349 nm) and a variation in the emission color from green to red for Ba₄BiTb_1−xEu_xO(BO₃)₄ (λ_ex = 377 nm) are visible. Based on the CIE diagrams, it is expected that when the Eu³⁺ doping amount is greater than 0.2, the luminescence color of Ba₄BiTb_1−xEu_xO(BO₃)₄ will change slightly from orange-red to red for λ_ex = 349 nm. In comparison, it will remain red for λ_ex = 377 nm, due to the high efficiency of Bi³⁺-to-Eu³⁺ and Tb³⁺-to-Eu³⁺ energy transfer. What's more, the quantum yields (QYs) of Ba₄BiTb_0.999Eu_0.001O(BO₃)₄ were determined by the integration sphere method to be approximately 0.06% for λ_ex = 349 nm. The QYs of Ba₄BiTb_1−xEu_xO(BO₃)₄ (x = 0, 0.001, 0.002, 0.005, 0.01, 0.05, 0.1, 0.2) for λ_ex = 377 nm were 0.25%, 0.51%, 0.63%, 1.02%, 1.49%, 3.65%, 4.75%, and 7.06%, respectively. The QYs of Ba₄BiTb_1−xEu_xO(BO₃)₄ phosphors (λ_ex = 377 nm) increase with the increase of Eu³⁺ concentration from x = 0 to 0.2, which is attributed to the efficient Tb³⁺ → Eu³⁺ energy transfer. The low QY values may be because the sample was prepared by a simple high-temperature solid-state reaction, and we believe that the luminous efficiency can be enhanced by optimizing the experimental conditions in future research.

Table 2 CIE chromaticity coordinates and CCT values of Ba₄BiTb_1−xEu_xO(BO₃)₄ (0 ≤ x ≤ 0.2) phosphors under the excitation of 349 and 377 nm

Ba4BiTb1−xEuxB4O13	CIE(x, y) at λex=		CCT(K) at λex=
	349 nm	377 nm	349 nm	377 nm
x = 0	0.2235	0.2785	14 437	6761
	0.3106	0.5058
x = 0.001	0.3365	0.4854	5242	2578
	0.2938	0.4389
x = 0.002	0.3886	0.5248	3173	2064
	0.3154	0.4235
x = 0.005	0.3829	0.5545	3009	1708
	0.2921	0.3947
x = 0.01	0.4231	0.5759	2275	1644
	0.3131	0.3946
x = 0.05	0.5476	0.6004	1642	1623
	0.3697	0.3889
x = 0.1	0.5703	0.6079	1620	1636
	0.3718	0.3865
x = 0.2	0.5714	0.6120	1652	1667
	0.3549	0.3797

---

Fig. 8 The CIE chromaticity diagrams of Ba₄BiTb_1−xEu_xO(BO₃)₄ (x = 0, 0.001, 0.002, 0.005, 0.01, 0.05, 0.1 and 0.2) phosphors (λ_ex = 349 nm for points A–H; λ_ex = 377 nm for points 1–8). The inset shows the photographs of the selected phosphors under the irradiation of 349 nm (point B) and 377 nm (points 1, 2, and 8), respectively.

