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[Hiroyo Segawa](https://orcid.org/0000-0002-7198-8410), Natalia A. Wójcik, [Kohsei Takahashi](https://orcid.org/0000-0002-6443-1534), [Takashi Takeda](https://orcid.org/0000-0003-2510-4562), Sharafat Ali

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This is the peer reviewed version of the following article: Elucidating photoluminescent properties of Eu‐doped Ca–Al–Si–O(–N) glasses and the local structures of Eu ions, which has been published in final form at https://doi.org/10.1111/jace.19615. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Elucidating photoluminescent properties of Eu‐doped Ca–Al–Si–O(–N) glasses and the local structures of Eu ions](https://mdr.nims.go.jp/datasets/aa8e7904-9df0-4517-8662-eece3e0d6676)

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Elucidating photoluminescent properties of Eu-doped Ca-Al-Si-O(-N) glasses and the local structures of Eu ionsHiroyo Segawa, *a Natalia. A. Wójcik, b, c Kohsei Takahashi, a Takashi Takeda, a Sharafat Ali ba National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.b Department of Built Environment and Energy Technology, Linnæus University, 35195 Växjö, Sweden.cAdvanced Materials Center, Institute of Nanotechnology and Materials Engineering, Gdańsk University of Technology, Narutowicza Street 11/12, 80–233 Gdańsk, Poland.* Correspoinding author: Hiroyo Segawa (E-mail SEGAWA.Hiroyo@nims.go.jp, Tel +81-29-860-4601)AbstractEuropium ions (Eu) doped luminescent materials have attracted considerable attention for their numerous optical applications. Eu-doped Ca-Al-Si-O(-N) glasses were synthesized from a mixture of oxynitride glasses and Eu2O3 powder using a standard melt-quenching technique in a radiofrequency furnace. The source Eu trivalent ions primarily changed to Eu2+ during melting, and the ratio of Eu2+ ions increased with an increase in Eu content in the starting mixture. All the prepared glasses exhibited photoluminescence (PL) owing to the 5d4f transition of Eu2+ ions. The absorption edge and PL wavelength shifted to longer wavelength with an increase in Eu content. Moreover, oxynitride glasses exhibited a longer wavelength than those of oxide glasses. The internal quantum efficiency increased with the increase in Eu content until it reached a maximum. X-ray absorption structure (XAS) and electron spin resonance (ESR) spectroscopies were used to determine the local structure of Eu ions, which confirmed that changes in the local structure of Eu ions were responsible for the shift in PL peak and the change in internal quantum efficiency. The development of the Eu- doped Ca-Al-Si-O-N glasses is highly inspiring for transparent phosphors. IntroductionInorganic rare-earth (RE) luminescent materials have attracted considerable attention owing to their numerous applications, such as light-emitting diodes, lasers, solar cells, photocatalysts, and biological imaging. 1–5 Among the RE ions, Eu ions act as excellent dopants because they exist in both trivalent, Eu3+, and divalent, Eu2+ states, depending upon the host materials. The Eu3+ and Eu2+ ions exhibit 4f4f and 4f5d transitions, respectively. The 4f4f transitions result in line-shaped emission spectra owing to the shielding effect of the 5s25p6 subshells, however, the Eu2+ ions exhibit a broad 4f5d transition emission, and their emission wavelength changes from blue to red depending on the surrounding coordination environment. 6–9 Glasses are more favorable than single crystals and ceramics as host materials for Eu ions, owing to their transparency in the visible region, high ion dispersive power, low cost, and ease of formation into various shapes. The transparent glasses doped with Eu ions can be potentially applied in lighting, displays, deep UV sensing, security printing, and solar light-downshifting materials as the glass absorbs energy from a light source, such as UV or visible light, and release that energy by emitting colored light. 