Correlation among photoluminescence and the electronic and atomic structures of Sr2SiO4:xEu3+ phosphors: X-ray absorption and emission studies

A series of Eu3+-activated strontium silicate phosphors, Sr2SiO4:xEu3+ (SSO:xEu3+, x = 1.0, 2.0 and 5.0%), were synthesized by a sol–gel method, and their crystalline structures, photoluminescence (PL) behaviors, electronic/atomic structures and bandgap properties were studied. The correlation among these characteristics was further established. X-ray powder diffraction analysis revealed the formation of mixed orthorhombic α'-SSO and monoclinic β-SSO phases of the SSO:xEu3+ phosphors. When SSO:xEu3+ phosphors are excited under ultraviolet (UV) light (λ = 250 nm, ~ 4.96 eV), they emit yellow (~ 590 nm), orange (~ 613 nm) and red (~ 652 and 703 nm) PL bands. These PL emissions typically correspond to 4f–4f electronic transitions that involve the multiple excited 5D0 → 7FJ levels (J = 1, 2, 3 and 4) of Eu3+ activators in the host matrix. This mechanism of PL in the SSO:xEu3+ phosphors is strongly related to the local electronic/atomic structures of the Eu3+–O2− associations and the bandgap of the host lattice, as verified by Sr K-edge and Eu L3-edge X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure, O K-edge XANES and Kα X-ray emission spectroscopy. In the synthesis of SSO:xEu3+ phosphors, interstitial Eu2O3-like structures are observed in the host matrix that act as donors, providing electrons that are nonradiatively transferred from the Eu 5d and/or O 2p–Eu 4f/5d states (mostly the O 2p–Eu 5d states) to the 5D0 levels, facilitating the recombination of electrons that have transitioned from the 5D0 level to the 7FJ level in the bandgap. This mechanism is primarily responsible for the enhancement of PL emissions in the SSO:xEu3+ phosphors. This PL-related behavior indicates that SSO:xEu3+ phosphors are good light-conversion phosphor candidates for use in near-UV chips and can be very effective in UV-based light-emitting diodes.


Results and discussion
,b shows the atomic structures of the (orthorhombic) α′ and (monoclinic) β phases of the SSO matrix, respectively. As the Eu 3+ doping concentration increases, β-phase SSO is likely converted into α′-phase SSO. The structure of SSO is similar to the non-close-packed structure of K 2 SiO 4 27 . As displayed in Fig. 1a,b, one unit cell of SSO comprises 26 atoms that share four formula units. Its structure can be best described as comprising corner-sharing SiO 4 tetrahedra in parallel chains. The oxygen ions are located at three types of nonequivalent lattice sites, and the Si ions are located at the center of the oxygen tetrahedron. When Eu 3+ is doped into the SSO matrix, the Eu 3+ ions may affect the SSO host lattice by changing the lattice constants and/or causing structural distortion, varying the ratio of the α′ phase to the β phase in the SSO matrix 18 . However, this modification of the SSO matrix and/or site occupancy can only be observed by X-ray diffraction (XRD) at a rather high Eu 3+ -dopant concentration (≥ 5%) in the host matrix 18 . Figure 1c presents XRD patterns of both the as-synthesized SSO:xEu 3+ phosphors with various concentrations of Eu 3+ ions and SSO, SrCO 3 (SCO), SiO 2 and Eu 2 O 3 for reference. The diffraction peak at (112) typically corresponds to the α′-SSO:xEu 3+ phase, and the (211) and (301) peaks typically correspond to the β-SSO:xEu 3+ phase. This result clearly shows that the crystal structures of the SSO:xEu 3+ phosphors and the SSO host were mixed phases of both the α′ and β-SSO:xEu 3+ phases. Prominent (211) and (301) diffraction peaks were observed at an Eu 3+ ion concentration of 1.0%, whereas the (112) peak became more prominent as the concentration of Eu 3+ ion increased above 1.0%, as was particularly evident for x = 5.0% and pure