Crystal structures and luminescence properties of Ca₃RE₂Si₄O₈N₄ (RE = Y, La, and Ce) activated by Ce³⁺

Abstract

The discovery of new compounds expands the scope of materials chemistry. In this study, we report a new series of oxynitride compounds, Ca₃RE₂Si₄O₈N₄ (RE = Y, La, and Ce), and their crystal structures. The samples were synthesized by a solid-state reaction method under an inert atmosphere. Single-crystal X-ray diffraction analysis revealed that these compounds are isostructural and crystallize in the cubic space group Pa3̄. The structures contain four crystallographically distinct cation sites and unique isolated 12-membered ring units constructed from Si(O,N)₄ tetrahedra. Ce³⁺-containing samples exhibit broad photoluminescence under ultraviolet excitation, and the emission colour changes from deep blue to bluish green with varying RE composition. Analysis of the emission spectra suggests that the luminescence originates from multiple crystallographic sites and exhibits thermal quenching behaviour. These results demonstrate that Ca₃RE₂Si₄O₈N₄ represents a new class of oxynitride phosphors with multi-site luminescence and tuneable emission properties.

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Introduction

White light emitting diodes (LEDs) have increasingly replaced conventional incandescent and fluorescent lighting in recent years. The performance of phosphor-converted LEDs, including emission colour, colour rendering, and luminous efficiency, is strongly governed by the properties of the phosphors employed in the devices. Phosphors activated by ions exhibiting 4f–5d transitions have been widely studied because their emission properties are highly sensitive to the local structure of the host compounds.^1–5 As a result, a wide variety of phosphors with tuneable emission colours have been developed. In compounds with multiple crystallographically distinct cation sites, activator ions can occupy different sites, resulting in multi-emission behaviour. Such multi-emission can lead to a variety of luminescence properties,^6–12 and, in some cases, white light emission from a single phosphor.^13–18 Therefore, the exploration of new host materials with multiple crystallographic sites for activators has attracted considerable attention.

The introduction of multiple anions into a host lattice provides an effective approach to modifying local coordination environments.¹⁹ In particular, the incorporation of anions with different valences, such as O²⁻ vs. N³⁻ and O²⁻ vs. F^–, alters both the local structures and the overall cation compositions compared with single-anion compounds, often leading to the formation of new structures and unusual coordination environments. Owing to the stronger nephelauxetic effect of N³⁻ relative to O²⁻, the introduction of nitrogen generally results in larger Stokes shifts and tuneable emission properties. Consequently, oxynitrides are considered to be promising host compounds.^11,20–28 For practical applications, phosphors must maintain their performance under elevated temperatures, typically up to 423 K.⁵ In addition to structural stability, it is crucial that both emission intensity and colour characteristics remain stable with increasing temperature. In general, rigid crystal frameworks are favourable for suppressing lattice vibrations and reducing thermal quenching. Silicate-based compounds possessing three-dimensional networks of Si(O,N)₄ tetrahedra are particularly suitable in this regard, because such networks contribute to structural rigidity and stability, thereby improving resistance to thermal quenching.^12,26–32

In our previous studies, we have explored a variety of silicon oxynitride phosphors.^{18,27,28,33–36} Among them, compounds containing both divalent alkaline earth and trivalent lanthanide cations exhibit unusual local environments and distinctive luminescence properties.¹⁸ In this study, we report a series of oxynitride compounds, Ca₃RE₂Si₄O₈N₄ (RE = Y, La, and Ce). Their crystal structures were determined by single-crystal X-ray diffraction analysis, revealing four crystallographically distinct cation sites available for Ca and RE ions. The Si(O,N)₄ tetrahedra form an isolated 12-membered ring unit; such a structural motif is rarely observed in silicates. Powder samples were also successfully synthesized, and their photoluminescence properties were investigated using Ce³⁺ as an activator. The emission colour under UV excitation varies depending on the RE element, and Ca₃Ce₂Si₄O₈N₄ exhibited bluish-cyan emission despite the high concentration of activator ions. To the best of our knowledge, only a limited number of silicon oxynitrides containing both alkaline-earth and lanthanide cations have been reported. These findings provide insight into the design of oxynitride phosphors with multiple emission centres.