9,10 Eu ions are incorporated in a trivalent state in the majority of oxide glasses prepared via the traditional melting method under air atmosphere; however, they are inserted in a divalent state in oxynitride glasses. Eu2+ ions shift toward longer wavelengths with an increase in the concentration of Eu ions in oxynitride glasses, as observed via photoluminescence (PL) analysis. 11–13 However, the PL properties of oxynitride glass have rarely been investigated. Therefore, an understanding of the PL properties of oxynitride glasses is required to demonstrate their potential as novel transparent phosphors.Silicon oxynitride glasses are prepared by heating powdered mixtures of the modifier oxide(s) and nitride compounds in crucibles under an Ar or N2 atmosphere. 14–16 Recently, hydrides and metals were utilized for the synthesis of oxynitride glasses with considerably higher nitrogen content than those obtained via conventional melting, owing to their high reactivity with nitrogen upon heating. However, the utilization of metal/metal hydrides reduced the oxides, such as silica in the melts, leading to the formation of metallic microparticles, including silicides, have resulted in decreased transparency and glass blackening. Although oxynitride glasses exhibit superior mechanical, chemical, and thermal properties compared to their oxide counterparts, their PL properties have not been extensively investigated. 12, 13, 17–19Recently, SiO2-Al2O3 gels doped with Eu ions were prepared via the sol-gel process and sintered in an ammonia atmosphere, resulting in the formation of Si-Al-O-N: Eu2+ glasses. 20 The Eu ions environment was controlled by varying the aluminum or nitrogen content, and the luminescence properties of the prepared glasses were investigated. The nitrogen content in the xerogel samples was less than 3 at%, whereas in the aerogel samples, it increased to 20 at% as the nitridation temperature increased. The emission spectra for all the samples showed a peak attributed to the 4f-5d transition of the Eu2+ ions. For the xerogel samples with 0.5 mol% Eu, the PL intensity increased with increasing Al content, and the peak wavelength shifted toward shorter wavelengths. Following nitridation, the xerogel sample with 10 mol% Al and 5 mol% Eu expanded in volume, and yellow emission was observed on the inner surface. The PL peak positions of the aerogel samples hardly depend on the nitrogen content, and a redshift of the PL peak was observed at an N/Eu ratio greater than 30 suggesting that the addition of nitrogen could shift the PL toward longer wavelengths when N/Eu is considerably high. However, the effects of Eu and N content on the PL properties have not been investigated in detail. Recently, one of the authors of this manuscript successfully obtained transparent and large-sized Ca-Al-Si-O-N glasses with high N content via melting process.21 Therefore, oxynitride glasses prepared via the melting process can potentially be used as host materials doped with Eu ions for transparent phosphors.In this study, Ca-Al-Si-O glass and Ca-Al-Si-O-N glass with approximately 5 at% nitrogen were remelted with Eu2O3 to investigate their PL properties. The resulting glasses were transparent and the Eu ions changed to a divalent state. The PL properties of the glasses were measured, and the local structures around the Eu ions were estimated. Consequently, the effects of the Eu and N contents on the PL properties were elucidated.ExperimentalAn oxide glass, E0, was prepared from SiO2, Al2O3, and CaO, reagents, and an oxynitride glass, NE0 was prepared using the same oxides and AlN, which was partially added as a nitrogen source instead of Al2O3. The preparation of the pristine glasses was carried out using a previously reported procedure. 