SSO. Apparently, as the Eu 3+ doping concentration increased, the SSO matrix became increasingly α′-rich SSO:xEu 3+ phase from β-rich SSO:xEu 3+28 . Figure 1c also shows a characteristic (222) diffraction peak at ~ 28.2° that arises from the Eu 2 O 3 phase. This peak is identified as the cubic phase of Eu 2 O 3 and confirmed by comparison with the JCPDS pattern (card No. . Apparently, XRD results show that doping with Eu 3+ ions does not significantly change the structure of the lattice. The peaks that are marked with '♦' in the XRD spectra indicate the presence of a tiny amount of SCO in the SSO:xEu 3+ phosphors. The variation in the weight ratio of α′-SSO/β-SSO phases with the concentration of Eu 3+ ions in the SSO:xEu 3+ phosphors and SSO was quantitatively analyzed. The analysis was based on the ratio of α′-SSO to β-SSO phase peaks and was performed using general structure analysis system (GSAS) software 29 . To qualitatively study the effect of Eu 3+ concentration on the crystal structure and the weight ratio of α′-SSO/β-SSO phases in the SSO:xEu 3+ phosphors, XRD spectra of the SSO:xEu 3+ phosphors were simulated by GSAS software, and the simulation of the x = 5.0% sample is presented in the bottom panel of Fig. 1c, along with the experimental results. Table 1 presents the weight percentages and ratios of α′-and β-SSO phases in the SSO:xEu 3+ phosphor and compares them with those reported elsewhere 19 . Table 1 indicates that the α′-SSO/β-SSO phase ratio was 0.70 at a Eu 3+ ion concentration of x = 1.0% and increased to 8.65 and 16.93 at Eu 3+ ion concentrations of x = 2.0 and 5.0%, respectively. However, the SSO host matrix had the highest (26.86) α′-SSO/β-SSO phase ratio. These results further confirm the formation of α′-rich SSO:xEu 3+ phosphors at higher Eu 3+ -doping concentrations in the SSO:xEu 3+ phosphors, which are associated with a more   Table 2 shows the lattice constants (a, b and c) of the α′-SSO and β-SSO phases, the crystal angle θ in the β-SSO phase and the volumes of both the α′-and β-SSO phases in the SSO:xEu 3+ phosphors. The parameters in Table 2 Fig. 1c, and this result can be explained by the fact that the Eu 3+ doping concentration in the SSO host matrix is too low to be detected by XRD. However, high-resolution transmission electron microscopy (HRTEM) clearly shows the cubic Eu 2 O 3 -phase in the SSO: xEu 3+ matrix, as presented in the lower inset of Fig. 4a, and will be discussed later. Figure 2a displays the PL spectra of SSO:xEu 3+ phosphors recorded at room temperature upon excitation at a wavelength (λ) of 250 nm (~ 4.96 eV). The PL spectra include rather broad features that are centered at ~ 590, 613, 652 and 703 nm, consistent with previously reported results 28 . The PL spectral features reveal emissions from Eu 3+ activators in the host matrix; the emission features are related to the intra-4f-4f 5 D 0 → 7 F J (J = 1, 2, 3 and 4) electronic transitions. The observed 5 D 0 → 7 F 1 ( 7 F 3 ) transitions at ~ 590 (652) nm and 5 D 0 → 7 F 2 ( 7 F 4 ) transitions at ~ 613 (703) nm are special; the first is a symmetry-sensitive transition and is known as the magnetic dipole transition (MDT) with the selection rule (ΔJ = 0, ± 1), and the second is a hypersensitive electric dipole transition (EDT) with ΔJ ≥ 2 18 . Both are sensitive to the local environment and depend on the symmetry of the crystal field of the Eu 3+ activators in the host matrix 3,9 . Typically, the most important parameter for understanding symmetry is the asymmetric ratio or asymmetric factor (I rat = I EDT /I MDT ), which is defined as the ratio of the integral intensity of EDT ( 5 D 0 → 7 F 2,4 ) to that of MDT ( 5 D 0 → 7 F 1,3 ). At a crystal site with inversion symmetry, MDT is usually the most intense emission feature, whereas at a site without inversion symmetry, EDT is the strongest emission feature because transitions with ΔJ = ± 2 are hypersensitive to small deviations from inversion symmetry. 