Modulating optical properties through cation substitution: composition-property relationships in M^I₃M^IIIP₃O₉N:Eu²⁺ (M^I = Na, K; M^III = Al, Ga, In)

Abstract

Developing phosphors with narrow photoluminescence emission peaks and high chromatic stability holds significant importance in light-emitting diode (LED) display technologies, where a wide color gamut is essential to achieve the Rec. 2020 specifications. This research focuses on the optical properties of a solid solution: M^I_2.97Eu_0.015M^IIIP₃O₉N [M^I = Na, K; M^III = Al, (Al_0.75Ga_0.25), (Al_0.5Ga_0.5), (Al_0.25Ga_0.75), Ga, (Ga_0.75In_0.25), (Ga_0.5In_0.5)] to understand how the narrow-emitting photoluminescence in K₃AlP₃O₉N:Eu²⁺ can evolve during host structure cation substitution. Photoluminescence measurements at low temperature (15 K) support that Eu²⁺ replaces three crystallographically independent Na⁺ sites in Na_2.97Eu_0.015AlP₃O₉N, similar to the parent K⁺ phosphor, but substituting Ga³⁺ and In³⁺ for Al³⁺ leads to a change in Eu²⁺ site preference, narrowing the full-width-at-half-maximum (fwhm) of the emission peak. The chromatic stability and photoluminescence quantum yield are also enhanced with higher Ga³⁺ content in the host but not with In³⁺. Thermoluminescence analysis indicates the relationship between trap states and the enhanced quantum yield with Ga³⁺ leads to the series’ best performance. The analysis of the M^I_2.97Eu_0.015M^IIIP₃O₉N series offers insight into the potential method for modulating optical properties with cation substitution in the host structure.

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1. Introduction

Pursuing sustainable alternatives to traditional lighting and more portable display technologies has led to the rapid development of phosphor-converted light-emitting diodes (pc-LEDs). Their energy efficiency and prolonged operating lifespan guarantee a reduction in electricity consumption and promise a substantial decrease in their environmental footprint.^1,2 Pc-LEDs generate white light by coating a blue LED (λ_ex = 450 nm) with yellow and red phosphors, which absorb the blue emission and down-shift it into a broad spectrum.^3,4 Alternatively, a combination of near-ultraviolet (n-UV) LED chips (λ_ex = 380 nm–420 nm) and blue, green, and red-emitting phosphors is another proposed strategy capable of generating higher-quality white light by covering a more significant portion of the visible spectrum. An added advantage of this approach is regulating the amount of blue LED light produced by the device in what is being called “human-centric” lighting.^5,6 Avoiding excessive exposure to blue light, which suppresses melatonin production and stimulates the brain, and replicating color temperature shifts throughout the day is valuable for regulating the human circadian rhythm. LED lighting can address these issues, provided phosphor conversion materials can be developed.

The U.S. Department of Energy has previously outlined the criteria for developing commercial-ready phosphors capable of generating white light.⁷ Phosphors must be capable of absorbing and efficiently down-shifted light from blue or violet LEDs and produce narrow photoluminescence emission with a full-width-at-half-maximum (fwhm) that is ≈30 nm (red = 794 cm⁻¹, green = 1148 cm⁻¹, blue = 1485 cm⁻¹). The narrow emission is vital to enhance lighting efficacy by minimizing the emission of photons in the near-IR and UV regions. It is also crucial for display applications to generate a wider color gamut and achieve the Rec. 2020 specifications.^8–10 Commercially relevant phosphors should also withstand average LED operating temperatures of 150 °C without succumbing to thermally induced changes in the optical properties. For instance, a phosphor's emission intensity should change by less than 50% (ideally less than 4%) from room temperature up to 150 °C to overcome thermal quenching.¹¹ The phosphor's emission color must also not vary as a function of temperature shifting by less than a 3-step MacAdam ellipse.¹²

Given such specific requirements, new materials are constantly being tested. (Oxy-)nitrides substituted with rare-earth (RE) activator ions, like Eu²⁺, are one major class of phosphors studied today motivated by their propensity to strongly absorb violet/blue light and re-emit these photons across the broad visible spectrum via the parity-allowed 4f⁷ ↔ 4f⁶5d¹ electronic transitions.^8,13,14 The energy position of these Eu²⁺ transitions is set based on the nature of the host structure's coordinating atoms and the geometry of the crystallographic substitution site. The resulting nephelauxetic effect stabilizes (lowers) the rare-earth 5d orbitals in energy by reducing the inter-electron repulsion of activator ions.^15–17 This effect depends mainly on the anion type, with N³⁻ causing a more considerable decrease in energy than O²⁻, while O²⁻ is more effective than F⁻. The 5d orbitals of the lanthanide ion are further separated by crystal field splitting (CFS) effects, which depend primarily on the coordination number, volume of the cation site, and symmetry of the site.^18,19 The combination of effects is exemplified by comparing the phosphorus oxynitride Ba₂PO₃N:Eu²⁺ with the [BaO₇N₃] and [BaO₇N₂] sites generating cyan emission (λ_em = 530 nm), whereas the phosphate Ba₂P₂O₇:Eu²⁺ shows blue-shifted emission (λ_em = 420 nm) from the [BaO₁₀] and [BaO₇] sites.^20,21