21 The pristine glass was crushed into a powder in an Al2O3 mortar and mixed with an appropriate amount of Eu2O3 powder. The mixed powders were re-melted in Nb crucibles at approximately 1700 °C for 1 h using a radio frequency furnace under the N2 atmosphere. Thereafter, the furnace was turned off and the melts were cooled to room temperature.The glass samples were dissolved in an HCl solution, and the Ca, Si, Al, and Eu contents were determined using inductively coupled plasma optical emission spectroscopy ICP-OES (SPS3520UV-DD, Hitachi-High tech Sci., Japan). Oxygen and nitrogen contents were determined using He gas-CO2 IR absorption, and thermal conductivity using LECO equipment (TC-436AR), respectively. X-ray absorption fine structure (XAFS) spectroscopy was used to collect the Eu LIII-edge spectra in the 6600–7600 eV energy range using the BL9A or BL12C beamlines at the Photon Factory (KEK, Japan). XAFS spectra were collected in fluorescence mode. The background was subtracted by fitting the lower spectral absorption edge (pre-edge) region in 6821–6951 eV. The spectra were then normalized for atomic absorption based on the average absorption coefficient of the spectral post-edge region in 7071–7571 eV. To confirm the valence states of the Eu ions, the absorption data for crystalline Eu2O3 and EuCl2 were measured as a reference. Data processing was performed in the Demeter system, 22 using Athena software.The electron spin resonance (ESR) spectra of the glass samples were measured to determine the local structure of Eu ions in the glasses. After the samples were crushed, approximately 1 g of the glass powder was placed in a glass tube and analyzed using ESR (FA-100, JEOL, Japan). PL spectra of the glass powders were measured using a multichannel photodetector (QE-2100, Otsuka Electronics Co., Ltd., Japan) with a Xe lamp as the excitation source. The glass powders were placed in 10 mm silica cells on each side, and the excitation light was irradiated from the bottom of the cell. The PL spectra were collected using an integrated sphere. The internal quantum efficiency (IQE) was calculated using the equation (1): 　     (1)where, E(λ)/hν, R(λ)/hν, and P(λ)/hν are the number of photons in the excitation, reflectance, and emission spectra, respectively. Further details of this calculation can be found in the literature. 23 The decay curves of some samples were measured using a streak camera (C14831-110, Hamamatsu Photonics Co., Ltd.) irradiating at 350 nm by Ti: Sapphire laser with optical parametric generator (Chameleon Vision-S, Coherent, Inc.).Results and discussionThe Eu-doped glasses appeared gray at low Eu content; however, as the Eu content increased, the glasses became transparent and changed to yellowish green. The analyzed glass compositions are listed in Table 1. Pristine oxide and oxynitride glasses before the addition of Eu2O3 are referred to as E0 and NE0, respectively, as described in a previous study. 21 The cation compositions were estimated for comparing the glasses before and after remelting. The Si content in the mother glasses is marginally higher than that in the Eu-doped glasses, whereas the Ca and Al contents are lower owing to different analytical calculation methods for pristine glasses (EDX) and remelted glasses with Eu (ICP-OES). The anion composition was estimated for NE-series, and the nitrogen content decreased by approximately 5 eq.%. The equivalent nitrogen concentration, in eq.%, is defined as (3[N]×100)/(3[N]+2[O]) where [N] and [O] are the concentrations of nitrogen and oxygen, respectively. The partial removal of N ions occurred according to the reaction (2) given as: 12, 24 6Eu3+ +2N3- → 6Eu2+ +N2                             (2)The valence states of the Eu ions were confirmed using X-ray absorption near-edge structure (XANES) spectra. XANES spectra of E-series are displayed in Fig. 1(a). The Eu2O3 and EuCl2 spectra are used as references for divalent and trivalent ions, respectively. The spectra of the NE-series samples are similar to those of the E-series samples, as shown in Fig. S1(a). All the samples show a peak at approximately 6973 eV, which is assigned to Eu2+ ions, whereas the shoulder at 6980 eV corresponds to Eu3+ ions. The ratio of Eu3+ ions to total Eu was determined by deconvolution of the spectra into two peaks assigned to Eu3+ and Eu2+ ions. The detailed deconvolution is presented in Fig. S1(b). 