18 The relative intensity I EDT /I MDT strongly depends on the local symmetry around the Eu 3+ activators and thus provides information about the degree of distortion from inversion symmetry of the local environment around the Eu 3+ activators in the host matrix. A lower symmetry around Eu 3+ activators results in a higher asymmetric ratio or asymmetric factor. In this work, I rat [I( 5 D 0 → 7 F 2 )/I( 5 D 0 → 7 F 1 )], which is the ratio of the integrated intensities in the regions 603 to 634 and 577 to 601 nm, changed from 1.29 (x = 1.0%) to 1.57 (x = 2.0%) to 1.61 (x = 5.0%) as the asymmetric field around the Eu 3+ activators increased, reflecting a small change in the lattice distortion, possibly caused by either substitution at the Sr sites or the formation of interstitial Eu-O associations in the SSO:xEu 3+ phosphors. Yin et al. 31 observed the PL properties of a Eu 2 O 3 /Tb 2 O 3 film that was deposited on a p + -type Si substrate with a small lattice mismatch using ITO and Ag as the negative and positive electrodes, respectively. These excitations ( 5 D 0 → 7 F 1 at 583 nm and 5 D 0 → 7 F 2 at 611 nm) yielded relatively broad-band features and improved the I rat , which can be associated with the CT band, owing to the injection of electrons and holes into the unoccupied Eu 5d and occupied O 2p states of the Eu 2 O 3 layer from ITO and Tb 2 O 3 films, respectively, thus enhancing the 4f-4f electronic transitions. The transition from 5 D 0 → 7 F 0 (J = 0) is forbidden by both ED and MD, and only weak transitions are observed as a result of the crystal field-induced J-mixing effect of the Eu 3+ activators. However, this transition ( 5 D 0 → 7 F 0 ) occurs because of the unique structural features of the orthorhombic polymorph of SSO. 1,9 In the atomic structure of SSO, as shown in Fig. 1a,b, the Si atoms in the SiO 4 tetrahedra form a parallel chain. The Sr 1 sites form more symmetric linear chains of Si-O-Sr 1 -O-Sr 2 , but Table 1. Weight percentages of β-SSO and α′-SSO phases and α′-SSO/β-SSO phase ratio.  Table 2. Lattice constants (a, b and c) obtained by fitting with the β-SSO and α′-SSO phases, crystal angle θ in the β-SSO phase and volumes for both the β-and α′-SSO:xEu 3+ phases.

Sample
Fitting with β-SSO Fitting with α′-SSO www.nature.com/scientificreports/ the Sr 2 sites form less symmetric zig-zag chains of Sr 1 -O-Sr 2 -O-Sr 1 along the b-axis. 1,3 In the present case, the 5 D 0 → 7 F 0 transition (~ 575 nm) is completely absent, suggesting that the Eu 3+ activators have rather asymmetric environments in the SSO:xEu 3+ phosphors. Notably, the total PL intensity in Fig. 2a is proportional to the ratio of α′-SSO/β-SSO phases, as presented in Table 1. Based on the above discussion, the enhanced PL emission is associated with not only the α′-SSO/β-SSO phase ratio (or the degree of symmetry of Eu 3+ activators) but also the effect on the CT band of the Eu-O associations in the SSO:xEu 3+ phosphors described here. The PL of selected rare-earth ions that are doped into host matrices is well known to be able to be used as a spectral probe of crystal structure, which is closely related to the crystal field of the activators and is determined by the valence-band maximum (VBM) and conduction-band minimum (CBM) of the matrix, as well as the partial electronic density of states (DOSs) of the luminescent centers and intrinsic/extrinsic defects, which function as trap centers in the matrix. 