The position of the electronic transitions and associated emission color is not necessarily binary. Forming a solid solution through meticulous elemental substitution is one approach that enables tuning of the phosphors’ photoluminescent properties.^2,22,23 These substitutions are valuable on either the anion or the cation sublattice. The most common is varying the cation composition to change the properties gradually. For example, the Rb₂(Ca_1−xSr_x)P₂O₇:Eu²⁺ system replaces the larger cation Sr²⁺(r_6-coord = 1.18 Å) for the smaller Ca²⁺(r_6-coord = 1.00 Å) expanding the unit-cell, generating a continuous blue-shifted emission from 612 nm to 567 nm.²⁴ Similarly, oxynitrides can also benefit from forming solid solutions, provided cation charge compensation mechanisms are available. The oxynitridosilicate Sr_2−xLa_xSiO_4−xN_x solid solution incorporates nitrogen with the cation charge balanced out by adding La³⁺ for Sr²⁺, resulting in a tunable red-shifted emission from 550 nm to 700 nm with increasing N³⁻ content.²⁵ Varying the composition does not just shift the maximum of the emission peak, but it can holistically impact the optical properties. In the La₅(Si_2+xB_1−x)(O_13−xN_x):Ce³⁺ system, the increase of N³⁻ shifts the emission from violet (λ_em = 421 nm) to blue (λ_em = 463 nm) region while simultaneously enhancing the thermal quenching temperature (T₅₀) from 120 °C (x = 0) to 202 °C (x = 0.7).²² Solid solution formation can also create more complex outcomes like changing rare-earth substitution site preference in the phosphor. The (Rb_1-xK_x)₂CaPO₄F:Eu²⁺ solid solution shows that decreasing the Rb⁺ site volume while expanding the Ca²⁺ site by incorporating K⁺ can cause significant changes by varying substitution site preference.²⁶ The pure Rb⁺ composition shows strong site preference on [Rb(2)O₈F₂] and no emission from [CaO₄F₂], whereas RbKCaPO₄F:Eu²⁺ shows a stronger emission peak from [Rb(1)O₈F₂]. Finally, a higher K⁺ ratio than Rb⁺ pushes the emission red-shifted by letting Eu²⁺ mostly occupy [CaO₄F₂] sites than other Rb⁺ sites. The study of solid solutions plays a crucial role in controlling the optical properties of phosphors.

One oxynitride discovered by our group where solid solutions may prove interesting for tuning the optical properties is the K₃AlP₃O₉N:Eu²⁺ system.²⁷ This material's narrow emission (fwhm = 45 nm, 2110 cm⁻¹) and reasonable photoluminescent quantum yield (PLQY) of 60(4)%, holding promise in violet excited human-centric display concepts. Based on this previous study, the complete cation substitution of K⁺ with a smaller Na⁺ is tested to tune the emission color and investigate the influence of M^I site substitution on the optical properties. Subsequent investigations into solid solution substitutions on the M^III site following Na_2.97Eu_0.015M^IIIP₃O₉N [M^III = Al, (Al_0.75Ga_0.25), (Al_0.5Ga_0.5), (Al_0.25Ga_0.75), Ga, (Ga_0.75In_0.25), (Ga_0.5In_0.5)] are performed to understand the chemistry of these Na⁺ analogs in depth. The properties are investigated using low-temperature emission, temperature-dependent emission, and thermoluminescence (TL) glow curve measurements. These results highlight the impact of elemental substitution on optical properties and provide guidance for designing materials with a narrow emission peak. Although the resulting phosphors studied here are only excitable by UV light, the insights gained support the future development of advanced pc-LED phosphors.

2. Experimental procedure

2.1. Materials synthesis

The solid solutions of (Na_1−2xEu_x)₃AlP₃O₉N (x = 0.001, 0.0025, 0.005, 0.01, 0.02) and Na_2.97Eu_0.015M^IIIP₃O₉N [M^III = Al, (Al_0.75Ga_0.25), (Al_0.5Ga_0.5), (Al_0.25Ga_0.75), Ga, (Ga_0.75In_0.25), (Ga_0.5In_0.5)] were synthesized by solid-state reactions starting from KPO₃, Na₆[(PO₃)₆] (Sigma-Aldrich, 96%), Al₂O₃ (Alfa Aesar, 99%), Ga₂O₃ (Alfa Aesar, 99.99%), In₂O₃ (Alfa Aesar, 99.9%), and Eu₂O₃ (Alfa Aesar, 99.99%). KPO₃ was prepared by dehydrating KH₂PO₄ (Alfa Aesar, 99.0%) at 350 °C for 12 h in air. Each component was weighed out in the appropriate stoichiometric ratio, and the starting reagents were mixed and ground in an agate mortar and pestle using acetone as a wetting medium. Powders were further milled for 30 min in a high-energy ball mill (Spex 800 M Mixer/Mill) using polystyrene vials. The mixtures were then pressed into a 6 mm diameter pellet and placed on a bed of sacrificial powder in a boron nitride crucible. The pellets were heated at 800 °C for 12 hours under flowing NH₃ gas, with heating and cooling rates of 3 °C min⁻¹, to facilitate the in situ nitridation of the host structure and the reduction of Eu²⁺.

2.2. Structural studies, density functional theory, and optical characterization

Powder X-ray diffractograms of all products were collected using an X'Pert PANalytical Empyrean 3 equipped with Cu Kα (λ = 1.5406 Å) as the radiation source. Le Bail refinements were performed using the General Structural Analysis System II (GSAS-II) software.²⁸ The background was described using a Chebyshev-1 function, and the peak shapes were modeled by a pseudo-Voigt function. Scanning electron microscopy (SEM) micrographs and energy-dispersive X-ray spectroscopy (EDS) elemental mappings were collected using a Phenom Pharos scanning electron microscope attached with a 25 mm² silicon drift detector energy-dispersive X-ray spectrometer (Thermo Fisher Scientific). An accelerating voltage of 15 kV and an emission current of 12 μA were used.

