Achievement of narrow-band blue-emitting phosphors KScSr_1−yCa_ySi₂O₇:Bi³⁺ by the migration of luminescence centers

Abstract

In recent years, efforts have been made to develop narrow-band emission phosphors with excellent performance. Herein, a series of KScSr_1−yCa_ySi₂O₇:0.07Bi³⁺ narrow-band phosphors were synthesized by a co-substitution method, and the crystal structure, the occupancy of activated ions and luminescence properties were studied in detail. The substitution of Ca²⁺ for Sr²⁺ ions resulted in the migration of the activated Bi³⁺ from the K site to Sr site, accompanied by the regulation of the emission peak from 410 nm to 455 nm, the peak emission half width from 52 nm to 40 nm, and the color purity from the original 78% to 88%. In addition, a warm white LED with low CCT = 3401 K, CRI = 95.5, and CIE color coordinates of (0.3447, 0.3682) has been obtained through the combination of KSS_0.6C_0.4S:0.07Bi³⁺ with a commercial green and red phosphor on a UV (370 nm) chip. The results not only provided a strategy based on the manipulation of chemical composition and crystal structure to tune spectral distribution, but also broadens the choice of activators of narrow-band blue-emitting phosphors.

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1. Introduction

Phosphor converted white light-emitting diodes (pc-LEDs) are emerging backlight units in modern liquid crystal display (LCD) applications.¹ Over the past decade, many efficient phosphors have been developed, most of these phosphors have wide-band emission. The wavelength of some phosphors exceeds the sensitive area of human eyes (700 nm), resulting in the reduction of the luminous efficiency of pc-LEDs. Therefore, over the years people have been committed to improve the resolution of the LCD, power consumption and color gamut. The luminescence characteristics of light, such as position of emission peak and full-width at half-maximum (FWHM), will directly affect the space of color gamut.^2–6 The improvement of color gamut is a hot issue because it can make the color of LCDs more vivid and attractive to consumers. Phosphors, often with narrow emission bands, can produce a wider gamut. Therefore, in order to obtain a large gamut in the pc-LED backlight, narrow-band emission phosphors with good performance need to be developed.

As early as a few years ago, the special luminescence characteristics of narrow-band phosphors attracted the attention of academic and industrial circles. For example, phosphors like Sr[Mg₃SiN₄]:Eu²⁺ (SMS) and Sr[Mg₂Al₂N₄]:Eu²⁺ (SMA) were reported by Schnick's group, and they presented narrow emission, good luminescence efficient and thermal stability.^7,8 Recently, Hubert's group reported KLi₃SiO₄:Eu²⁺ with a broad-band, near warm white emission (FWHM = 49 nm, λ_em = 515 nm) in the same oxide-based system.^9,10 These studies tend to focus on the effects of symmetric cation sites and highly condensed skeletons on the emissive bands, and most of them are doped Eu²⁺ or Mn⁴⁺ ions. However, these elements, especially Eu²⁺, are usually reabsorbed in the visible region, resulting in a decrease in luminous efficiency. Therefore, it is necessary to develop a pair of narrow-band blue luminescent phosphors that is highly efficiently absorbed in the NUV region and matched well with NUV commercial LED chips to further improve the performance of LCD.

As a favored sensitizer or an activator in luminescent materials, the spectroscopy of Bi³⁺ ions has been extensively investigated.¹¹ Compared with rare earth luminescence centers, the absorption of Bi³⁺ is mainly concentrated in the NUV region, with no reabsorption and high energy efficiency. Depending of the nature of the host lattice in which Bi³⁺ ion is inserted, this ion may show either a regular luminescence, a cluster-related luminescence, and so on. The regular Bi³⁺ luminescence results from intra-ionic transitions between the ground ⁶s₂ and first excited ⁶s₁⁶p₁ configurations. Absorption takes place from ¹S₀ fundamental state to ³P₁, ³P₂ and ¹P₁ excited states.^12,13 Benefiting from its naked 6s and 6p orbits, Bi³⁺ has a broadly adjustable luminescence spectrum, affected by different crystal field environments, which opens up great possibilities for ion regulation. For instance, Peng's team reported CaMoO₄:Bi³⁺ phosphors, half peak width is about 200 nm, mainly studies the cationic alternative implementation color adjustable launch in 2014.¹⁴ Moreover, they obtained (Y, Sc)(Nb, V)O₄:Bi³⁺ phosphors, which also changed the half-peak width from 120 nm to 230 nm through ion regulation in 2016;¹⁵ In summary, Bi³⁺ ions can not only achieve efficient absorption in the NUV region, but also its naked orbits is very conducive to the matrix composition regulation, so as to obtain phosphors with excellent performance.