25–27 The plot of the estimated ratio of Eu3+ to total Eu as a function of Eu contents is shown in Fig. 1(b). The relative Eu3+ content to total Eu gradually decreases as the Eu content increases and becomes nearly constant at 7 % when the Eu content exceeds 0.4 at%. This ratio does not depend upon the presence or absence of nitrogen. When Eu2O3 is added to the calcium aluminosilicate glasses, Eu3+ is partially reduced, and approximately 25% of Eu2+ ions are formed under a vacuum atmosphere. 28 However, in this study, reduction occurs more easily by flowing nitrogen, and majority of the Eu3+ ions are reduced to Eu2+ ions during remelting. To compare oxide and oxynitride samples with similar Eu ion concentrations, the XANES spectra for two sets of samples: E5 and NE5, E50 and NE47, are determined as shown in Fig. S2. For samples with the same Eu content, the ratio of Eu3+ ions to the total Eu calculated from the normalized spectra (Fig. 1(a) and Fig. S1(a)) are similar for the E- and NE-series samples. However, the shoulder of the E-series samples without nitrogen is slightly higher than that of the NE-series containing nitrogen (Fig. S2), suggesting that marginal Eu3+ reduction occurs in line with the reaction (2), however, the reduction effects are not dominant as the melting is carried out in nitrogen atmosphere. Additionally, extended X-ray absorption fine structure (EXAFS) measurements were used to determine the local structures of Eu ions. The local structures of Eu ions estimated from the k3-weighted experimental EXAFS signals (Fig. S3) do not depend on the presence of nitrogen atoms or Eu ions. Fig. 2(a-1) and (a-2) show the absorptance spectra of the E- and NE-series, respectively. The absorption edge gradually shifts to the longer-wavelength with increasing Eu content. The absorption band of Eu2+ ions appears in the 326–413 nm range. 29 The absorption in the 470–550 nm region decreases at Eu content higher than 0.238 %, indicating an improvement in the transparency of the glass. The improvement in transmittance could be attributed to the suppression of reduction reaction of not only Eu ions, as shown in eq. (2), but also silicon ions. Consequently, the glass color changes from gray to yellowish green. Fig. 2(b-1) and (b-2) show the PL spectra of the E- and NE-series, respectively, excited at 300 nm. The PL spectra show a peak in the 400–600 nm range, with a maximum at approximately 480 nm. The intensity of the peak increases with increasing Eu content. The highest peak intensity is observed for E50 in the E-series and for NE24 in the NE-series. However, in the NE series, the peak intensity decreases with an increase in Eu content for the NE47 and NE92 samples. Moreover, the peak position gradually shifts to longer-wavelengths as the Eu content increases, for both the series. The wavelength of the PL peak excited at 300 nm is plotted as a function of Eu content in Fig. 2(c). The peak wavelength of the NE-series is longer than that of the E-series for the same Eu content, and the difference in the peak wavelengths between the NE- and E-series is marginally higher when the Eu content is low. Fig. 2(d) shows the IQE of the samples excited at 300 nm for different Eu contents. The IQE increases with increasing Eu content, reaching a maximum of 0.3 for approximately 0.3 at% of Eu, and then gradually decreases. A clear difference between the IQE of the NE- and E-series is not observed. Although the IQE of the samples in this study is not as high as that of ceramic phosphors such as SiAlON, 23 it is higher than that of silicate glass. 30 To estimate the sites of the Eu ions, the lifetimes for the two sets of samples: E5 and NE5, E50 and NE47, and NE24 with the highest IQE, were measured. The decay curves are shown in Fig. 3. All the decay curves were fitted for time, t, using a second-order exponential equation (3).   (3)where I is the luminescence intensity, A1 and A2 are constants, and 1 and 2 are the rapid and slow lifetimes of the exponential components, respectively. The average lifetime (*) was determined using equation (4) as follows, 31     (4)The average lifetimes obtained are listed in Table 2. The decay curves do not depend on the Eu content and presence of nitrogen. However, the average lifetime gradually decreases with increasing Eu content. For NE24, the emission peak is deconvoluted into two peaks, as shown in Fig. S4, which suggests that the Eu ions have two sites with different emission energies, although there are many distinguishable sites for the Eu2+ ions in the glasses. When the Eu content increases, the emission peak shifts to longer wavelengths, i.e., toward the lower energy. Thus, Eu ions in low energy sites increase with the increase in the Eu content. Considering the relationship between the emission peak and the lifetime, it is apparent that the Eu2+ ions at low-energy sites have short lifetimes, which is in contrast to that observed previously for silicate glasses.30 The increase in Eu content at the low-energy site, which has a short lifetime, might decrease the IQE owing to the quick energy loss. In contrast, the PL intensity, as shown in Fig. 2, increases with increasing Eu content till 0.2 at% approximately. The increase in PL intensity could be attributed to emission enhancement owing to an increase in Eu content. Thus, the IQE increases when the Eu content is lower than 0.2 at%.Fig. 4 shows the ESR spectra, which reveal the electronic states of the Eu ions in the glasses. The Eu3+ ions are not paramagnetic and do not exhibit ESR signals. In contrast, Eu2+ ions have an electronic spin of S=7/2 and a nuclear spin of I=5/232, 33, and display ESR signals at room temperature. The signals at g~6, g~2.8, and g~2.0 in Fig. 4(a-1) and (b-1), are assigned to the U spectrum of the Eu2+ ions indicating a moderate distortion of the cubic, tetrahedral, or octahedral crystal field. 34 The ESR signal at g~4.6 is assigned to dominant asymmetric resonance and represents a crystal field of orthorhombic symmetry of Eu2+ ions. 34 Considering the origin of the signals and the ESR analysis of Ga3+ ions, Malchukova and Boizot reported two Eu2+ sites: a high-symmetry site represented by a signal at g~4.6 and a low-symmetry site represented by the U spectrum. The signal at g~4.6 was assigned to the site as a network former, and the signal at g~6 was assigned to the site as a network modifier. 35 Thus, the intensities of both the signals at g~6, Ig~6 and g~4.6, Ig~4.6 of NE- and E-series are plotted for Eu content in Fig. 5(a) and 5(b), respectively. The intensity of the signal gradually increases with increasing Eu content. The ratios of the intensities of both signals, Ig~6/Ig~4.6, are shown in Fig. 5(c). The Ig~6/Ig~4.6 in the E-series drastically decreases with an increase in Eu content when the Eu content is lower than 0.1 at%, whereas that of the NE-series does not depend on the Eu content. In the E-series doped with a low Eu content, Eu ions are introduced preferentially to the low-symmetry site as a network modifier. However, in the NE-series, the Eu site is introduced to a higher symmetry, even when the samples are doped analogous to the E-series, which could be attributed to the difference in optical basicity between the oxynitride and oxide glasses. The incorporation of nitrogen increases the optical basicity, 36, 37 eliminating the need for Eu2+ ions to function as network modifiers in low-Eu-content glasses. The Eu site might exhibit a larger nephelauxetic effect in the NE-series, than that in the E-series, resulting in a shift to longer PL peak wavelengths. Some samples, such as E3 and E10 exhibit large signal at g~2.3 approximately. The signal varies between the samples, however, it does not depend on the Eu content. Although the origin of the signal is not clear, it is possibly linked to paramagnetic impurities. 