Results and discussion

Crystal structures of Ca₃RE₂Si₄O₈N₄

Single-crystal X-ray diffraction analysis revealed that the compounds crystallize in the cubic space group Pa3̄ (No. 205) irrespective of RE (1, 2, and 3, Table S1), indicating that they are isostructural. The lattice parameters a were refined to 14.8435(1), 15.1267(1), and 15.0831(2) Å for RE = Y, La, and Ce, respectively. These values follow the expected trend based on the ionic radii of trivalent lanthanide ions in ninefold coordination (Y³⁺: 1.08 Å, La³⁺: 1.22 Å, and Ce³⁺: 1.20 Å). Fig. 1 shows the refined crystal structures and coordination environments of Ca₃Y₂Si₄O₈N₄ as a representative example. The distribution of oxygen and nitrogen atoms cannot be reliably determined from X-ray diffraction data because of their similar atomic scattering factors.¹⁹ Considering the cation compositions determined by energy-dispersive X-ray spectroscopy, charge balance, and the number of anion sites, the O/N ratio was assumed to be 8/4. In silicon-containing oxynitrides, Si–N bonds are generally longer than Si–O bonds.^35,37–40 According to Pauling's second rule,⁴¹ nitrogen tends to have a higher coordination number with silicon than oxygen. Therefore, nitrogen atoms were assigned to anion sites with larger coordination numbers and longer Si–(O,N) bond distances (Fig. 1, Table S2), although the assignment of the anion distribution remains tentative because of the limited reliability of distinguishing O and N atoms by X-ray diffraction. For Ca²⁺ and RE³⁺, there are four crystallographically distinct cation sites, M1–M4, with coordination numbers of 9, 7, 7, and 6, respectively (Fig. 1, Table S3). Although Ca²⁺ and RE³⁺ have different valences, they can occupy the same crystallographic sites because of their similar ionic radii (Ca²⁺: 1.18 Å in ninefold coordination).^35,37 The M4 site was refined as being exclusively occupied by Ca²⁺. When RE was introduced at the M4 site in the refinement, its occupancy was below 0.05 and the reliability factors changed negligibly (R-factor: 1.83% → 1.81% for RE = Y). Considering the small bond valence sum at the M4 site (<2), the assignment is reasonable. The other cation sites are occupied by Ca²⁺ and RE³⁺ ions in a disordered manner. In general, RE³⁺ ions preferentially occupy more spacious sites with higher coordination numbers due to their larger ionic size and higher charge. In Ca₃Y₂Si₄O₈N₄, Ca²⁺ can occupy the nine-coordinated M1 site because of its larger ionic radius relative to Y³⁺. In contrast, La³⁺ and Ce³⁺ are larger than Ca²⁺; therefore, the M1 site is predominantly occupied by RE³⁺ in Ca₃La₂Si₄O₈N₄ and Ca₃Ce₂Si₄O₈N₄. A similar tendency is observed for the M2 and M3 sites (Table S3). The bond valence sums at each site agree well with the expected average cation valences, supporting the validity of the present structural model.

Fig. 1 Refined crystal structures of Ca₃Y₂Si₄O₈N₄ (1) drawn with Ca (pink), Y (pale green), Si (blue), O (red), N (black), and Si(O,N)₄ tetrahedra. Coordination environments of M1–M4 sites and Si are also depicted.

In another single-crystal X-ray diffraction measurement using a different crystal for RE = Y, a large positive residual electron density was observed around (0.2441, 0.2441, 0.2441), corresponding to the Wyckoff position 8c (site symmetry .3.) (1′, Table S1, Fig. S1). When Ca²⁺ was assigned to this 8c site, the refined occupancy was approximately 0.1, indicating that ∼90% of the site is vacant. Although partial occupation may occur, this site was treated as vacant due to its low occupancy.