In our work, a series of narrow-band blue-emitting phosphors KScSrSi₂O₇:xBi³⁺ (0 < x < 0.13) and KScSr_1−yCa_ySi₂O₇:0.07Bi³⁺ (0 < y < 0.05) have been prepared by high temperature solid state reaction. Take advantage of the sensitivity of Bi³⁺ ions to the surrounding coordination environment and crystal field, Ca²⁺ ions were introduced into KScSrSi₂O:0.07Bi³⁺ to replace Sr²⁺, control the occupying selection of activated ion Bi³⁺ from K site to Sr site and the luminescence color was adjusted from purple to blue, the structure rigidity and lattice distortion of KScSr_1−yCa_ySi₂O₇:0.07Bi³⁺ were changed by cationic component regulation, and a narrow-band emission with a half-peak width of 40 nm was achieved.

2. Experimental section

2.1 Sample preparation

A series of KScSrSi₂O₇:xBi³⁺ (KSSS:xBi³⁺) (0 < x < 0.13) and KScSr_1−yCa_ySi₂O₇:0.07Bi³⁺ (KSS_1−yC_y:0.07Bi³⁺) (0 < y < 0.5) phosphors were synthesized by high temperature solid-state method. The raw materials used are K₂CO₃ (99.99%), Sc₂O₃ (99.99%), SrCO₃ (99.9%), SiO₂ (99.9%, D50∼10 μm), CaCO₃ (99.9%) and Bi₂O₃ (99.99%). The raw materials are weighed and mixed in an agate mortar according to certain stoichiometric and fully ground for 30 minutes. Then the mixed powder samples are transferred into an alumina crucible and sintered at a high temperature of 1200 degrees for 5 h. Finally, all obtained powder samples are naturally cooled to room temperature in the furnace and ground for further experimental measurement.

2.2 Characterization

The phase structure of the sample was characterized by Bruker D8 X-ray diffractometer. The corresponding parameters were as follows: the radiation source was CuKα (λ = 0.15406 nm), the working voltage and current were 40 kV and 40 mA, respectively. The scanning range was 10–80° and the step was 0.02° per s. And the Findit software and Crystalmaker software were used for the structure. Crystal structure refinements were implemented using General Structure Analysis System (GASA). The emission and excitation spectra of samples were recorded by using a Hitachi F-7000 spectrophotometer equipped with a 450 W xenon lamp as the excitation source. The scanning speed was 1200 nm min⁻¹ with a step of 0.2 nm. At room temperature, the decay curves were measured by Horiba FL3 with the width of the incident and outgoing slits is 1 nm, and the width of the slit of the sample is 29 nm.

3. Results and discussion

3.1 Crystal structure characterizations

The XRD patterns of KSSS:xBi³⁺ (0 < x < 0.13) and KSS_1−yC_y:0.07Bi³⁺ (0 < y < 0.5) are shown in Fig. S1.† The diffraction peaks of all the samples were consistent with the structure of KSSS standard card, indicating that all the samples were pure phase and the introduction of Bi³⁺ and Ca²⁺ ions did not change the crystal structure of the matrix. According to the rule of ion radius matching, Bi³⁺ ions were expected to occupy the positions of Sr²⁺ and K⁺ ions. On account of Bi³⁺ was very sensitive to different coordination environments, the crystal field environment around Bi³⁺ can be changed to achieve the regulation of the luminescence performance of Bi³⁺ doped phosphors. In this paper, the luminescence properties of phosphors were improved by using cationic substitution of Ca²⁺ for Sr²⁺ to adjust the coordination environment around Bi³⁺ ions.