The electronic bandgaps of the Na₃M^IIIP₃O₉N (M^III = Al, Al_0.5Ga_0.5, Ga, Ga_0.5In_0.5) host structures were calculated using density functional theory (DFT). The calculations employed the Vienna ab initio simulation (VASP) package within the projector-augmented wave (PAW) method.^29,30 Structural optimization was first performed with exchange and correlation treated using the generalized gradient approximation (GGA) Perdew–Burke–Ernzerhof (PBE) functional.³¹ The calculations employed a plane wave energy cut-off of 500 eV, as well as force and energy convergence criteria of 1 × 10⁻⁸ eV and 0.01 eV Å⁻¹, respectively, and an 8 × 8 × 8 Gamma-centered k-point grid.³² The solid solutions (M^III = Al_0.5Ga_0.5, Ga_0.5In_0.5) were approximated using a 1 × 1 × 1 “supercell” with the M^III atoms decorated among the single crystallographic site with all possible permutations structurally optimized and the lowest energy structure selected for subsequent band gap and band structure calculations. A hybrid exchange and correlation functional (HSE06) with similar cut-off energy and energy convergence criteria but a 4 × 4 × 4 Gamma-centered k-point grid was used for these calculations.³³ The band structure and density of states plots are provided in Fig. S1.†

Steady-state photoluminescence and quantum yield measurements involved mixing the polycrystalline products in an optically transparent silicon epoxy (United Adhesives Inc., OP 4036) and depositing the combination onto a quartz slide (Chemglass). The excitation and emission spectra were obtained using a PTI fluorescence spectrophotometer equipped with a 75 W xenon arc lamp excitation source. The PLQY was determined following the method of de Mello et al.³⁴ using a Spectralon-coated integrating sphere (150 mm diameter, Labsphere) with an excitation wavelength of 340 nm. Temperature-dependent photoluminescence was recorded between 15 K and 600 K using a FLS1000 Fluorescence Spectrometer from Edinburgh Instruments Ltd. The samples were mounted on a copper holder in a closed-cycle helium cryostat from Lake Shore Cryotronics, using Silver Adhesive 503 glue from Electron Microscopy Sciences. A 450 W continuous xenon arc lamp was used as the excitation source for these measurements. A TMS302-X double grating excitation and emission monochromators with 2 × 325 mm focal lengths were used, and the luminescence signal was recorded with a Hamamatsu R928P high-gain photomultiplier detector thermoelectrically cooled to −22 °C. The excitation spectra were corrected for the incident light intensity, and the emission ones were corrected for the emission channel spectral sensitivity.

Thermoluminescence (TL) experiments were performed using Lexsyg Research Fully Automated TL/OSL Reader from Freiberg Instruments GmbH. An X-ray lamp VF-50J RTG with W-anode operated under 12 kV and 0.1 mA was used as a charging source. The phosphor samples’ irradiation with X-rays (12 kV, 0.1 mA) was performed for 10 seconds, and then TL glow curves were recorded using a 9235QB-type photomultiplier from ET Enterprises in the range from 303 K to 773 K. All experiments were controlled through LexStudio 2 software.

3. Results and discussion

3.1. Distinctive optical behavior in M^I_2.97Eu_0.015AlP₃O₉N (M^I = Na, K)

The previously reported phosphor, K₃AlP₃O₉N:Eu²⁺, crystallizes in cubic space group P2₁3 (No. 198), and is part of a broader isostructural series following M^I₃M^IIIP₃O₉N (M^I = Na; K, M^III = Al; Ga; Cr; Fe; Mn).³⁶ Illustrated in Fig. 1a, the crystal structure was originally solved by single-crystal X-ray diffraction with three [PO₃N] tetrahedra connected through a nitrogen-occupied vertex that creates a [N(PO₃)₃] unit. The [AlO₆] octahedra share an O atom from six different [PO₃N] tetrahedra, producing an Al[N(PO₃)₃] backbone. There are three crystallographically independent M^I sites in the crystal structure: [M^I(1)O₉], [M^I(2)O₆N], and [M^I(3)O₆]. Although the [M^I(1)O₉] site was reported as a six-coordinated site in the original reference, a closer re-analysis suggests a nine-coordinated environment as a better description based on the bond lengths of the adjacent atoms and space-filling in the unit cell.²⁷

Fig. 1 (a) Crystal structure of M^I₃AlP₃O₉N (ICSD #78373) and three crystallographically independent Na⁺/K⁺ sites for Eu²⁺ substitution. A bond length of <2.96 Å creates a [M^I(1)O₆] polyhedron that is severely elongated and distorted. However, when the bond length is ≈2.98 Å, a 9-coordinated polyhedron [M^I(1)O₉] creates a more reasonable site geometry in the unit cell that is more coherent with the photoluminescent characteristics. Photoluminescence deconvoluted emission spectrum of (b) Na_2.97Eu_0.015AlP₃O₉N and (c) K_2.97Eu_0.015AlP₃O₉N at low temperatures. Gaussian deconvolution was made by first converting the spectra into energy scale by means of Jacobian transformation.³⁵

The previous K₃AlP₃O₉N:Eu²⁺ study showed a 454 nm (blue) emission at room temperature with a bandwidth of 45 nm (2110 cm⁻¹) under 400 nm excitation. The relatively narrow photoluminescence stemmed from two factors: site-selective excitation (λ_ex = 405 nm) efficiently removes the emission from [K(1)O₉], and a preferential thermal quenching of the [K(3)O₆] site at room temperature. As a result, only Eu²⁺ on the [K(2)O₆N] site contributes to the emission under violet excitation. Building on this prior investigation, this work first explores the substitution of K⁺ with the smaller Na⁺ ions (K⁺: r_6-coord = 1.38 Å, r_7-coord = 1.46 Å, r_9-coord = 1.55 Å; Na⁺: r_6-coord = 1.02 Å, r_7-coord = 1.12 Å, r_9-coord = 1.24 Å)³⁷ to understand the relationship between composition and the effects of cation substitution while tuning the emission color.