9 The Eu 3+ ion has six electrons in the 4f shell, which is not an entirely filled f orbital. Figure 2b presents a typical energy diagram of Eu 4f-4f electronic transitions, and the Eu 3+ activators usually comprise emission features in the red spectral region, where the emission transition is caused by the crystal field splitting of the 7 F J levels. In addition to these emission features, emissions from higher 5 D levels, viz. 5 D 1 (green), 5 D 2 (blue) and even 5 D 3 , are commonly observed. However, their presence or absence depends principally on the host lattice. 9 Nevertheless, in this study of the origin of the Eu 3+ luminescence of SSO:xEu 3+ phosphors, the emission features mostly correspond to transitions from the excited 5 D 0 level to the 7 F J levels in the Eu 4f 6 configuration and are affected strongly by the CT band and/or the local electronic/atomic structures of the Eu 3+ ions in the formation of the Eu 3+ -O 2− associations and the host lattice. Principally, the PL is a three-step process: (1) absorption of a UV photon, (2) transfer of the excitation energy (or electrons) to the luminescent centers, and (3) radiative emission from the luminescent centers. To illustrate the effects of the luminescent centers of Eu 3+ activators that are involved in the correlation between the CT band and the Eu 3+ -O 2− associations and of the bandgap of the host lattice on the absorption/excitation and emission processes, Fig. 2b schematically depicts the excitation of electrons from the valence band (VB) to the conduction band (CB) of the host matrix upon excitation by UV light (λ = 250 nm, ~ 4.96 eV); the excitation by UV light presumably exceeds the energy gap (the energy separation between the VBM and the CBM), E g , of the SSO:xEu 3+ phosphors because the bandgap of the α′-phase/β-phase SSO is close to 4.49/4.11 eV 19 , and that of the mixed α′-and β-phases of SSO is close to 4.12 eV 32 , yielding free electrons and holes in the CB and VB, respectively. This process also involves the relative energy levels of Eu 3+ -O 2− associations, since the valence electrons of the activators/Eu 3+ -O 2− associations can also be excited directly by UV or by energy that is transferred from the host lattice, so the overall excited free electrons above/near the CBM thermally cross and relax non-radiatively (as indicated by the dashed line), being transferred to the lower excited state of 5 D 0 . This process is followed by de-excitation to the ground states via radiative electronic transitions, corresponding to the PL from the 5 D 0 → 7 F J transitions (J = 1, 2, 3 and 4), which are yellow (~ 590 nm), orange (~ 613 nm) and red (~ 652 nm and 703 nm), respectively, as observed in Fig. 2a.
Understandably, the position of the PL spectral features does not change significantly with the Eu 3+ doping concentration, but their overall intensity increases. The enhancement of PL is related to the transfer of electrons from the CT band, which is related to the Eu 3+ -O 2− associations. Hypothetically, the Eu 3+ -O 2− associations provide extra electrons that are transferred from Eu 3+ -O 2− states with free electrons as a result of excitation from VB, resulting in nonradiative transfer to lower 5 D 0 levels.  Fig. 3a. By the dipole-transition selection law, the main absorption near-edge feature of SSO:xEu 3+ phosphors at the Sr K-edge represents the transition of electrons from Sr 1 s to 5p unoccupied states and is slightly more (x = 1.0 and 5.0%) or less (x = 2.0%) intense than that of the reference SSO, suggesting no significant change in the electronic unoccupied Sr 5p states of Eu-doped SSO:xEu 3+ phosphors relative to that of SSO. Figure 3b shows the magnitudes of the Fourier transformed (FT) Sr K-edge EXAFS of the SSO:xEu 3+ phosphors and SSO. The upper inset shows the corresponding k 3 -weighted k 3 χ oscillating spectra. The selected k-range for the FT spectra was ~ 2.9-10.7 Å −1 . To provide more comprehensive insight into the local atomic structures at the Sr 1 and Sr 2 sites as the Eu concentration in the SSO:xEu 3+ phosphors