As shown in Fig. 1(a), the crystal structure of KScSrSi₂O₇ belongs to the KMRESi₂O₇ family (M = Ba²⁺, Sr²⁺, RE = Y³⁺, Sc³⁺), which belongs to the P₁₂₁/n₁ space group and consists of a tetrahedron and octahedron containing Si₂O₇ groups and octahedral coordination with Sc atoms (〈Sc–O〉 = 2.12 Å). The framework of KMRESi₂O₇ is based on a ScO₆ octahedron which shares each of its apices with an oxygen atom of the Si₂O₇ group. Adjacent ScO₆ octahedra is connected to each other through Si₂O₇ units, whose Si–Si vectors are oriented parallel to the c axis. This connection creates layers of ScO₆ polyhedra in the x–y plane, furthermore, a three-dimensional framework with 8-coordinated Sr atoms (〈Sr–O〉 = 2.71 Å) and 9-coordinated K cations (〈K–O〉 = 2.86 Å) located in voids. The three-dimensional connectivity generates a rigid lattice exhibiting highly condensed network. And the two cations are “stacked” in narrow channels, and they are arranged in strict order.¹⁶ In the KMRESi₂O₇ family, KSSS has a relatively small bond angle and bond length, which infers that it has a more rigid structure. There are three cation sites in KSSS, K⁺ (r = 1.55 Å, CN = 9), Sr²⁺ (r = 1.26 Å, CN = 8) and Sc³⁺ (r = 0.745 Å, CN = 6). Due to the exposed electron configuration in the outermost layer of Bi³⁺ ion, the luminescence of Bi³⁺ is very sensitive to the surrounding crystal field environment. The different symmetry of multivalent cations (Sr²⁺/K⁺/Sc³⁺) provide infinite possibilities for controlling the selection of Bi³⁺ at different sites and the regulation of luminescence performance. In order to further prove Bi³⁺ successfully doped into the crystal structure, KSSS:xBi³⁺ were refined with GSAS software, and the refining results were shown in Fig. 1(b). The cross represents the experiment raw data, the red solid line represents the calculated data, the blue solid line represents the difference between the observed data and calculated data, the green solid line represents the Bragg position. This refined result shows that the parameters meet the requirements, in other words, all samples are single phase. Other concentration refining results are shown in the Fig. S2.† It is known that the generation of cationic vacancies should contribute to the shrinkage of the crystal cell volume. The cationic vacancies should be produced when the Sr²⁺ ions positions were occupied by Bi³⁺ ions because of the integral charge balance. It can be expressed by the following formula (1):¹⁷

Fig. 1 (a) The crystal structure of KScSrSi₂O₇; (b) Rietveld structure refinement patterns of KScSrSi₂O₇:0.07Bi³⁺.

3.2 Luminescence properties of KScSrSi₂O₇:xBi³⁺

Fig. 2(a) shows the emission spectra of KSSS:0.07Bi³⁺ at different excitation wavelengths of 340 nm and 370 nm, as well as the excitation spectra corresponding to the main emission peak of 415 nm and 500 nm. It is obvious that KSSS:0.07Bi³⁺ has a site-selective excitation and its emission spectrum varies with the excitation wavelength. And the excitation spectrum presents a wide 260–400 nm excitation band centering at 280 nm and 370 nm, which indicates that it can be well matched with commercial near ultraviolet LED chips. There are three different cation sites in KSSS crystal, and Bi³⁺ is extremely sensitive to the surrounding coordination environment, therefore, the asymmetric emission may be caused by the substitution of Bi³⁺ for different cation sites. It is generally believed that the doping of Bi ions at trivalent or bivalent sites with coordination number CN larger than or equal to 6 is the most probable.¹⁸ In order to further determine the number of potential luminous centers, the radius percentage difference between the doped ion Bi³⁺ and the potentially substituted ion (Sr²⁺/K⁺/Sc³⁺) in KSSS is calculated by using formula (2), and is listed in Table 1.¹⁹where D_r is the percentage difference of ion radius, CN is the coordination number, R_m (CN) and R_d (CN) are the radii of substituted ions and doped ions, respectively. Since the data of CN = 9 of Bi³⁺ ion is insufficient, a reasonable approximation is provided here. Table 1 presents the R_m (CN), R_d (CN) and D_r values. The corresponding D_r values between Bi³⁺ ions and cations (K⁺/Sr²⁺/Sc³⁺) are 20%, 7.1% and 38.2%, respectively, among them, the possibility of Sc³⁺ being replaced is very small and can be ignored. And Bi³⁺ occupies K and Sr sites in KScSrSi₂O₇.

Fig. 2 (a) The different PLE and PL spectra of KSSS:0.07Bi³⁺. (b) Gaussian fitting of KSSS:0.07Bi³⁺ emission spectra; the inset shows the average life of two sub-emission peaks (410 nm and 455 nm). (c) The PLE spectra of KSS_1−yC_yS:0.07Bi³⁺ under 410 nm and 455 nm different emission. (d) The PL spectra of KSS_1−yC_yS:0.07Bi³⁺ under 370 nm excitation; the inset shows the relative excitation intensity vs. various Ca²⁺ contents (y) at λ_em = 410 nm and λ_em = 455 nm.