35In contrast, the signals at approximately g~2.0 are some possibilities except for the U spectrum of Eu2+ ions. Therefore, the ESR spectra shown in Fig. 4(a-1) and (b-1) are enlarged in the g~2 region and given in Fig. 4(a-2) and (b-2), respectively. Two signals are observed at g~2.00 and g~1.96 with different peak-to-peak linewidths, Hpp. In this region, defects other than the U spectrum of Eu are observed. 38, 39 Different types of defects have been previously identified on calcium aluminosilicate (CAS) glasses after irradiation; 40–42 including oxygen-associated defects, aluminum oxygen hole centers (Al-OHCs), silicone-related hole centers, and isotropic resonance with a negative lobe due to the Ca+ centers. The intensity and Hpp of the signal at g~1.96 and g~2.00 are plotted in Fig. 6(a) and (b), respectively. As shown in Fig. 6(a), the Hpp values increase linearly from several to 30 mT with increasing Eu content, consistent with the previous reports. 43 Thus, the signal at g~1.96 could be assigned to the U spectrum of the Eu ions. In the case of the NE-series, the signal shape represents the content of Eu2+ ions, which increases with increasing Eu content. However, the signal at g~2.00 is narrower than that at g~1.96 and does not depend on the Eu content. In silicon oxynitride films, the signal associated with defects was observed at g~2.00 and Hpp value was approximately 0.5 mT. 44, 45 Thus, the signal at g~1.96 could be assigned to the silicon-related defects in glass networks. The changes in the E-series are complex, but the signal intensity in the NE-series decreases, and Hpp increases with increasing Eu content, resulting in a decrease in the number of defects in the glass matrix. Consequently, the effects of the Eu content and nitrogen addition on the PL peak could be elucidated as follows: First, the PL peak shifts to longer wavelengths with increasing Eu content in both series, which is consistent with previous research. 12 The UV absorption edge shifts to longer wavelengths with increasing Eu content, as shown in Fig. 2(a). Thus, the PL emission shifts to longer wavelengths with increasing Eu content. The shift in the PL peak (Fig. 2(c)) suggests that the environmental change with increasing Eu content is not different between the two series with or without nitrogen. The addition of nitrogen influenced the shift toward longer wavelengths. Moreover, the high polarizability of nitrogen ions induces a centroid shift of the 5d levels of Eu2+ ions.10, 11, 46 Thus, the PL peak of the NE-series shifts to a longer wavelength than that of the E-series, as shown in Fig. 2(c). The decay curves (Fig. 3) and EXAFS spectra (Fig. S3) suggest that the influence of the Eu environment on the addition of nitrogen is insignificant because of the slight effects on the environment of the Eu ions. The addition of nitrogen in glasses results in an increase in the density and the densities of E0 and NE0 are calculated as 2.63 and 2.84 g/cm3, respectively, 21 which implies that the NE-series has a denser structure than that of the E-series when the Eu contents are the same. Therefore, the difference in the local structures causes differences in the electron states measured via ESR and shifts the PL peak toward longer wavelengths.ConclusionsThis study investigated the PL properties of Eu-doped Ca-Al-Si-O(-N) glasses synthesized via a two-step melting process. Eu ions were doped as trivalent ions using Eu2O3, which changed to Eu2+ ions after melting. Particularly, the ratio of Eu2+ to total Eu increased with an increase in the Eu sources shifting the absorption edge to longer wavelengths. All studied glasses exhibited PL owing to the 5d4f transition of Eu2+ ions. The PL wavelength shifted to the longer -wavelength with an increase in Eu content, and the NE-series glasses showed longer wavelengths than those of the E-series owing to the nephelauxetic effect. The IQE