The reported synthesis of Na₃AlP₃O₉N used a synthesis temperature of 800 °C with AlN in a molten flow of sodium polyphosphates.^38,39 In the study here, polycrystalline samples of (Na_1−2xEu_x)₃AlP₃O₉N (x = 0.001, 0.0025, 0.005, 0.01, 0.02) were obtained at 800 °C under flowing NH₃, which is used for supplying nitrogen in the target compositions and for reducing Eu³⁺ to Eu²⁺. The products had no notable impurities, and their peaks matched the reference pattern, as confirmed by powder X-ray diffraction (Fig. S2a†). Analyzing the unit-cell dimensions using Le Bail refinements shows that the lattice parameters increase with the substitution of the larger Eu²⁺ ion for the smaller Na⁺ ion (Eu²⁺r_6-coord = 1.17 Å, r_7-coord = 1.2 Å, r_9-coord = 1.3 Å) (Fig. S2b†). The refined lattice parameters are provided in Table S1.†

Substituting Eu²⁺ into the solid solution (Na_1−2xEu_x)₃AlP₃O₉N generates deep blue luminescence with an excitation band ranging from 280 nm to 400 nm (see Fig. S2c†). The excitation spectra for x = 0.001, 0.0025, and 0.005 show little change in shape; however, a distinct shoulder appears around 375 nm in the excitation spectrum for x = 0.01 and 0.02. A similar trend is observed when collecting the emission spectrum using λ_ex = 340 nm, with a shoulder extending from 450 nm to 550 nm. As a result, there is a slight red shift in the deep blue emission, with λ_em,max = 427 nm at x = 0.001 shifting to λ_em,max = 432 nm at x = 0.02 as the concentration of Eu²⁺ increases. Generally, a higher concentration of larger Eu²⁺ ions expands the unit cell and the volume of the substituted sites, leading to a blue shift in the λ_em,max position accompanied by lower crystal field splitting (CFS). However, in this case, the presence of shoulders in both photoluminescent excitation and emission spectra suggests that Eu²⁺ prefers to substitute onto [Na(2)O₆N] or [Na(3)O₆], resulting in a longer emission peak wavelength at higher rare earth concentrations.

The optimal Eu²⁺ concentration in the (Na_1−2xEu_x)₃AlP₃O₉N series was determined by measuring the PLQY under 340 nm excitation (Fig. S2d†). At x = 0.005, PLQY reaches the highest value among the series with 21(2) %. Beyond x = 0.01, the PLQY drops rapidly due to the increased probability of the energy transfer between Eu²⁺ ions. In higher Eu²⁺ concentrations, there is less separation between Eu²⁺ ions, increasing the likelihood of energy transfer and enabling electrons to find quenching sites, inducing a non-radiative relaxation. The critical distance of energy migration, R_c, between two activators can be calculated with eqn (1),

where V is the volume of the unit cell, x_c is the critical concentration of the activator at maximum emission intensity, and n is the sum of the Wyckoff positions of the crystallographically independent substitution sites.⁴⁰ The calculated R_c here is 26.2 Å. The long-distance non-radiative energy transfer generally occurs through electron exchange interaction or electric multipolar interaction. To decide what type of interaction among dipole–dipole (d–d), dipole–quaternary (d–q), and quaternary–quaternary (q–q) interactions causes the quenching, the emission intensity (I) at each activator concentration is analyzed with eqn (2),where x is the activator concentration above the critical concentration, k and β are constants of a given host, and θ is the electric multipolar character.⁴¹ The energy transfer mode of electric multipolar interaction can be confirmed by the values of θ = 6, 8, and 10, which correspond to d–d, d–q, and q–q interaction, respectively. With an assumption of β(x)^θ/3 ≫ 1, eqn (2) can be simplified as a linear equation with slope –θ/3.

log I/x = k′ − θ/3 log x(k′ = log k − log β) (3)

The plot of log I/x versus log x of Na_3-2xEu_xP₃O₉N shows the slope of −2.0(2), indicating θ = 6 (Fig. S3†). Thus, the electric dipole–dipole interaction is the main source of non-radiative concentration quenching, producing the low PLQY in this system at high Eu²⁺ concentrations.

Additional differences in the photoluminescent properties of K₃AlP₃O₉N:Eu²⁺ and Na₃AlP₃O₉N:Eu²⁺ are immediately apparent at low temperatures. The emission of the Na⁺ analog is blue-shifted and broader (λ_em = 429 nm, fwhm = 58 nm, 3217 cm⁻¹) compared to the reported K₃AlP₃O₉N:Eu²⁺ (λ_em = 445 nm, fwhm = 54 nm, 2865 cm⁻¹) under 340 nm excitation and at low temperature (77 K). Deconvoluting the spectra into their components indicates that Na₃AlP₃O₉N:Eu²⁺ (Fig. 1b) and K₃AlP₃O₉N:Eu²⁺ (Fig. 1c) can both be fit by three Gaussian peaks with the peak center arising from Eu²⁺ on the [M^I(1)O₉] position shifting from 416 nm to 410 nm, Eu²⁺ on the [M^I(2)O₆N] position moving from 447 nm to 432 nm, and Eu²⁺ on the [M^I(3)O₆] position moving from 465 nm to 462 nm, for the K⁺ and Na⁺ analogs, respectively. The observed blue shift of each peak is somewhat of a surprise, considering the unit cell volume decreased from 905.5 ų to 801.8 ų when fully substituting K⁺ with Na⁺. This would normally suggest an expected redshift of the emission spectrum to the cyan or even green region of the visible spectrum due to stronger CFS. This unexpected blue shift, therefore, must be attributed to weaker CFS stemming from local distortions as the much larger Eu²⁺ is stuffed on the smaller Na⁺ site. Such changes are not uncommon. In fact, the most popular example is the blue shift between the commercial yellow-emitting phosphor Y₃Al₅O₁₂:Ce³⁺ and its derivative Lu₃Al₅O₁₂:Ce³⁺. Despite replacing the larger Y³⁺ with the smaller Lu³⁺, Lu₃Al₅O₁₂:Ce³⁺ shows a blue-shifted green emission because of local distortion coming from the dodecahedral [LuO₈] site than [YO₈] and generates smaller crystal field splitting (ε_cfs,YAG = 3.34 eV, ε_cfs,LuAG = 3.12 eV).^42,43

Further differences in the optical properties are revealed by monitoring changes in the emission spectrum under various excitation wavelengths (Fig. 2). On the high-energy side of the emission spectrum, the emission from [Na(1)O₉] remains unchanged with respect to excitation wavelength, unlike the [K(1)O₉] site, which exhibits a significant narrowing of the emission peak. Conversely, longer excitation wavelengths promote longer-wavelength emission from [Na(3)O₆], whereas this phenomenon is not observed for [K(3)O₆]. The result is that site-selective excitation and preferential thermal quenching fail to narrow the M^I = Na analog. In fact, the emission broadens with excitation, in contrast to the narrower emission obtained for M^I = K with longer excitation wavelengths. The excitation spectra of M^I = Na and M^I = K for λ_em = 420 nm and λ_em = 450 nm were also measured (Fig. S4†). In both cases, longer wavelength emissions arise from longer wavelength excitations. The shift in excitation of M^I = Na is less than M^I = K, moving from 329 nm for λ_em = 420 nm to 340 nm for λ_em = 450 nm, while M^I = K moves from 346 nm to 352 nm.