Table 1 Ionic radii difference percentage (D_r) between matrix cations and doped Bi³⁺

Matrix cation (CN)	Rm (Å)	Doped ion (CN)	Rd (Å)	Dr (%)
K^+ (9)	1.55	Bi^3+ (9)	1.24	20
Sr^2+ (8)	1.26	Bi^3+ (8)	1.17	7.1
Sc^3+ (6)	0.745	Bi^3+ (6)	1.03	38.2

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According to the number of luminescence centers, the emission band of KSSS:0.07Bi³⁺ can be fitted to two sub-emission peaks, which are located at 410 nm and 455 nm, respectively, as shown in Fig. 2(b). The insets depicts the fluorescence lifetime of the two sub-emission peaks excited by 370 nm pulse laser (nano-LED), which both show second order exponential decay tendency as follows:^21,22

where I(t) is the intensity at t, and τ₁ and τ₂ represent the short and long-decay components while A₁ and A₂ are fitting constants. The average lifetime τ* can be calculated by the following eqn (4):^20,22

τ* = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (4)

The lifetimes of the two sub-emission peaks (410 nm and 460 nm) are 0.2571 μs and 0.5065 μs, respectively, which are consistent with the lifetimes of Bi³⁺. The difference in lifetime between different emission peaks confirms the existence of two possible emitting centers. In order to determine the origin of the asymmetric emission peaks, the crystallographic occupancy of ions in the matrix was calculated by using Van Uitert theoretical formula:²³

where E represents the peak (cm⁻¹) of Bi³⁺ ions, Q is the free Bi³⁺ ion at the lower energy edge position, V and E represent the valence and electron affinity of Bi³⁺ ions, respectively. N and R represent the coordination number of the central cation and the radius of the matrix central cation replaced by the activated ion, respectively. Therefore, in the Van Uitert equation, Q, V and E are all inherent attribute parameters of the matrix, hence the size of E only depends on the product of N and R. For K and Sr, the value of N × R is 13.95 and 10.08, respectively. Since the E value is positively correlated with the N × R, the origin of the two peaks can be qualitatively determined. According to the relation between E value and N × R, the two sub-peaks at 410 nm and 455 nm are generated by Bi³⁺ ions occupying the K and Sr positions, respectively.

For further demonstrate the source of sub-emission peak, the following formula is used to calculate the distance of the centroid shift values of each emission center in KSSS:0.07Bi³⁺ and the crystal field splitting values and total shift values (D(A)) of Bi³⁺ ions at 6s6p level, as follows:^24–27

ε_cfs = β_polyR⁻² (8)

Formula (6) and (7), 〈r〉 stands for the radial radius of an electron in 6s6p configuration or 6s² configuration, e is the elementary charge, and ε₀ is the permittivity of vacuum. Thus, the value of is constant. R_i stands for the distance between the center atom and the anion in the lattice. The summation is over all N anions that coordinate Bi³⁺. ½ΔR represents a correction for lattice relaxation around Bi³⁺, and ΔR for two kinds of cation and activate ion in KSSS radius difference. α^o_sp is the spectroscopic polarizability of the anion, and its value is 2.82 × 10⁻³⁰ m⁻³.χ_av is the electronegativity of the cation. In formula (8), β_poly is a constant that depends on the type of the coordination polyhedron. In this case, for KSSS:0.07Bi³⁺, β_poly = 1.35 × 10⁹ pm² cm⁻¹. In formula (9), ε_c(A) is the centroid shift, ε_cfs(A) is the crystal field splitting. Finally, by formula (9), the total displacement value of energy level in KSSS:0.07Bi³⁺ shows D(K) < D(Sr), the related data are listed in Table 2. Combined with the previous analysis, it is verified again that the sub-emission peaks at 410 nm and 455 nm are derived from the emission of Bi³⁺ ions at K and Sr sites, respectively.

Table 2 The relative value of the energy level displacement in KSSS:0.07Bi³⁺

Occupying	K^+	Sr^2+
av bond (R)	2.890	2.690
ΔR	0.031	0.009
εc(A)	0.477	0.605
εcfs(A)	2.001	2.309
D(A)	D(K) < D(Sr)

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3.3 Controlling site-selection of Bi³⁺ via Ca²⁺ substitution for Sr²⁺