increases, the nearest-neighbor (NN) coordination number (N 1 /N 2 ), Sr 1 -O/Sr 2 -O bond length (R 1 / R 2 ), and mean square fluctuation of the Debye-Waller factor (DWF, σ 1 2 /σ 2 2 ) at the Sr 1 /Sr 2 sites were obtained by fitting Sr K-edge EXAFS spectra. All spectra were analyzed by standard procedures using the Athena program package 33,34 to extract quantitative local information about the local atomic structures at the Sr 1 /Sr 2 sites in the SSO:xEu 3+ phosphors and SSO.    www.nature.com/scientificreports/ the local atomic structures (N 1 /N 2 , R 1 /R 2 and σ 1 2 /σ 2 2 ) at the Sr 1 /Sr 2 sites in the matrix with various Eu 3+ doping concentrations remain nearly unchanged, revealing that the SSO:xEu 3+ phosphors easily tolerate Eu incorporation without significant distortion of the host lattice. Clearly, the coordination number of N 1 /N 2 , the NN Sr 1 -O/ Sr 2 -O bond length and the corresponding DWF σ 1 2 /σ 2 2 in the SSO:xEu 3+ phosphors and SSO are also very similar, although the former have slightly longer bond lengths and a smaller DWF than the latter [the slight shift in the magnified scale of the over plotting first main FT features is easily observed in the lower inset of Fig. 3b]. With reference to Fig. 3a, b, the similarity of the general line shapes in the XANES/the first main FT spectral feature at the Sr K-edge of the SSO:xEu 3+ phosphors relative to those of the SSO host reveals that Eu 3+ -doping in the SSO:xEu 3+ phosphors does not significantly distort the local electronic and atomic structures at the Sr sites in the matrix. Since Eu 3+ does not replace the Sr site and exists in the host lattice, the host lattice provides space to accommodate Eu 3+ ions in the SSO:xEu 3+ matrix. Importantly, these results suggest that the Eu 3+ activators may not substitute at both/either the Sr 1 and/or the Sr 2 sites, so the CT band of O 2− → Eu 3+ does not initially undergo electron transfer by polyhedral SrO 10 /SrO 9 (Sr 1 -O/Sr 2 -O → Eu) in the SSO:xEu 3+ phosphors 1,3,16 . Furthermore, as shown in the lower inset of Fig. 3b, the intensities of the first main feature (NN Sr-O bond length) in the FT spectra of the SSO:xEu 3+ phosphors overall exceed that of the SSO, primarily because the former contains fewer defects/oxygen vacancies, resulting in less structural disorder and/or DWF than in the SSO host matrix. 35 This phenomenon follows from the fact that the NN Sr-O shell around Sr sites in the SSO: xEu 3+ phosphors has fewer defects/oxygen vacancies, so the DWF is smaller than that of SSO ( Table 3). As stated above, the defects or oxygen vacancies that act as trap centers are primarily attributed to the presence of the luminescent activators [13][14][15] , but as shown in the lower inset in Fig. 3b, the maximum intensity of the feature associated with the NN Sr-O bond length in the FT spectra of the SSO:xEu 3+ phosphors is slightly greater than that of SSO. Apparently, the Sr K-edge EXAFS studies do not support the claim that defects or oxygen vacancies are formed by doping with luminescent Eu activators, which critically determine the PL property in SSO:xEu 3+ phosphors [13][14][15] . Figure 4a displays XANES spectra at the Eu L 3 -edge of the SSO:xEu 3+ phosphors (x = 1.0, 2.0 and 5.0%) and Eu 2 O 3 , obtained in total fluorescence yield mode. The upper inset shows the magnification of the near-edge feature from ~ 6,960 to 7,000 eV following subtraction of the arctan background from the near-edge feature, as indicated by the dashed line in Fig. 4a. Clearly, the XANES spectra of SSO:xEu 3+ phosphors yield a sharp line-shaped feature at the Eu L 3 -edge, which is almost identical to but much stronger than that of pure Eu 2 O 3 , demonstrating the formation of Eu 2 O 3 -like structures rather than Eu substitution at Sr sites in the SSO:xEu 3+ phosphors reported elsewhere 16 Fig. 2a,b. To conclusively elucidate the formation by Eu 3+ ions of interstitial Eu 2 O 3 -like structures, rather than the substitution of these ions at Sr sites in the SSO:xEu 3+ phosphors, Fig. 4b shows the FT spectra of k 3 χ data of the SSO:xEu 3+ phosphors (x = 1.0, 2.0 and 5.0%) and Eu 2 O 3 from k = 2.7 to 9.5 Å −1 at the Eu L 3 -edge. The inset presents the Eu L 3 -edge EXAFS k 3 χ data for the SSO:xEu 3+ phosphors and Eu 2 O 3 . The feature in the FT spectra at ~ 2.4 Å corresponds to the NN Eu-O bond length, and the second main feature at ~ 3.2-3.3 Å corresponds to the Eu-Eu bond length. The first (second) main FT feature of SSO:xEu 3+ reflects a slightly longer (shorter) Eu-O (Eu-Eu) bond length in the SSO:xEu 3+ phosphors than in Eu 2 O 3 . The general FT spectra at the Eu L 3edge of SSO:xEu 3+ phosphors and Eu 2 O 3 exhibit similar line shapes of FT features in Fig. 4b, confirming that the local atomic structures of Eu 3+ ions in the SSO:xEu 3+ phosphors are comparable to that of the cubic phase of Eu 2 O 3 . To quantitatively elucidate the local atomic structures around Eu 3+ ions in SSO:xEu 3+ phosphors and Eu 2 O 3 , the fitting results concerning the NN coordination number (N), Eu-O bond length (R) and corresponding DWF (σ 2 ) were also obtained using the Athena program package 33,34 and are presented in Table 4. Although the amount of Eu 2 O 3 -like structures in the host matrix is very small and not detectable by long-range sensitive XRD measurements, as presented in Fig. 1c, they were indeed incorporated as interstitial Eu 2 O 3 -like structures in the host matrix, as revealed by the short-range sensitive XANES, EXAFS and HRTEM techniques, as shown in Fig. 4a,b, respectively. Notably, the general XANES line-shapes/FT spectral features at the Eu L 3 -edge in Fig. 4a,b and the results concerning the local atomic structures (N, R and σ 2 ) in Table 4 clearly differ from the Sr K-edge XANES line-shapes/FT spectral features in Fig. 3a,b and Table 3, conclusively revealing that the Eu 3+ ions formed Eu 2 O 3 -like structures in the host matrix. No evidence supports substitution of the Eu 3+ ions for the Sr 2+ ions at the Sr 1 and/or Sr 2 sites in the SSO:xEu 3+ phosphors [16][17][18][19]36,37 . Ultimately, the covalent bond energy of Eu-O (2.95 eV) exceeds that of Sr-O (2.66 eV), 18 implying that Eu forms much stronger bonds with neighboring O atoms than does Sr, so Eu 2 O 3 -like interstitial structures are formed preferentially in the SSO:xEu 3+ matrix.
As discussed above, when the phosphor is excited by a suitable wavelength/energy, it can exhibit high PL intensity with desirable emission chromaticity, which is strongly related to the electronic structures between the luminescent activators and the host matrix. To reveal the role of the Eu 3+ -activator in SSO:xEu 3+ phosphors, Fig. 5 shows the normalized O K-edge XANES and K α XES spectra there of (x = 1.0, 2.0 and 5.0%) and references SSO and Eu 2 O 3 . These spectra show the correlations among PL, DOSs near the VBM/CBM and the bandgap of the SSO:xEu 3+ phosphors, as well as the differences between the electronic structures and band gap of the SSO:xEu 3+ phosphors from those of the SSO host lattice. As displayed in Fig. 5, based on the first-principles calculations by Pan et al. 19 , the DOSs near/at the CBM (features A 1 and A 2 ) of SSO mostly involve O 2p and Si 3p states, while the DOSs near/at the VBM (features B 1 and B 2 ) are dominated by O 2p and Si 3p states; the Sr 4p states at ~ 15 eV are far below the VBM of SSO. The α′ and β phases of SSO generally exhibit similar total and partial DOSs in the lattice. Theoretical calculations based on first principles indicate that Eu 2 O 3 can exist as three stable structures (cubic, monoclinic and hexagonal phases) under ambient pressure. The novel correlation property of Eu 2 O 3 is strongly related to f-f interactions and highly localized electrons in the 4f states of Eu ions in the compound, generating several metastable electronic configurations that depend on the partial occupation of 4f states. 