Fig. 2 Emission spectra of (a) K₃AlP₃O₉N:Eu²⁺ and (b) Na_2.97Eu_0.015AlP₃O₉N under different excitation wavelengths recorded at room temperature show that site-selective excitation on [Na(1)O₉] and preferential quenching on [Na(3)O₆] do not happen in M^I = Na, unlike M^I = K.

3.2. Fine-tuning the emission spectrum through solid-solutioning: Na_2.97Eu_0.015M^IIIP₃O₉N (M^III = Al, Ga, In)

Substitution on the M^I site clearly results in significant changes to the optical properties. Therefore, the full extent of these changes was further probed by exchanging the M^III site. Synthesizing a series of compositional derivatives following Na_2.97Eu_0.015M^IIIP₃O₉N [M^III = Al, (Al_0.75Ga_0.25), (Al_0.5Ga_0.5), (Al_0.25Ga_0.75), Ga, (Ga_0.75In_0.25), (Ga_0.5In_0.5)] was possible under the same synthetic conditions. The purity of polycrystalline products was confirmed by powder X-ray diffraction (Fig. 3a). The peak positions shift to smaller 2θ when Al³⁺ is substituted by Ga³⁺ and Ga³⁺ is substituted by In³⁺, indicating the larger M^III ion replaces the smaller ion (Al³⁺r_6-coord = 0.535 Å, Ga³⁺r_6-coord = 0.62 Å, In³⁺r_6-coord = 0.8 Å).³⁷ All attempts to extend the Na_2.97Eu_0.015M^IIIP₃O₉N series increasing the In³⁺ content to M^III = Ga_0.25In_0.75 and M^III = In led to impurities. Indeed, the diffractogram of M^III = Ga_0.25In_0.75 (Fig. S5a†) includes unmatched impurity peaks, while analysis by SEM suggests that the product possibly contained indium metal as the ammonia reduced In₂O₃ (Fig. S5b†). In the case of M^III = In, the indium metal particles were large enough to be observed by eye.

Fig. 3 (a) Le Bail refinements of Na_2.97Eu_0.015M^IIIP₃O₉N (M^III = Al, Ga, In) starting from Na₃AlP₃O₉N structure (ICSD #78373). Peak positions shift towards smaller values in Q space along with increments of larger M^III elements, Ga³⁺ than Al³⁺ and In³⁺ than Ga³⁺, which are emphasized by a greyline around 1.6 Å⁻¹. (b) Calculated volumes of the entire series demonstrate a linear increase according to the weighted average radius of M^III. (c) Normalized emission spectra of Na_2.97Eu_0.015M^IIIP₃O₉N under λ_ex = 340 nm. (d) The peak positions and fwhm of these emission spectra show that the increment of Ga³⁺ induces more blue-shifted and narrower emissions. However, the substitution of Ga³⁺ by In³⁺ barely influences the emission properties. (e) Emission spectra under different excitations of M^III = Ga reveal no significant changes compared to M^I = K and M^I = Na.

Le Bail refinements (Table S2†) of the Na_2.97Eu_0.015M^IIIP₃O₉N series were performed using the Na₃AlP₃O₉N crystal structure file (ICSD #78373) as the starting point. The calculated unit cell volumes of the solid solution series show a linear expansion along the weighted average radius of M^III, supporting that Ga³⁺ and In³⁺ replaced Al³⁺ (Fig. 3b). Micrographs collected on an SEM and a subsequent EDS analysis of Na_2.97Eu_0.015AlP₃O₉N and Na_2.97Eu_0.015GaP₃O₉N (Fig. S6†) also show that all elements are uniformly distributed in the examined particles and there is no apparent phase or elemental separation. Since the Eu²⁺ concentration (∼0.09%) is below the instrument's detection limit, the EDS result for Eu²⁺ is not included in the analysis.

This new Na_2.97Eu_0.015M^IIIP₃O₉N solid solution shows the counterintuitive relationship between their unitcell volume and photoluminescence properties similar to M^I_2.97Eu_0.015AlP₃O₉N. As plotted in Fig. 3c, the solid solution between Al³⁺ and Ga³⁺ (i.e., M^III = Al, Al_0.75Ga_0.25, Al_0.5Ga_0.5, Al_0.25Ga_0.75, Ga) reveals blue-shifted emission spectra (M^III = Al, λ_em = 429 nm; M^III = Ga, λ_em = 409 nm) with increasing Ga³⁺ (Fig. 3d). This shift seems like following the unit cell expansion of ∼2.5% with the incorporation of the larger Ga³⁺ion, inducing a smaller CFS by enlarging the Eu²⁺ substitution site volume. However, the inclusion of In³⁺ does not lead to an additional shift in the emission spectra despite the unit cell volume expanding another 2.2%. A change in the bandwidth is also observed, with M^III = Ga having a narrower fwhm = 42 nm (2493 cm⁻¹) compared to M^III = Al having a fwhm = 58 nm (3217 cm⁻¹). The narrower emission when M^III = Ga originates from the lack of [Na(3)O₆] site emission, supported by Gaussian fitting. The emission spectra are all identical for each excitation spectrum, unlike M^III = Al (Fig. 3e). Exchanging Ga³⁺ for In³⁺ again does not change the peak shape.