As is known to all, Bi³⁺ ion as a commonly used transition metal doped element, it's 6s and 6p electrons are in the outermost layer of the electron shell, which makes its luminescence properties are easily affected by the surrounding environment.^28,29 As a consequence, Ca²⁺ ions are introduced to regulate the local lattice and improve the luminescence properties. Fig. 2(c) shows the excitation spectra of KSS_1−yC_yS:0.07Bi³⁺. When monitored at 410 nm, the resulting PLE spectrum contained two excitation band peaks at 370 nm (in the 350–420 nm range) and 290 nm (in the 200–350 nm range), and when monitored at 455 nm, the resulting PLE spectrum contains one excitation band peaks at 370 nm (in the range of 300–420 nm), which could be attributed to the electronic transitions from the ground state (¹S₀) to the excited state (³P₁) of Bi³⁺ ions. With the concentration of Ca²⁺ (y) increasing, the emission intensities at 410 nm gradually decrease, while the emission intensities of 455 nm gradually increase and reach its maximum value when y = 0.4, and finally the concentration quenching occurs, as shown in Fig. 2(d). When Ca²⁺ concentration increased, it is speculated that Bi³⁺ ions may be transferred from one luminescence center to another in KSS_1−yC_yS crystals. The relationship between the emission intensity (fitting intensity at 410 nm and at 455 nm) and Ca²⁺ concentration is plotted in the inset of Fig. 2(d). The trend of monotonic decrease of emission intensity at 410 nm is in sharp contrast to that at 455 nm, which indicates that the occupation of Bi³⁺ transfers from one crystallization position to another in KSS_1−yC_yS.

In the above study, the occupying position of Bi³⁺ in the host was confirmed, that is, the 410 nm emission peak comes from the K site occupied by Bi³⁺ ions, and the 455 nm emission peak originates from the Sr site occupied by Bi³⁺ ions. Since the luminescence properties of Bi³⁺ are easily influenced by the surrounding crystal field environment, it is necessary to investigate the structural changes caused by the incorporation of Ca²⁺ ions. The main reason for the structural change is that Ca²⁺ has a smaller ionic radius compared with Sr²⁺. Firstly, at the Sr²⁺ site, Ca²⁺ with small radius cannot completely coordinate with 8O²⁻ ions, leading to Ca²⁺ ions moving towards the surface and the gap of SrO₈ polyhedrons and coordinating with 3O²⁻ ions to form an isosceles triangular plane. Secondly, the addition of Ca²⁺ increases the distance between the cations, providing space to accommodate more activated ions. Finally, the introduction of small Ca²⁺ ions not only change the local environment of the lattice but also promote the mobility of the activated ions. Importantly, we used the modification of the SrO₈ polyhedron to stimulate the sensitivity of the activated ion Bi³⁺ to its surrounding crystal field environment, leading to the redistribution of Bi³⁺ at two different cation positions. In order to understand the redistribution of Bi³⁺ ions more accurately, KSS_1−yC_yS:0.07Bi³⁺ was refined with GASA software. Similarly, KSS_1−yC_yS:0.07Bi³⁺ were refined with GSAS software, and the refining results were shown in Fig. S3.† All parameters met the experimental requirements. When the small radius of Ca²⁺ (r = 1.12 Å, CN = 8) is used to replace the large radius of Sr²⁺ (r = 1.26 Å, CN = 8) in the crystal, the cell should shrink and the cell parameters decrease. Fig. 3(a) shows the changes of cell volume V and cell parameters a, b and c with Ca²⁺ concentration y increasing, respectively. Curiously, when y increased from 0 to 0.2, both cell parameters and cell volume showed a decreasing trend, but when the replacement concentration was greater than 0.2, each parameter presented an increasing trend. The changes of polyhedron SrO₈ are shown in the illustration. Obviously, the volume of polyhedron SrO₈ also shows a trend of first increase and then decrease, which is due to the gradual entry of Bi³⁺ (r = 1.17 Å, CN = 8) ions with a larger radius into the larger Sr site. In other words, with the gradual substitution of Ca²⁺ for Sr²⁺ ions, Bi³⁺ ions gradually transferred from the K to the Sr site. The schematic diagram of the redistribution of Bi³⁺ in Sr and K sites are presented in Fig. 3(b). When y < 0.2, since the radius of Ca²⁺ is smaller than that of Sr²⁺, and the occupying transfer of Bi³⁺ ion is in the initial stage, the contraction degree of the crystal cell is greater than the expansion degree of the crystal cell, as a result all the parameters of the crystal cell show a state of decrease. When the substitution concentration y > 0.2, the expansion caused by the entry of Bi³⁺ ions with large radius into the Sr site is greater than the lattice contraction caused by the replacement of Sr²⁺ ions with Ca²⁺, hence the crystal cell shows the state of expansion.