Silicon occupies two crystallographically distinct sites. Six SiO₂N₂ tetrahedra are connected through their vertices to form an isolated 12-membered ring unit, [Si₆O₁₂N₆] (Fig. 1). A compound, Ca₃Si₂O₄N₂, crystallizing in the Pa3̄ space group, has previously been reported (Fig. S2).⁴² Its formula can be rewritten as Ca₆Si₄O₈N₄. Considering charge neutrality, substitution of three Ca²⁺ ions by two RE³⁺ ions yields the present compounds, Ca₃RE₂Si₄O₈N₄. Although this suggests a structural relationship between Ca₃RE₂Si₄O₈N₄ and Ca₃Si₂O₄N₂, their frameworks are fundamentally different. In Ca₃Si₂O₄N₂, Si(O,N)₄ tetrahedra form isolated [Si₁₂O₁₂N₁₂] units (Fig. S2), whereas the present compounds contain [Si₆O₁₂N₆] ring units.

Powders and solid solutions

Fig. 2 shows the Rietveld refinement profiles of Ca₃RE₂Si₄O₈N₄. The refined crystallographic parameters and atomic coordinates are summarized in Tables S4–S6. Ca₃La₂Si₄O₈N₄ was refined as a single phase, whereas Ca₃Y₂Si₄O₈N₄ and Ca₃Ce₂Si₄O₈N₄ contained Ca-substituted Y₄Si₂O₇N₂ (Ca_0.9Y_3.1Si₂O_7.9N_1.1) and Ca₂Ce₈(SiO₄)₆O₂ as impurity phases, respectively. Due to the formation of lanthanide-rich secondary phases, compositions were refined to be Ca_3.1Y_1.9Si₄O_8.1N_3.9 for RE = Y and Ca_3.5Ce_1.5Si₄O_8.5N_3.5 for RE = Ce. However, the target compounds were obtained as the dominant phases. The average bond lengths and bond valence sums were consistent with those obtained from single-crystal analysis (Tables S3 and S6). Fig. 3 shows the XRD patterns of Ca₃(Y_1−xCe_x)₂Si₄O₈N₄ and Ca₃(La_1−yCe_y)₂Si₄O₈N₄. Most diffraction peaks can be indexed to Ca₃RE₂Si₄O₈N₄ and shift toward lower angles with increasing x or decreasing y. The main phases correspond to Ca₃RE₂Si₄O₈N₄, although Ca_0.9Y_3.1Si₂O_7.9N_1.1 and Ca₂Ce₈(SiO₄)₆O₂ are observed as secondary phases in Ca₃(Y_1−xCe_x)₂Si₄O₈N₄ at x ≤ 0.5 and Ca₃(La_1−yCe_y)₂Si₄O₈N₄ at y ≥ 0.5, respectively. For x ≥ 0.5, Ca₃(Y_1−xCe_x)₂Si₄O₈N₄ contains Ca₂Ce₈(SiO₄)₆O₂ or a closely related phase. The gradual shift of diffraction peaks with changing x and y values reflects lattice expansion (Fig. S3). This trend is consistent with the ionic radii of Y³⁺ (0.96 Å at seven coordinate), La³⁺ (1.10 Å), and Ce³⁺ (1.07 Å). These results indicate the formation of continuous solid solutions.

Fig. 2 Rietveld profiles of (a) Ca₃Y₂Si₄O₈N₄, (b) Ca₃La₂Si₄O₈N₄, and (c) Ca₃Ce₂Si₄O₈N₄. Red cross marks, black lines, and blue lines represent the observed, calculated, and difference profiles, respectively. Green ticks correspond to the Bragg reflection positions. Orange ticks indicate the Bragg reflection positions of (a) Ca_0.9Y_3.1Si₂O_7.9N_1.1 and (c) Ca₂Ce₈(SiO₄)₆O₂, respectively. The refined structural parameters are summarized in Tables S4–S6.

Fig. 3 XRD patterns of (a) Ca₃(Y_1−xCe_x)₂Si₄O₈N₄ and (b) Ca₃(La_1−yCe_y)₂Si₄O₈N₄. Simulated XRD patterns of Ca_3.1Y_1.9Si₄O_8.1N_3.9, Ca₃La₂Si₄O₈N₄ and Ca_3.5Ce_1.5Si₄O_8.5N_3.5 are shown for reference.