Collecting the emission spectra of Na_2.97Eu_0.015M^IIIP₃O₉N [M^III = Al, (Al_0.75Ga_0.25), (Al_0.5Ga_0.5), (Al_0.25Ga_0.75), Ga, (Ga_0.75In_0.25), (Ga_0.5In_0.5)] at low temperature (15 K) and comparing these results to room temperature provides additional insights into the photoluminescent properties. Deconvoluting each emission demonstrates that the photoluminescence of the M^IM^IIIP₃O₉N system is independent of the unit cell volume. The M^III = Al emission spectrum with Gaussian function at 15 K reveals a good fit with three curves (Fig. 4a). Three peaks are also used to describe the data at 300 K, with the only differences being thermally induced spectral broadening and each peak position slightly blue-shifted, which can be explained by unit-cell thermal expansion (Fig. 4b) and/or contribution from transitions from thermally populated higher-energy vibronic levels of the 5d₁ emitting level. In M^III = Al_0.5Ga_0.5, the emission intensity at 15 K (Fig. 4c) and 300 K (Fig. 4d) increases for Eu²⁺ on the [Na(1)O₉] site whereas the emission from Eu²⁺ on the [Na(2)O₆N] and [Na(3)O₆] sites diminishes. When Al³⁺ is fully exchanged for Ga³⁺, the emission from Eu²⁺ on the [Na(1)O₉] increases further, Eu²⁺ on the [Na(2)O₆N] shows a greater decrease, and the emission from Eu²⁺ on the [Na(3)O₆] is entirely lost even at low temperature. Meanwhile, the peak positions of Eu²⁺ on [Na(1)O₉] and Eu²⁺ on [Na(2)O₆N] are mostly preserved at around 410 nm and 430 nm, respectively. As a result of the strong preference for Eu²⁺ to occupy the [Na(1)O₉] site, the total emission spectrum is noticeably blue-shifted and narrowed. It can be explained by the possibility that Ga³⁺ induces more distortion on the smallest 6-coordinated cation site, making it too small to be substituted by Eu²⁺ or the nearest neighbor impact. Interestingly, incrementing In³⁺ into Ga³⁺ generates no remarkable differences in the emission spectrum (Fig. 4g). The emission of M^III = Ga_0.5In_0.5 at T = 300 K also shows a narrowed spectrum with a slightly shifted [Na(2)O₆N] emission peak.

Fig. 4 In the Na_2.97Eu_0.015M^IIIP₃O₉N series, emission spectra are measured under 340 nm excitation at T = 15 K and T = 300 K. Emission spectra of M^III = Al (a) at T = 15 K, (b) at T = 300 K, and M^III = Al_0.5Ga_0.5 (c) at T = 15 K, (d) at T = 300 K are deconvoluted in three peaks from each Na⁺ sites. With the absence of Al³⁺, the peak from [Na(3)O₆] is quenched in M^III = Ga (e) at T = 15 K, (f) at T = 300 K, and M^III = Ga_0.5In_0.5 (g) at T = 15 K, (h) at T = 300 K.

3.3. Thermal and chromatic stability changes in Na_2.97Eu_0.015M^IIIP₃O₉N

The operating temperature of a commercial LED package is approximately 423 K.⁷ Thus, the emission intensity should be thermally robust against any thermal quenching processes at such a temperature. This is often qualitatively analyzed by the thermal quenching temperature (T₅₀), in which the relative emission intensity drops by 50%. Temperature-dependent emission measurements were conducted to study the series’ thermal stability (Fig. 5a–d). At M^III = Al, the T₅₀ shows an extrapolated value of 610 K, which is relatively high compared to other reported commercial thermally stable phosphors (YAG:Ce³⁺, T₅₀ ≈ 632K; BAM:Eu²⁺, T₅₀ ≈ 650K).^44,45 Although the T₅₀ decreases with an incremental Ga³⁺ to 470 K for M^III = Al_0.5Ga_0.5 and to 455 K for M^III = Ga, their values are still reasonably thermally stable, showing higher values than 423 K. Here, In³⁺ dramatically reduces the photoluminescence thermal stability, decreasing the T₅₀ for M^III = Ga_0.5In_0.5 to 270 K with almost zero intensity at 423 K. To explain the reduced thermal stability, changes in the bandgap of hosts along their composition alteration should be examined first. The bandgap of Na₃M^IIIP₃O₉N was calculated by DFT. Theoretically, M^III = Al should have the highest E_g because of aluminum's smaller electronegativity (EN) than Ga³⁺ or In³⁺ (χ_Al = 1.61, χ_Ga = 1.81, χ_In = 1.78), which makes a more considerable EN difference between phosphorus, oxygen, and nitrogen. The calculated E_g,DFT values are 6.75 eV for M^III = Al, 6.38 eV for M^III = Al_0.5Ga_0.5, 6.21 eV for M^III = Ga, and 5.70 eV for M^III = Ga_0.5In_0.5 (Fig. 5e). Based on the bandgap values, the decreasing T₅₀ can be explained by the thermally activated photoionization, which occurs more readily with a smaller bandgap.⁴⁶

Fig. 5 Temperature-dependent emission of Na_2.97Eu_0.015M^IIIP₃O₉N series at (a) M^III = Al, (b) M^III = Al_0.5Ga_0.5, (c) M^III = Ga, and (d) M^III = Ga_0.5In_0.5. T₅₀ decreases along the diminishing host structure's bandgap. (e) Calculated E_g using diffuse reflectance spectrum and E_a values using temperature-dependent emission spectra of Na_2.97Eu_0.015M^IIIP₃O₉N series (f) Changes in chromaticity by increasing temperature in CIE 1976 space of M^III = Al and M^III = Ga plotted with 3-step MacAdam ellipse at 300 K.