Fig. 3 (a) The changes of cell parameters (V, a, b, c) of KSS_1−yC_yS:0.07Bi³⁺ with y increasing; the inset presents the variation of the volume of the SrO₈ polyhedron. (b) Schematic diagram of the redistribution of Bi³⁺ in Sr and K sites with y increasing.

According to the above analysis, the emission peaks at 410 nm and 455 nm are originated from Bi³⁺ which occupied the K site and Sr site, respectively. Therefore, the migration of Bi³⁺ ions well explains the change of PL spectrum in Fig. 2(d). As more Bi³⁺ ions transfer from K site to Sr site, the intensity of 455 nm emission increases continuously, while that of 410 nm decreases gradually. When most of Bi³⁺ ions enter the Sr site, the emission intensity will not increase with the increase of Ca²⁺ concentration.

Generally, the luminescence of Bi³⁺ ion has a large absorption cross section and a wide emission due to its s–p transition.³⁰ Surprisingly, the emission spectra of KSS_0.6C_0.4S:0.07Bi³⁺ shows a narrow-band blue-emission under the excitation of 370 nm, the FWHM is only 40 nm, as shown in Fig. 4(a). Generally, the coordination symmetry and rigidity of the crystal structure are the main factors affect the half-peak width of the emission spectrum, the highly symmetric and rigid structure are more likely to have a narrow emission peak. Conversely, the lower the symmetry of crystal structure, the wider the half-peak width.^31–33 For a polyhedron, the symmetry of the polyhedron is usually characterized by the dispersion degree of the side length. The smaller the dispersion degree of the side length, the higher the symmetry of the polyhedron. The crystal distortion can be expressed by using the set of following equations:^34–36

Fig. 4 (a) The emission spectrum of KSS_0.6C_0.4S:0.07Bi³⁺ (λ_ex = 370 nm). (b) Variation of the distortion degree of KSS_1−yC_yS:0.07Bi³⁺ samples. (c) Temperature-dependent emission spectra of KSS_0.6C_0.4S:0.07Bi³⁺ phosphors under 370 nm excitation. (d) The temperature-dependent normalized integrated emission intensities of KSSS:0.07Bi³⁺ and KSS_0.6C_0.4S:0.07Bi³⁺; The inset shows the functional relationship of ln[(I₀/I_T) − 1] versus 1/kT of KSS_0.6C_0.4S:0.07Bi³⁺.

Among them, TO_i is the distances between cations and individual ligand anions, TO_m is the average bond length. The distortion D(TO) is the average deviation of the bond length. D_w is the degree of distortion of the entire crystal, D(TO_K) and D(TO_Sr) refer to the torsion resistance of polyhedra KO₉ and SrO₈, respectively, and the results are listed in Table 3. Obviously, when Sr²⁺ ions are replaced by Ca²⁺ ions, the distortion degree of SrO₈ polyhedron in KSS_1−yC_yS:0.07Bi³⁺ crystal is significantly reduced (As shown in Fig. 4(b)), which indicates that the introduction of Ca²⁺ ions greatly improved the structural rigidity of Sr polyhedron, enhanced the overall symmetry of crystal cells and caused the redistribution of Bi³⁺ ions on the occupied space. With the increase of Ca²⁺ content, the number of Bi³⁺ entering the Sr site increases, while the number of Bi³⁺ at the K site decreases gradually, thus realizing the transfer of the main emission peak from 410 nm to 455 nm. The enhancement of structural rigidity and symmetry of the crystal makes the half-peak width of the emission peak significantly narrower.

Table 3 Variation of the distortion degree of KSS_1−yC_yS:0.07Bi³⁺ samples

	y = 0	y = 0.04	y = 0.08	y = 0.10	y = 0.20
DK	0.112	0.112	0.111	0.112	0.112
DSr	0.166	0.068	0.068	0.067	0.068
DW	0.139	0.090	0.090	0.090	0.090

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Debye temperatures for crystallographically distinct atoms (θ_D,i) were calculated with isotropic atomic displacement parameters using the high temperature approximation:^37,38

where i represents the atomic species K or Sr, and A_i is the atomic weight of the atom. The U_iso,i values employed were the average of the individual atomic species in the unit cell, weighted by their Wyckoff multiplicities. Then, according to the stoichiometric coefficients of each atom in the formula KSS_1−yC_yS, the Debye temperature is roughly calculated. It can be seen from the formula that Debye temperature is inversely proportional to atomic displacement parameter (U_iso). Relevant parameters can be obtained by using the previous refinement data. The corresponding U_iso of K site and Sr site before and after Ca replacement is 0.0397, 0.0419 and 0.0242, 0.0121, respectively. Relevant data are listed in Table S1.† Obviously, when Ca ions replaces Sr²⁺ ions, the average atomic displacement of K site and Sr site decreases, and the reduction of Sr site is particularly significant. The smaller U_iso corresponds to the higher Debye temperature. Furthermore, it is proved that when Bi³⁺ ions is transferred from K site to Sr site, the local environment of around Bi³⁺ ions becomes more symmetric and more rigid, which further proves the cause of narrow-band emission after replacement.