Photoluminescence

Fig. 4 shows the photoluminescence (PL) and PL excitation (PLE) spectra of Ca₃(Y_1−xCe_x)₂Si₄O₈N₄ and Ca₃(La_1−yCe_y)₂Si₄O₈N₄. All samples exhibited broad emission bands. 1 mol% Ce-doped Ca₃Y₂Si₄O₈N₄ and Ca₃La₂Si₄O₈N₄ showed deep-blue emission with a peak at 416 nm and bluish-cyan emission with a peak at 465 nm, respectively, under excitation at 330 nm. With increasing Ce concentration, the excitation peaks shifted to longer wavelengths, and the emission peak of Ca₃Ce₂Si₄O₈N₄ appeared at 480 nm, resulting in bluish-green emission. As discussed in the previous section, the samples contained impurities such as Ca_0.9Y_3.1Si₂O_7.9N_1.1 and Ca₂Ce₈(SiO₄)₆O₂. These compounds exhibit emission under excitation at 250–400 nm due to Ce³⁺ activation, and their emission spectra overlap with those of the Ca₃(Y_1−xCe_x)₂Si₄O₈N₄ and Ca₃(La_1−yCe_y)₂Si₄O₈N₄ samples (Fig. S4). The emission peaks of Ce-doped Ca_0.9Y_3.1Si₂O_7.9N_1.1 and Ca₂Ce₈(SiO₄)₆O₂ appear at 455 nm and 480 nm, respectively, and the emission intensity of Ca₂Ce₈(SiO₄)₆O₂ is approximately one order of magnitude weaker than that of the present compounds. Considering both impurity contents and intensity differences, the observed PL and PLE spectra are mainly attributed to the target compounds. The emission spectra of three samples—1 mol% Ce-doped Ca₃Y₂Si₄O₈N₄, 1 mol% Ce-doped Ca₃La₂Si₄O₈N₄, and Ca₃Ce₂Si₄O₈N₄—were deconvoluted into two, four, and four components, respectively, as shown in Fig. 5. The Ce³⁺ emission based on the 4f–5d transition typically consists of two components due to spin–orbit splitting; 5d → ²F_5/2 and 5d → ²F_7/2. These results suggest the presence of multiple emission centres in 1 mol% Ce-doped Ca₃La₂Si₄O₈N₄ and Ca₃Ce₂Si₄O₈N₄.^35,43,44 The two components observed for 1 mol% Ce-doped Ca₃Y₂Si₄O₈N₄ can be attributed to single-site emission. Structural refinement of Ca₃Ce₂Si₄O₈N₄ indicates that Ce ions preferentially occupy the M1, M2, and M3 sites in descending order of preference (M1 > M2 > M3), while the M4 site is not occupied (Tables S3, S5 and S6). In 1 mol% Ce-doped Ca₃Y₂Si₄O₈N₄, most Ce ions occupy the M1 site, resulting in emission peaks at 3.08 eV and 2.85 eV. The spectral shape is therefore dominated by emission from Ce³⁺ at the M1 site. Considering the emission properties of the impurity phase (Fig. S4), a minor contribution from the impurity phase cannot be completely excluded; however, its influence on the overall emission behaviour is considered negligible because of its small amount. In contrast, four emission peaks at 3.27, 2.97, 2.70 and 2.48 eV are observed for 1 mol% Ce-doped Ca₃La₂Si₄O₈N₄, suggesting the presence of multiple crystallographic emission sites in the target compound, because this sample is essentially single phase. In general, a longer average Ce³⁺–(O,N) bond length leads to weaker crystal field splitting and thus higher 4f–5d transition energy.⁴⁵ Because the La³⁺ ion is the largest cation among the RE ions investigated in the present study, the corresponding bond lengths are the longest. La ions preferentially occupy the largest M1 site as discussed above (Tables S3, S5 and S6); therefore, Ce³⁺ ions are distributed over the M2 and M3 sites. With increasing Ce concentration, the occupation of smaller sites such as the M3 site increases, leading to lower emission energy and a redshift in the overall emission spectra. Additionally, the 4f–5d transition energy decreases with increasing Ce concentration due to interactions between adjacent Ce³⁺ 5d orbitals.^46,47 In the La system, the emission peaks generally shifted to longer wavelengths with increasing Ce concentration (increasing y); however, the emission of Ca₃Ce₂Si₄O₈N₄ appears at a shorter wavelength than those of the samples with y = 0.50 and 0.75. This non-linear emission behaviour might be attributed to the presence of multiple emission sites (Fig. S5). Although detailed discussion is limited due to the presence of impurity phases, the emission colour can be tuned by varying the composition.