Although the E_g values nicely explain the thermal stability for M^III = Al, Al_0.5Ga_0.5, and Ga, further explanation is required for the low thermal stability of M^III = Ga_0.5In_0.5. The position of the Eu²⁺ 5d orbital energy levels is splitting due to CFS, and once the electron on the most stabilized 5d₁ level is excited by the thermal energy, it can be transferred into the host's conduction band, creating non-radiative relaxation and diminishing the photoluminescence intensity.^47,48 The smaller bandgap decreases the energy level of the conduction band, enhancing photoionization quenching. The activated energy (E_a), which is the energy gap between the edge of the conduction band and the 5d₁ level of Eu²⁺, determines the degree of the quenching process. From the temperature-dependent emission spectra, the E_a values can be estimated by the Arrhenius dependence, as depicted by eqn (4),

where I₀ is the initial intensity, E_a is the activation energy, k is the Boltzmann constant, and T is the temperature. For M^III = Al, E_a is 0.31(3) eV, while M^III = Al_0.5Ga_0.5 and M^III = Ga gives lower values of 0.20(1) eV and 0.27(3) eV (Fig. S7†). The E_a of M^III = Ga_0.5In_0.5 shows the lowest value of 0.15(2) eV, corresponding well with the thermal quenching tendency in the series. These estimated E_a values support the lower thermal stability of the In³⁺ containing hosts since thermal ionization can occur more easily with smaller E_a.

Chromatic stability with the increasing temperature is also crucial in the actual application, as is the thermal stability. Since M^III = Ga shows less shift in peak position along the temperature due to the lack of [Na(3)O₆] emission, as shown in Fig. 4e, the chromatic stabilities of Na_2.97Eu_0.015AlP₃O₉N and Na_2.97Eu_0.015GaP₃O₉N are calculated based on the temperature-dependent emission at each temperature. Coordinates of six different temperatures in the 100–600 K are plotted in CIE 1976 u′v′ space with a 3-step MacAdam ellipse, which is presented as a white oval centered at each phosphor's 300 K color coordinate (Fig. 5f). Compared to M^III = Al, the shift in emission color of M^III = Ga is noticeably smaller for all the temperature ranges. To quantify the color changes, the concept of Δu′Δv′, the distance between the color coordinates at 300 K and the average color coordinate at six different temperatures , is calculated by following eqn (5).⁴⁹

Na_2.97Eu_0.015AlP₃O₉N has the higher value of Δu′Δv′ = 0.0119 than Na_2.97Eu_0.015GaP₃O₉N whose value is Δu′Δv′ = 0.0038, supporting M^III = Ga is more chromatically stable.

3.4. Quantum yield analysis through thermoluminescence of Na_2.97Eu_0.015M^IIIP₃O₉N

Even though the phosphor's thermal stability is satisfactory, the photoluminescent quantum yield should be higher for application. The most common commercial blue, BAM:Eu²⁺, and green, β-SiAlON:Eu²⁺, phosphors often show a PLQY > 90%. The PLQY of the Na_2.97Eu_0.015M^IIIP₃O₉N solid solution was measured to investigate how the cation substitution in the host affects PLQY. Interestingly, PLQY increases from 21(2)% to 54(2)% between Al³⁺ to Ga³⁺ compositions, inversely related to the E_g and E_a tendencies (Fig. 6a). However, the introduction of In³⁺ rapidly quenches the PLQY to 2.2(4)%, indicating that the low quantum yield is related to the small E_a and room-temperature thermal photoionization. The trend in quantum yield as a function of M^III cation was also investigated in greater depth.

Fig. 6 (a) Room-temperature PLQY of Na_2.97Eu_0.015M^IIIP₃O₉N. M^III = Ga shows the highest value, which is almost three times as large as M^III = Al. The increment In³⁺ significantly drops PLQY. TL glow curves of the solid solution at (b) M^III = Al, (c) M^III = Al_0.5Ga_0.5, (d) M^III = Ga, and (e) M^III = Ga_0.5In_0.5. Each curve is deconvoluted with the Randall–Wilkins equation.

Due to the aliovalent substitution between Na⁺ and Eu²⁺, the inherent defect should occur to compensate for the charge valence. The most probable charge-neutral defect configurations are depicted in Kröger–Vink notation,

where V refers to a vacancy site, subscript refers to the cation site, and superscript means the cation position, ×, ˙, and ′ refer to neutral, positive, and negative charge, respectively.⁵⁰ Other common types of defects, like point defects and antisite defects, also can impact the Na_2.97Eu_0.015M^IIIP₃O₉N series photoluminescence. To understand how the influence of defects along the cation substitution on M^III differs in the host system, TL glow curves are analyzed (Fig. 6b–e). The intensity of each TL curve was divided by a thermal quenching profile to correct the temperature dependency (TD) before being analyzed. Since the emission of M^III = Ga_0.5In_0.5 is mostly quenched after 400 K, the noise in its TL curve after 500 K is exaggerated by the TD correction; thus, the fitting was done before 500 K.

The peak positions and dosimetry parameters of trap states are determined using the glow curve deconvolution method. Deconvolution of each TL glow curve is performed with the GlowFit2 software using an approximation of the first-order kinetics Randall–Wilkins equation (eqn (8)).^51,52

where I is the TL intensity, n₀ is the number of trapped electrons, s is the frequency factor, E is trap depth, k is Planck constant, T is temperature, and β is the heating rate. The area of each curve represents the bound electron population. The deconvolution parameters of each TL curve are plotted in Table 1. The values of E and s can evaluate the reliability of fitting. Additionally, since the s reflects the interaction between the trap-bounded electron and the lattice phonon, the maximum value of s should be the same or lower than the lattice vibration of the host. In the Na₃M^IIIP₃O₉N (M^III = Al, Ga, In), the vibrations of O–N, O–P, N–P, and M^III–O are generally between 1000–1100 cm⁻¹, 900–1000 cm⁻¹, 1100–1200 cm⁻¹, and 400–650 cm⁻¹, respectively, which correspond to the range from 1.2 × 10¹³ to 3.6 × 10¹³ s⁻¹.^53–56 The s values extracted from the fitting are primarily between 1 × 10¹¹ and 3 × 10¹², supporting the validity of TL deconvolution.