Generally, the enhancement of crystal structure rigidity tends to improve the stability of phosphors to a certain extent. To further verify that the introduction of Ca ions increases the structural rigidity of the overall lattice, we measured the temperature spectra of KSSS:0.07Bi³⁺ and KSS_0.6C_0.4S:0.07Bi³⁺. Fig. 4(c) shows the temperature dependent PL spectra of representative KSS_0.6C_0.4S:0.07Bi³⁺ measured from 300 K to 500 K with a step of 25 K, and the temperature dependent PL spectra of KSSS:0.07Bi³⁺ is shown in Fig. S4.† It is observed that the emission intensity gradually decreases with increasing temperatures due to thermal quenching. The temperature stability of phosphor after Ca²⁺ substitution increased by 12% at a high temperature of 423 K shown in Fig. 4(c). The PL intensity can be fitted by the Arrhenius equation:^39,40

ln(I₀/I) = ln A − E_a/kT (12)

where, I₀ and I are emission intensities of the sample at 298 K and other test temperatures, respectively. A is a constant, k is Boltzmann's constant, T is the thermodynamic temperature. When taking 1/kT as the x-coordinate and ln(I₀/I) as the y-coordinate, linear fitting was performed on the obtained data. The slope of the fitted line is the activation energy E_a. As is shown in an illustrated of Fig. 4(d), the activation energy E_a of the activated ions in KSSS:0.07Bi³⁺ and KSS_0.6C_0.4S:0.07Bi³⁺ were 0.203 eV and 0.270 eV respectively, which indicated that the phosphors with Ca²⁺ introduction had better thermal stability. It is also further proved that the rigidity of the structure is improved after the introduction of Ca ions, which creates a high rigidity environment for the activated ions.

The luminescence properties of phosphors (e.g. peak emission wavelength and full-width at half-maximum (FWHM)) directly affect the color gamut space.⁴¹ As a result, phosphors with narrow half-peak width tend to produce higher color purity, which will be more suitable for high-power pc-LEDs and wide-color backlight applications.³⁶ The color purity of the phosphor can be defined by the percentage of the chromaticity coordinates of the measured light source and the linear distance between the white light source, which can be calculated by the following formula (13):^42,43

where (x, y) represents the color coordinates of phosphor. C(x_C, y_C) represents the white light source whose CIE color coordinate is (0.3333, 0.3333), as shown in Fig. 5(a), D(x_D, y_D) is the color coordinate of the main wavelength corresponding to the measured light source. It connects the measured light source S₁ (KSSS:0.07Bi³⁺) with white light source C and extends the intersection with the spectral trajectory line at point D. Main wavelength of 465 nm, the comparison wavelength and CIE1931 color coordinates table (x_D, y_D) for (0.1355, 0.0399), S₂ is the color coordinates of KSSCS:0.07Bi³⁺. As a result, the color purity of KSSS:0.07Bi³⁺ and KSS_0.6C_0.4S:0.07Bi³⁺ are 78% and 88%, respectively. Obviously, the KSS_0.6C_0.4S:0.07Bi³⁺ phosphors showed better color purity after controlling the cationic component, which indicates that the color purity of the phosphor has been greatly improved by the substitution of Ca²⁺ for Sr²⁺. Table 4 presents the comparison of luminescent specialties between some phosphors reported previously and KSS_0.6C_0.4S:0.07Bi³⁺. It is obvious that the color purity of KSS_0.6C_0.4S:0.07Bi³⁺ is at a medium level. However, comprehensively comparing data such as excitation and emission wavelength, FWHM, color purity and thermal activation energy, it can be found that the performance of KSS_0.6C_0.4S:0.07Bi³⁺ is at an advantage.

Fig. 5 (a) CIE 1931 color coordinates of main wavelength positions of KSSS:0.07Bi³⁺ and KSS_0.6C_0.4S:0.07Bi³⁺, color space of NTSC standard (red dotted line) and white LED device (black line), and luminescence photograph of KSS_0.6C_0.4S:0.07Bi³⁺ phosphors under 365 nm UV lamp. (b) The EL spectrum of LED device fabricated with KSS_0.6C_0.4S:0.07Bi³⁺ and the commercial phosphors on an UV LED chip (370 nm) under a voltage of 3.2 V and current of 5 mA.