Fig. 4 PL (solid lines) and PLE (dotted lines) spectra of (a) Ca₃(Y_1−xCe_x)₂Si₄O₈N₄ and (b) Ca₃(La_1−yCe_y)₂Si₄O₈N₄. Excitation wavelength was 330 nm.

Fig. 5 Decomposition of emission curves of (a) Ca₃(Y_0.99Ce_0.01)₂Si₄O₈N₄, (b) Ca₃(La_0.99Ce_0.01)₂Si₄O₈N₄ and (c) Ca₃Ce₂Si₄O₈N₄. Excitation wavelength was 330 nm.

The internal quantum efficiency (IQE), external quantum efficiency (EQE), and absorption rate were 19.2%, 7.9%, and 41.2%, respectively, for 1 mol% Ce-doped Ca₃Y₂Si₄O₈N₄, and 23.2%, 13.8% and 59.2%, respectively, for 1 mol% Ce-doped Ca₃La₂Si₄O₈N₄, (Fig. S6). The maximum EQE for Ca₃La₂Si₄O₈N₄ is observed at 1 mol% Ce (y = 0.01), whereas the highest EQE (24.5%) for Ca₃Y₂Si₄O₈N₄ is obtained at 20 mol% doping. The emission intensity decreased with increasing temperature while largely maintaining the spectral shapes, as shown in Fig. 6. The plateau-like feature observed in the emission spectrum of the La compound at 473 K may be related to the presence of multiple emission centres, although the exact origin remains unclear. The integrated intensities at 423 K were approximately 32% and 30% of those at 298 K for the Y and La compounds, respectively. The activation energies estimated from Arrhenius plots are 0.265 eV and 0.256 eV for Y and La systems, respectively. These values are comparable to those of thermally stable phosphors;^27,48,49 however, significant thermal quenching is observed. This behaviour is attributed to thermal ionization, that is, electron transfer from the excited state to the conduction band.^50,51

Fig. 6 Thermal dependence PL spectra of 1 mol% Ce³⁺-doped (a) Ca₃Y₂Si₄O₈N₄ and (b) Ca₃La₂Si₄O₈N₄. Excitation wavelength was 330 nm.

In the present compounds, the 12-membered ring units, [Si₆O₁₂N₆], formed by corner-sharing tetrahedra are isolated and do not form a rigid framework. Because the units are isolated rather than forming a three-dimensional rigid framework, lattice vibrations are not effectively suppressed, leading to severe thermal quenching. The energy gaps estimated from absorption spectra are approximately 4.77 eV and 4.59 eV for Ca₃Y₂Si₄O₈N₄ and Ca₃La₂Si₄O₈N₄, respectively (Fig. S7). The smaller band gap of the La compound facilitates thermal ionization, leading to stronger thermal quenching.^{27,31,50–52}

Conclusions

We have synthesized a series of Ca₃RE₂Si₄O₈N₄ (RE = Y, La, and Ce) compounds and characterized their crystal structures by single-crystal and powder X-ray diffraction analysis. The structures consist of Si(O,N)₄ tetrahedra forming unique isolated 12-membered ring units, which have rarely been reported in silicates. Four crystallographically distinct cation sites are present for Ca and RE ions, and Ce³⁺ ions can occupy up to three of these sites. Ce-containing samples exhibit photoluminescence under UV excitation, and the emission colour can be tuned from deep blue to bluish green by varying the composition. Ca₃Ce₂Si₄O₈N₄ exhibits photoluminescence despite the high concentration of Ce ions. Deconvolution of the emission spectra suggests that the observed emission may originate from multiple crystallographic cation sites, although definitive assignment of the individual emission components remains difficult. Because the Si(O,N)₄ tetrahedra do not form a rigid three-dimensional framework, the compounds exhibit significant thermal quenching caused by thermal ionization, which limits their applicability as phosphors. Nevertheless, the present results demonstrate that Ca₃RE₂Si₄O₈N₄ provides a new structural platform for multi-site luminescence and offers insight into the design of oxynitride phosphors with tuneable emission properties.