3.9. Thermal stability

Thermal stability is an indispensable parameter for novel phosphor material because the LED chips are always operated in a high-temperature environment. The temperature-dependent PL spectra of the phosphor Ba₄BiTb_0.999Eu_0.001O(BO₃)₄ were recorded under two excited wavelengths of 349 and 377 nm, respectively, and the corresponding normalized emission intensities were calculated, as shown in Fig. 9(a) and (b). The PL intensity decreases monotonically with the temperature increasing from 303 to 483 K due to the thermal quenching effect. When the temperature reaches 423 K (150 °C), the PL intensity retains ∼42% (λ_ex = 349 nm) and ∼17% (λ_ex = 377 nm) of that at room temperature (303 K), respectively, indicating that the thermal quenching is more obvious for λ_ex = 377 nm than λ_ex = 349 nm. To better understand the temperature dependence of photoluminescence, the activation energy of thermal quenching (ΔE) was calculated by the Arrhenius equation given as:⁷⁰where I and I₀ represent the PL intensity at temperatures of T and 303 K, respectively, and A and k are constants (k = 8.62 × 10⁻⁵ eV K⁻¹). Fig. 9(c) and (d) show the plots of ln(I₀/I − 1) against 1/kT for this phosphor under two excitations. The data can be well-fitted to straight lines with slopes of −0.156 and −0.281, respectively. Thus, the obtained ΔE value of 0.281 eV for λ_ex = 377 nm is greater than that of 0.156 eV for λ_ex = 349 nm, and both are comparable to those of several previously reported Bi³⁺/Tb³⁺/Eu³⁺ co-doped phosphors, such as Ca_2.24ZrSi₂O₉:0.17Eu³⁺, 0.09Bi³⁺, 0.50Tb³⁺ (ΔE = 0.195 eV), Gd_1.474MoB₂O₉:0.30Bi³⁺, 0.20Tb³⁺, 0.026Eu³⁺, (ΔE = 0.17 eV) and LaMoBO₆:0.4Tb³⁺, 0.005Eu³⁺, 0.02Bi³⁺ (ΔE = 0.19 eV).^30,31,71

Fig. 9 Temperature-dependent PL spectra of the Ba₄BiTb_0.999Eu_0.001O(BO₃)₄ phosphor upon excitation at 349 nm (a) and 377 nm (b), respectively; the insets show the relative PL integral intensities of the phosphor. The linear fitting curves of ln(I₀/I − 1) versus 1/kT for λ_ex = 349 nm (c) and λ_ex = 377 nm (d) are also depicted in the figure.

This phosphor exhibits strong temperature sensitization when excited at 377 nm, and its CIE coordinates shift significantly from (0.4540, 0.4052) to (0.2891, 0.3308) as the temperature increases from 303 to 483 K, indicating its potential application in temperature sensing. Fig. 10 shows the temperature-dependent Eu³⁺/Tb³⁺ fluorescence intensity ratio FIR [FIR = I₅₉₃/I₅₄₃ = I(Eu³⁺:⁵D₀ → ⁷F₁)/I(Tb³⁺:⁵D₄ → ⁷F₅)] for this phosphor upon excitation at 377 nm. The calculated FIR-value monotonically decreases with rising temperature from 303 to 483 K, which can be fitted to the correlation coefficient R² = 0.979 employing the linear function described by the equation:

FIR = I₅₉₃/I₅₄₃ = A + BT = 3.126 − 0.0057T (7)

Fig. 10 Temperature-dependent FIR (FIR = I₅₉₃/I₅₄₃, where I₅₉₃ and I₅₄₃ are the integrated emission intensities of Eu³⁺:⁵D₀ → ⁷F₁ transition from 585 to 600 nm and Tb³⁺:⁵D₄ → ⁷F₅ transition from 535 to 565 nm, respectively) of the Ba₄BiTb_0.999Eu_0.001O(BO₃)₄ phosphor upon excitation at 377 nm. The fitted line for the experimental data is also provided in the figure.

Thus, the sensitivity of Ba₄BiTb_0.999Eu_0.001O(BO₃)₄ for temperature sensing is about 0.57% per K. Considering this change in the emission intensity and chromaticity coordinates with temperature, we expect that Ba₄BiTb_1−xEu_xO(BO₃)₄ phosphors excited at 377 nm may be used as luminescent thermometers at this temperature range.^72,73 A detailed investigation of this aspect is beyond the scope of this article and will be provided elsewhere.