Table 4 Summary of excitation wavelength, emission wavelength, FWHM, color purity and activation energy (ΔE) of KSS_0.6C_0.4S:0.07Bi³⁺, some blue narrow-band phosphors and Bi³⁺ doped phosphors previously reported

Samples	λex (nm)	λem (nm)	FWHM (nm)	Color purity (%)	ΔE (eV)	Ref.
Na3Sc2(PO4)3:Eu^2+	370	458	50	89	—	44
Ba9Lu1.5Al0.5Si6O24:Ce^3+	400	488	117	—	0.3000	45
NaHfSi2O7:Eu^2+	397	460	54	90	0.1394	46
RbNa3(Li3SiO4)4:Eu^2+	400	471	22.4	83.3	—	47
Ba5SiO4Cl6:Eu^2+	345	440	32	93	0.1425	48
BaMgAl10O17:Eu^2+	350	450	52	91.3	—	49
Sr4OCl6:Eu^2+	370	446	46	—	0.1378	50
Ca5(BO3)3F:Bi^3+	322	459	100	—	—	51
Cs2AgInCl6:Bi^3+	380	560	160	—	—	52
KScSr0.6Ca0.4Si2O7:Bi^3+	370	455	40	88	0.2700	—

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3.4 Applications in white LED backlight display

The wide excitation, narrow emission, and excellent thermal stability make KSS_0.6C_0.4S:0.07Bi³⁺ a potential candidate for backlight display applications. A white LED was fabricated with KSS_0.6C_0.4S:0.07Bi³⁺ and the commercial green and red phosphor on an UV (370 nm) chip under a voltage of 3.2 V and current of 5 mA, whose EL spectrum and photograph are shown in Fig. 5(b). It can be seen a warm LED with low CCT = 3401 K, CRI = 95.5, and CIE color coordinate of (0.3447, 0.3682) has been obtained. For a comparison, the EL spectrum and photograph of the commercial blue phosphor BaMgAl₁₀O₁₇:Eu²⁺combined with UV (370 nm) chip are given in Fig. S5.† It is noticeable that the CCT of LED with KSS_0.6C_0.4S:0.07Bi³⁺ (3401 K) is lower than that of BaMgAl₁₀O₁₇:Eu²⁺ (5064 K), CRI up to 95.5% and CIE color coordinate of (0.4165, 0.4074) had more application potential. The color gamut of tour white LED device can cover the color space of 75% NTSC in CIE 1931 (as shown in Fig. 5(a)), which belongs to the broad gamut category. All these results indicate that KSSCS:0.07Bi³⁺ is a promising narrow-band blue phosphor for display backlight.

4. Conclusions

A series of blue narrow-band phosphors KScSr_1−yCa_ySi₂O₇:0.07Bi³⁺ were obtained by introducing Ca²⁺ ions into KScSrSi₂O₇:Bi³⁺. With Ca²⁺ concentration increasing, the original crystal field environment changed, which lead to the migration of Bi³⁺ from K site to Sr site. Consequently, the main emission peak wavelength changed from 410 nm to 455 nm, and the luminescence color turned from purple to blue. Importantly, the distortion of the polyhedron in the crystal was reduced, the rigidity of the crystal structure was increased, and the symmetry was improved. As a result, under the excitation of 370 nm, KScSr_0.6Ca_0.4Si₂O₇:0.07Bi³⁺ phosphor presented a blue narrow band emission with a FWHM of 40 nm, and the color purity increased from the original 78% to 88%. And moreover, the LED device using KScSr_0.6Ca_0.4Si₂O₇:0.07Bi³⁺ phosphor performs a low CCT of 3401 K and a R_a of 95.5. The results demonstrate that the blue narrow-band phosphor KScSr_0.6Ca_0.4Si₂O₇:0.07Bi³⁺ has a bright prospect for application in pc-LEDs.

Conflicts of interest

The authors declare that they have no conflicts of interest.

Acknowledgements

The work is supported by the National Natural Science Foundation of China (No. 51672066, 51902080), the Funds for Distinguished Young Scientists of Hebei Province, China (No. A2018201101), and the Natural Science Foundation of Hebei Province, China (No. E2019201223), the personnel training project of Hebei Province, China (No. A201902005).

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Footnote

† Electronic supplementary information (ESI) available: Table S1 and Fig. S1–S5. See DOI: 10.1039/d1ra01375k

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