increased with the increase in Eu content and reached a maximum at 30 % value when the Eu content was approximately 0.2 at%. The decay curves suggested that Eu2+ ions existed at two different sites: high-energy and low-energy, and that the Eu2+ ions with low-energy sites increased with the increasing Eu content. The increase in Eu2+ ions on the low-energy sites shifted the PL peak with the maximum IQE at 0.2 at%. ESR analysis revealed the local structure of the Eu ions; in the case of the E-series, the Eu sites changed from low to high symmetry with increasing Eu content, whereas the Eu sites in the NE-series were highly symmetrical, irrespective of the Eu content. The difference in the sites affected the PL peak position. These Eu-doped Ca-Al-Si-O-N glasses can be potentially used as transparent glass phosphors.Author ContributionsHiroyo Segawa: Conceptualization, Investigation, Writing–review & editing, Funding acquisition. Natalia Wójcik: Investigation, Writing–review & editing. Kohsei Takahashi: Investigation, Review & editing. Takashi Takeda: Investigation, Review & editing.Sharafat Ali: Investigation. Writing–review & editing, Funding acquisition, Supervision.Conflicts of interestThere are no conflicts to declare.AcknowledgmentsThis work was supported by the NIMS overseas researcher dispatch program and was partially supported by the MEXT Elements Strategy Initiative to Form the Core Research Center for Electronic Materials: Tokodai Institute of Elements Strategy, Japan, Grant Number JPMXP0112101001. The XAFS measurements were supported by assignment numbers2020G105 and 2022G095 from the Photon Factory, KEK, Japan. Technical assistance from Ms. Kazuko Nakajima and Ms. Fumie Inoue is also acknowledged. SA acknowledges financial support from the KKL Advanced Materials, LNU (Grant No. 87202002), and Crafoord Foundation (Grant No. 2022-0692).References1. Eliseeva SV, Bünzli J-CG. Rare earths: jewels for functional materials of the future. New J Chem. 2011;35(6):1165–76.2. Bunzli JC, Piguet C. Taking advantage of luminescent lanthanide ions. Chem Soc Rev. 2005;34(12):1048-77.3. 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J Lumin. 2003;104(4):239-60.Table 1 Analyzed glass composition. Sample name Cation ratio (at%) 　 Anion concentration  Ca Si Al Eu 　 N/O (%) N (eq%) E0 23.8 32.9 43.3 0.000    E3 24.2 27.9 47.9 0.026    E5 24.0 28.1 47.8 0.051    E10 24.1 28.1 47.7 0.099    E50 23.9 27.9 47.8 0.507            NE0 24.8 29.3 45.9 0.000  13.8 17.1 NE1 23.9 28.3 47.8 0.011  9.9 12.9 NE5 23.4 29.7 46.9 0.047  9.1 12.0 NE9 23.5 28.9 47.5 0.094  8.5 11.3 NE24 23.7 28.9 47.2 0.238  9.6 12.6 NE47 23.8 28.5 47.3 0.468  9.3 12.3 NE92 23.7 28.1 47.3 0.916 　 9.0 11.9Table 2 Average lifetime of samples.  E-series * (s) NE-series * (s) E5 0.41 NE5 0.37   NE24 0.24 E50 0.17 NE47 0.16Figure CaptionsFig. 1  Eu LIII-edge XANES spectra of the samples; (a) E-series, (b) NE-series, and those of Eu2O3 and EuCl2 powder are shown as references. (c) Plots of Eu3+ ratio to total Eu estimated from the fitting of XANES spectra.Fig. 2  Absorptance spectra of E series, (a-1) and NE series, (a-2). PL spectra of E series, (b-1) and NE series, (b-2) excited at 300 nm. (c) The wavelength of PL peak excited at 300 nm vs Eu contents and (d) Internal quantum efficiency (IQE) vs Eu contents.Fig. 3  Decay curve of the E5, NE5, E50, NE47, and NE24 samples excited at 350 nm and monitored in the 411–657 nm range. (b) expanded (a). Fig. 4  ESR signals of (a) E-series and (b) NE-series. (a-2) and (b-2) are expanded in the 320–360 mT range of (a-1) and (b-1), respectively.Fig. 5  (a) Plots of ESR line intensity at g~6 and g~4.6 for Eu contents of NE-series. (a-2) is expanded in the low Eu contents of (a-1). (b) Plots of intensity at g~6 and g~4.6 for Eu contents of E-series. (c) Plots of Ig~6/Ig~4.6 for Eu contents. Fig. 6  (a) Intensity and linewidth of signal at g~1.96 in E-series, (a-1) and NE-series, (a-2) and (b) at g~2.0 in E-series, (a-1) and NE-series, (a-2).15