40 Systematic studies of electronic band structures using the GW with the Hubbard U corrected local-density Table 4. Parameters obtained by best-fitting the Eu L 3 -edge EXAFS data in R-space mode from ~ 1.7 to 3.0 Å: NN coordination number (N), Eu-O bond length (R) and corresponding DWF (σ 2 ) at the Eu ions in the SSO:xEu 3+ phosphors and Eu 2 O 3 .   www.nature.com/scientificreports/ To reveal the enhancement of DOSs within the band gap of SSO:xEu 3+ phosphors that is caused by doping of the SSO host matrix with Eu, the lower panel in Fig. 5 displays the difference between unoccupied/occupied states of the CB/VB of SSO:xEu 3+ phosphors and those of the SSO. As revealed by the XRD data in Fig. 1c, the minor/impurity SCO phase may also contribute to the O K-edge XANES and K α XES spectra of the SSO:xEu 3+ phosphors and the SSO, but its intensity can be treated as an equal quantity, canceling out in the different XANES and XES spectra, as shown in the lower panel. A large difference in the O K-edge XANES and K α XES spectra around the A 1 (B 1 ) and A 2 (B 2 ) features was clearly observed between the spectra near/at the CBM (VBM) of the SSO:xEu 3+ phosphors and the SSO. The enhanced features near/at the CBM (VBM) in the right (left) lower panel arise from the increased density of the unoccupied O 2p-Eu 4f/5d states (occupied O 2p-Eu 4f states). In particular, the large enhancement of the A 2 feature (O 2p-Eu 5d states) reveals an increase in the DOSs in the bandgap of SSO:xEu 3+ phosphors upon doping with Eu. The electron excitation from the VB to the CB by excitation by UV light (λ = 250 nm, ~ 4.96 eV) exceeds the energy gap E g (~ 3.7 eV) of SSO:xEu 3+ phosphors, yielding extra free electrons and holes at the CB and VB, respectively, owing to incorporation of Eu 2 O 3 -like structures in the host matrix. The increase in the intensities of the features in the O K-edge XANES and K α XES spectra demonstrate the contribution of O 2p-Eu 4f/5d hybridized states (mostly from unoccupied Eu 5d states at the CB and occupied O 2p-Eu 4f states at the VB, respectively) in the bandgap of the host matrix. As shown in Fig. 2b, the O 2p-Eu 4f/5d (major of Eu 5d states, Eu 3+ -O 2− associations) above/at CBM likely act as donor levels for the nonradiative transfer of electrons to 5 D 0 levels, generating unoccupied states, as consistently observed at the O K-edge and Eu L 3 -edge XANES (Figs. 4a, 5), and yielding intra 4f-4f electronic transitions followed by excited 5 D 0 → 7 F J (J = 1, 2, 3 and 4) transitions, which enhance the PL of SSO:xEu 3+ phosphors upon Eu 3+ -doping. Again, based on the above results, the CT band plays a critical role in the radiative emission of Eu 3+ -activators because Eu 2 O 3 -like structures formed interstitially in the SSO:xEu 3+ matrix rather than at polyhedral SrO 10 /SrO 9 sites in the SSO:xEu 3+ matrix 1,3,18 . Additionally, due to the slightly varied crystal field splitting of Eu activators in the matrix, the PL property of SSO:xEu 3+ phosphors is caused by the Eu 2 O 3 -like structures, yielding all of the PL emission features (~ 590, 613, 652 and 703 nm) at slightly higher wavelengths than those of Eu 2 O 3 thin films (~ 583, 611, 648 and 694 nm) 31 and at lower wavelengths than those of Sr 1.9 SiO 4 :0.1Eu powder (~ 592, 620, 654 and 704 nm) 45 . Since Eu 2 O 3 -like structures act as interstitial sites in the SSO:xEu 3+ phosphors, SSO is a host material with a large bandgap and stable lattice, making it a suitable host for accommodating Eu 3+ -activator phosphors.