4. Conclusions

The investigation of the BaO (and SrO)–Bi₂O₃–Tb₂O₃–B₂O₃ system led to the discovery of two new phases, Ba₄BiTbO(BO₃)₄ and Ba_1.54Sr_2.46BiTbO(BO₃)₄. They are layered compounds, where Bi₂O₁₃ groups share corners with each other and edges with BO₃ triangles to create a 2D [Bi(BO₃)₂O]_n⁵ⁿ⁻ layer. There exists another kind of 2D layer, [Tb(BO₃)₂]_n³ⁿ⁻, constructed from TbO₆ octahedra and BO₃ triangles. These layers and their equivalent partners are stacked alternately along the [001] direction, with the interlayer spaces and intralayer tunnels filled by alkali-earth cations. This study also reveals that Ba₄BiLnO(BO₃)₄ (Ln = trivalent rare-earth ions) forms a large family of compounds, in which Ba²⁺ can be partially replaced by other divalent cations, such as Pb²⁺ and Sr²⁺. Additionally, a series of phosphors, Ba₄BiTb_1−xEu_xO(BO₃)₄ (0 ≤ x ≤ 1) and Ba₄BiYO(BO₃)₄, were synthesized at 830 °C. Based on analyses of IR/Raman and UV-vis diffuse reflection spectra, the geometric deviation of the BO₃ group from the D_3h symmetry was verified, and the indirect and direct band gaps of Ba₄BiLnO(BO₃)₄ (Ln = Y, Tb, Eu) were determined. Upon 349 nm excitation, the PL spectra of Ba₄BiTb_1−xEu_xO(BO₃)₄ contain blue Bi³⁺:³P₁ → ¹S₀, green Tb³⁺:⁵D₄ → ⁷F_6,5 and orange Eu³⁺:⁵D₀ → ⁷F_1,2,3,4 emissions. With an increase in Eu³⁺ content (x), the emission intensities of Bi³⁺ and Tb³⁺ decline steadily, whereas that of Eu³⁺ initially increases until x = 0.05, and then decreases. When λ_ex = 377 nm, only Tb³⁺ and Eu³⁺ emissions appear simultaneously, and the emission intensity of Tb³⁺ decreases, while that of Eu³⁺ increases along with increasing x. Thus, a single-component near-white emission and a color-tunable emission from green to red can be realized in Ba₄BiTb_1−xEu_xO(BO₃)₄ by manipulating the Tb³⁺/Eu³⁺ ratio and adopting different excitation wavelengths. Moreover, the Eu³⁺/Tb³⁺ fluorescence intensity ratio (I₅₉₃/I₅₄₃) for Ba₄BiTb_0.999Eu_0.001O(BO₃)₄ upon 377 nm excitation can be linearly related to the temperature between 303 and 483 K, indicating the potential of the phosphor as a promising temperature sensor. These findings indicate that exploring new borates based on element substitution is feasible. By doping rare-earth activators onto the newly prepared borate matrix, multifunctional fluorescent materials can be obtained, which may find potential use for w-LEDs and temperature sensor applications. This is a promising field of research that is worth further studying.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key R&D Program of China (2021YFB3501502) and the National Natural Science Foundation of China (92163107). We thank Mr Taifeng Lin from the large scientific instrument-sharing platform of the Beijing University of Technology for the luminescent property measurements.

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Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic files for Ba₄BiTbO(BO₃)₄ and Ba_1.54Sr_2.46BiTbO(BO₃)₄ in CIF format; atomic coordinates and equivalent isotropic displacement parameters; selected bond lengths and angles; Rietveld refinement results of Ba₄BiTb_1−xEu_xO(BO₃)₄ (0 ≤ x ≤ 1) and Ba₄BiYO(BO₃)₄; SEM images and EDX spectra of the single-crystals; Rietveld plots of Ba₄BiTb_1−xEu_xO(BO₃)₄ (0.001 ≤ x ≤ 0.2) and Ba₄BiYO(BO₃)₄; IR, Raman, XPS and UV-vis absorption spectra. CCDC 2311747 and 2311748. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3ra08265b

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