Methods

Synthesis

Single crystals and powders of Ca₃RE₂Si₄O₈N₄ (RE = Y, La, and Ce) were synthesized using CaCO₃ (Kanto Chemical, 99.99%), Y₂O₃ (Wako Pure Chemical, 99.99%), La₂O₃ (Kanto Chemical, 99.99%), CeO₂ (Kanto Chemical, 99.5%) SiO₂ (Kanto Chemical, 99.5%), and Si₃N₄ (Kojundo Chemical, 99.9%). Prior to use, La₂O₃ was calcined at 1173 K because La₂O₃ easily transforms into a hydroxide form under ambient conditions. A stoichiometric ratio of reagent powders was mixed in an agate mortar with ethanol. For Ce³⁺-doped samples, a portion of the raw material Y₂O₃ or La₂O₃ was replaced with CeO₂. Pellets of the mixtures were placed on an alumina boat with a carbon sheet and heated at 1773 K for 4 h under a 100 ml min⁻¹ N₂ flow. It was cooled to 1373 K at a rate of 30 K h⁻¹ and then to 1173 K at a rate of 100 K h⁻¹ followed by natural cooling to room temperature. Colourless crystals (RE = La and Y) and pale-yellow crystals (RE = Ce) were selected from the roughly crushed pellet. For powder samples, quenching processes were omitted.

Characterization

Single crystals were analysed by single crystal X-ray diffraction (single crystal XRD, Rigaku, XtaLAB Pro MM007) using Mo Kα radiation. Data collection, raw data conversion, empirical corrections for absorption, and the merging of equivalent reflections were carried out using CrysAlisPro. All calculations were conducted using the WinGX software package.⁵³ The structures were solved by direct methods using SHELXT,⁵⁴ and calculations and refinement were performed using SHELXL.⁵⁵

The obtained powders were characterized by powder XRD (Bruker AXS, D2 Phaser or Rigaku SmartLab-9 kW) and Rietveld refinement using the TOPAS 4.2 program (Bruker). Crystal structures were depicted using VESTA.⁵⁶ PL and PLE spectra were recorded at room temperature using a fluorescence spectrometer (Hitachi, F-7100). Another fluorescence spectrometer (Jasco, FP-6500) was also used for thermal quenching and quantum efficiency analyses. An absorption spectrum of non-doped sample was recorded using an ultraviolet-visible-near infrared spectrometer (Shimadzu, UV-3100) with an integrating sphere and the obtained spectrum was converted by the Kubelka–Munk method.

Author contributions

Makoto Kobayashi: conceptualization, data curation, investigation, project administration, supervision, validation, visualization, writing – original draft, and writing – review & editing. Takuya Yasunaga: investigation and methodology. Chikako Nagahama: investigation. Kotaro Fujii: data curation, investigation, and writing – review & editing. Masatomo Yashima: validation and writing – review & editing. Minoru Osada: resources and writing – review & editing, Hideki Kato: conceptualization, project administration, supervision, validation, and writing – review & editing. Masato Kakihana: project administration, supervision, resources, and writing – review & editing.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: crystallographic data, structural parameters, cation site parameters and atomic coordinates of Ca₃RE₂Si₄O₈N₄ refined by single-crystal XRD and Rietveld analysis; refined crystal structure of 1′; crystal structure of Ca₃Si₂O₄N₂; lattice parameters of powder samples; PL and PLE spectra of impurity phases; deconvoluted emission spectra; summary of absorbance, IQE and EQE, and DRS of Ca₃Y₂Si₄O₈N₄ and Ca₃La₂Si₄O₈N₄. See DOI: https://doi.org/10.1039/d6dt01213b.

CCDC 2553962–2553964 contain the supplementary crystallographic data for this paper.^57a–c

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgements

This work was supported in part by JSPS KAKENHI “Mixed anion” Grant Numbers JP16H06438 and JP16H06439, and DEJI²MA, the Joint Usage/Research Program of the Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University, the Cooperative Research Program of “Network Joint Research Center for Materials and Devices” from MEXT, Japan, and the Nippon Sheet Glass Foundation for Materials Science and Engineering.

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