Table 1 Parameters of calculated trap states using thermoluminescence glow curves

(a) Na2.97Eu0.015AlP3O9N				(b) Na2.97Eu0.015Al0.5Ga0.5P3O9N
Trap no.	T (K)	E (eV)	s (s^−1)	Trap no.	T (K)	E (eV)	s (s^−1)
1	349.00	0.79	1.73 × 10^11	1	338.83	0.77	1.86 × 10^11
2	378.81	0.92	3.39 × 10^12	2	357.71	0.87	1.47 × 10^12
3	397.98	0.94	3.54 × 10^11	3	372.56	0.90	1.24 × 10^12
4	424.00	0.95	1.03 × 10^11	4	396.30	0.97	1.54 × 10^12
5	450.00	1.01	1.35 × 10^11	5	426.01	0.99	3.38 × 10^11
6	483.90	1.04	4.86 × 10^10	6	538.80	1.19	6.05 × 10^10
7	522.60	1.05	6.43 × 10^9	7	572.50	1.29	9.43 × 10^10
8	589.50	1.12	1.26 × 10^9

---

(c) Na2.97Eu0.015GaP3O9N				(d) Na2.97Eu0.015Ga0.5In0.5P3O9N
Trap no.	T (K)	E (eV)	s (s^−1)	Trap no.	T (K)	E (eV)	s (s^−1)
1	354.12	0.90	6.19 × 10^12	1	345.61	0.77	1.47 × 10^11
2	370.02	0.91	2.17 × 10^12	2	362.30	0.90	2.45 × 10^12
3	390.48	0.98	3.09 × 10^12	3	385.96	0.99	7.34 × 10^12
4	418.69	1.03	1.50 × 10^12	4	415.19	1.04	2.83 × 10^12
5	448.24	1.07	5.80 × 10^11	5	464.21	1.13	1.22 × 10^12
6	542.69	1.15	1.97 × 10^10

---

Notably, M^III = Al exhibits eight pronounced peaks (Fig. 6b), and at least those numbered 3–8 are deep enough to immobilize the excited carriers for basically infinite time under ambient conditions. This phenomenon elucidates the low quantum yield observed in M^III = Al, as a significant portion of the excited electron population becomes trapped in various states, inhibiting photoluminescence despite the host material possessing the largest band gap and activation energy within the solid solution series. Along with the increasing Ga³⁺, the number of trap states and the portion of higher energy trap states decrease noticeably, supporting the highest PLQY at M^III = Ga, for which no significant TL appears above 400 K. Incrementally increasing In³⁺ does not show noticeable changes in trap states, indicating its low PLQY mainly stems from thermal quenching rather than defect quench. These results enable the construction of a luminescence mechanism for each composition. Combining the electronic structure (calculated E_g), photoluminescence, and thermoluminescence for the Na_2.97Eu_0.015M^IIIP₃O₉N in Fig. 7 demonstrates that the balance of band gap, trap depths, and 4f↔5d transition give M^III = Ga the best overall optical properties with the narrowest emission.

Fig. 7 Emission mechanism diagrams for (a) M^III = Al, (b) M^III = Al_0.5Ga_0.5, (c) M^III = Ga, and (d) M^III = Ga_0.5In_0.5.

4. Conclusion

This series of Eu²⁺-substituted Na₃M^IIIP₃O₉N (M^III = Al, Al_0.75Ga_0.25, Al_0.5Ga_0.5, Al_0.25Ga_0.75, Ga, Ga_0.75In_0.25, Ga_0.5In_0.5) phosphors was successfully synthesized using gas reduction nitridation. Powder X-ray diffraction confirmed the quality of the products from the synthesis while photoluminescence measurement at low temperature supports that Eu²⁺ replaces three crystallographically independent Na⁺ sites in Na_2.97Eu_0.015AlP₃O₉N. Interestingly, incorporating Ga³⁺ shifts the site selectivity so Eu²⁺ prefers only two substitution sites, [Na(1)O₉] and [Na(2)O₆N]. Na_2.97Eu_0.015AlP₃O₉N generates 429 nm violet emission with fwhm = 58 nm (3217 cm⁻¹). As Ga³⁺ increases, it appears there must be a distortion in the crystal structure, generating more blue-shifted emission further compounded by the lack of Eu²⁺ on the [Na(3)O₆] crystallographic site. This allows narrower emission with fwhm = 42 nm (2493 cm⁻¹) in Na_2.97Eu_0.015GaP₃O₉N with λ_em = 409 nm. The presence of Ga³⁺ also provides a decent T₅₀ = 455 K, while Na_2.97Eu_0.015GaP₃O₉N shows high chromatic stability. Additionally, PLQY of Na_2.97Eu_0.015GaP₃O₉N is the highest among the series, with a reasonable value of 54(2) %, worthy of further optimization efforts. Thermoluminescence analysis supports that the highest quantum yield of M^III = Ga originates from a smaller number of deep trap states that minimize their deleterious impact on excited electrons. This study of the Na_2.97Eu_0.015M^IIIP₃O₉N series offers that cation substitution on the host can accompany the preferred cation site selectivity, generating narrow and thermally and chromatically stable emission. This may be taken as proof that deliberately executed cation substitution may be an efficient approach for modeling and tuning important phosphors’ properties, among them for designing narrow-emitting and chromatically stable phosphors.

Author contributions

Nakyung Lee: conceptualization, data curation, formal analysis, investigation, visualization, writing – original draft. Justyna Zeler: data curation, formal analysis, funding acquisition, investigation, writing – review & editing. Małgorzata Sójka: conceptualization, writing – review & editing. Eugeniusz Zych: resources, writing – review & editing. Jakoah Brgoch: conceptualization, funding acquisition, project administration, supervision, writing – review & editing.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The National Science Foundation (DMR-2349319) and the Welch Foundation (E-2181) for supporting this work. J. Z. acknowledges support from the National Science Center (NCN) Poland project 2023/49/B/ST5/04265.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00410a

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