conclusion
In conclusion, PL measurements show that the wavelengths of the emission spectra do not significantly vary with Eu 3+ doping concentration in SSO:xEu 3+ phosphors. However, the PL intensity increases with increasing Eu 3+ doping concentration. The PL intensity is associated with the α′-SSO/β-SSO phase weight ratio in the SSO:xEu 3+ phosphors. At higher Eu 3+ contents, the luminescence is stronger because more Eu 2 O 3 -like structures are present in the host matrix, favoring the nonradiative transfer of electrons from Eu 5d states above/at the CBM to the 5 D 0 level, which observably increases the absorption intensity at Eu L 3 -edge XANES spectra of SSO:xEu 3+ phosphors. Furthermore. The results of the O K-edge XANES and K α XES spectra clearly demonstrate that the unoccupied O 2p-Eu 4f/5d and occupied O 2p-Eu 4f states within the bandgap (or near/at the CBM and the VBM) of the matrix promote the nonradiative transfer of electrons from the O 2p-Eu 4f/5d hybridized states (mostly Eu 5d states above/at the CBM) to the 5 D 0 level, facilitating electronic transitions from the excited 5 D 0 to 7 F J (J = 1, 2, 3 and 4) levels. This CT process is primarily responsible for the enhancement of the PL of SSO:xEu 3+ phosphors with increased Eu 3+ -doping in the matrix. Significantly, this study demonstrates that the Eu activators (or Eu 3+ -O 2− associations) interact weakly with the host matrix, which provides sufficient room to accommodate the Eu 3+ dopants as luminescence centers without substitution at the Sr sites in SSO: xEu 3+ phosphors.

Experimental methods and characterizations
The SSO:xEu 3+ phosphors in the α′ phase and the β phase with various Eu 3+ concentrations (x = 1.0, 2.0 and 5.0%) were synthesized using a sol-gel route at ~ 1,100-1,200 °C with SCO, SiO 2 and Eu 2 O 3 as starting materials. In this sol-gel process, stoichiometric ratios of SCO, SiO 2 and Eu 2 O 3 were taken in the solvent into an aqueous solution (called sol). The solutions were then hydrolyzed and condensed. The slow aggregation among colloidal particles formed a three-dimensional network structure with a poorly liquid gelatin (called gel). The resulting gel was then dried and sintered at high temperature (~ 1,100-1,200 °C) to form the final powder product of SSO:xEu 3+ . The formed crystal structures were mostly SSO with a tiny amount of SCO phases. The atomic structures of the (orthorhombic) α′ and (monoclinic) β phases of the SSO matrix in Fig. 1a, b were drawn using VESTA software 46 after considering the SSO raw crystallographic files (https ://www.mater ialsp rojec t.org/). XRD patterns were obtained using Cu K α (λ = 1.5418 Å) radiation at 40 kV. Energy-dispersive X-ray spectroscopy measurements for elemental mapping analysis showed the presence of Eu, but it is very difficult to obtain an accurate quantitative number for Eu concentrations due to the very low Eu content in each SSO:xEu 3+ phosphor. Hence, the "Eu-content %" mentioned in the text is the stoichiometric % taken during the synthesis process. The PL spectra were recorded at room temperature within the wavelength range of ~ 200-800 nm on a Hitachi F-4500 fluorescence spectrophotometer equipped with a 150 W Xe lamp, with emission upon excitation at a wavelength of 250 nm (~ 4.96 eV). Eu L 3 -, Sr K-edge XANES/EXAFS and O K α XES spectra were measured at the Taiwan Photon Source (TPS) 44A and 45A-undular beamlines 47 ; O K-edge XANES was performed at the Taiwan Light Source (TLS) 20A beamline of the National Synchrotron Radiation Research Center, Taiwan. The O K α XES spectra were obtained at an excitation energy of 550 eV with a resolution of better than 0.2 eV; the intensity of the XES features in the energy range between ~ 517 and 515 eV was normalized to unity. The O K-edge XANES spectra were obtained in fluorescence yield mode with a resolution greater than 0.1 eV and were normalized to the incident beam intensity following pre-edge background subtraction, with the area under the spectra in the Scientific RepoRtS | (2020) 10:12725 | https://doi.org/10.1038/s41598-020-69428-7 www.nature.com/scientificreports/ energy range between ~ 550 and 555 eV fixed. The energy resolutions of Eu L 3 -and Sr K-edge XANES were set to ~ 0.5 and 1 eV, respectively.