Optical Properties of Ce-Doped Li4SrCa(SiO4)2: A Combined

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Optical Properties of Ce-Doped Li4SrCa(SiO4)2: A Combined Experimental and Theoretical Study Rui Shi,† Xiaoxiao Huang,‡ Tiantian Liu,† Litian Lin,† Chunmeng Liu,† Yan Huang,§ Lirong Zheng,§ Lixin Ning,*,‡ and Hongbin Liang*,† †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China ‡ Anhui Province Key Laboratory of Optoelectric Materials Science and Technology, Department of Physics, Anhui Normal University, Wuhu, Anhui 241000, China § Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China S Supporting Information *

ABSTRACT: Investigation of optical properties of Ce3+activated phosphors is not only of practical importance for various applications but also of fundamental importance for providing a basis to understand relevant properties of other lanthanide ions in the same host. We report herein a combined experimental and theoretical study of optical properties of Ce3+ in Li4SrCa(SiO4)2. Photoluminescence properties of the material prepared by a solid-state reaction method are investigated with excitation energies in the vacuum-ultraviolet (VUV) to ultraviolet (UV) range at low temperatures. The band maxima in the excitation spectra are assigned with respect to 4f → 5d transitions of Ce3+ at the Sr and Ca sites, from comparison between experimental and ab initio predicted transition energies. As a result of the two-site occupation, the material displays luminescence at 300−500 nm with a high thermal quenching temperature (>500 K), consistent with the calculated large gaps (∼1.40 eV) between the emitting 5d levels and the bottom of the host conduction band. On the basis of experimental and calculated results for Ce3+ in Li4SrCa(SiO4)2, the energy-level diagram for the 4f ground states and the lowest 5d states of all trivalent and divalent lanthanide ions at the Sr and Ca sites of the same host is constructed and discussed in association with experimental findings.

1. INTRODUCTION Lanthanide-activated silicate phosphors have been widely investigated for applications in lighting and displays, and scintillators, due to favorable characteristics of host materials such as good chemical stability, low cost, and environmental friendliness.1,2 Recent years have seen much interest in alkaline earth orthosilicate phosphors activated by Ce3+,3−5 where the relevant 5d → 4f emissive transitions are spin- and parityallowed with a radiative lifetime of several tens of nanoseconds. Due to the strong interaction of the excited 5d electron with the local environment, the transition energies depend mainly on the chemical composition and geometric structure of the host, which enables tuning of luminescence properties for target applications. Among the orthosilicates, Li4SrCa(SiO4)2 (abbreviated as LSCSO hereafter) has an orthorhombic structure with the space group Pbcm (No. 57).6 The Sr and Ca sites are coordinated by 10 and 6 oxygen atoms with Cs and Ci symmetries, respectively. Luminescence properties of the material singly doped with Eu2+, Tb3+, or Ce3+ have been previously reported.7−10 In the Ce-doped system, the near-UV © XXXX American Chemical Society

emission at 345 nm and the blue emission at 420 nm were ascribed to Ce3+ located at the Sr and Ca sites, respectively, based on an empirical relationship between the lowest 4f → 5d transition energy and the local coordination structure.10 Since the measurements were performed at room temperature (RT), the fine spectral features that were required for a quantitative analysis of the structure−property relationship were missing in the UV excitation spectra, as a result of the temperatureinduced band broadening. Information on the split 5d energy levels of Ce3+ at a given site is also interesting from a fundamental viewpoint. According to the theoretical model developed by Dorenbos,11 the shift of the Ce3+ 5d centroid energy derived therefrom with respect to the free ion value is connected with the energy difference between the 4f ground levels of Eu2+ and Eu3+ at the same site. This energy difference is a crucial parameter used to construct the energy-level diagram for the 4f ground states and the lowest Received: October 5, 2017

A

DOI: 10.1021/acs.inorgchem.7b02561 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) Experimental (×) and calculated (red solid line) XRD patterns and the difference (green solid line) for the Rietveld fit of LSCSO by using the TOPAS program, with the short vertical black lines representing Bragg reflection positions of the calculated pattern. (b) The concentration-dependent lattice parameters of LSCSO:xCe. (c) The occupancy ratios of Ce3+ at Sr/Ca sites with the increase of Ce contents. (d) The temperature-dependent lattice parameters of LSCSO.

5d states of Ln3+ and Ln2+ ions relative to the host valence and conduction bands. With the availability of the energy-level diagram, we may interpret or even predict optical properties of the ions in a given host. In the present work, we perform a comprehensive study on optical properties of Ce-doped LSCSO through a combined experimental and theoretical approach. The purpose is twofold: (i) to gain a better understanding of the structure−property relationship which is important for further performance improvement of the material, and (ii) to obtain background information which is useful to evaluate optical properties of other Ln3+ and Ln2+ ions doped in the same host. With the use of synchrotron radiation at low temperatures, the fine structures of the excitation spectra are observed, extending from UV to VUV region. The excitation band maxima are assigned by comparison with theoretical Ce3+ 4f → 5d transition energies obtained with wave-function-based ab initio methods at the spin−orbit level. Moreover, electronic properties are calculated to understand the thermal stability of Ce3+ luminescence. On the basis of experimental and calculated results, the energy-level diagram for the 4f ground states and the lowest 5d states of Ln3+ and Ln2+ ions in LSCSO is constructed and discussed in association with experimental observations.

down to RT and ground into powder. The phase purity of samples was examined by X-ray diffraction (XRD) using Cu Kα radiation (λ = 0.15405 nm) on a BRUKER D8 ADVANCE powder diffractometer. High quality XRD data for Rietveld refinement were collected over a scanning angle range from 5° to 110°. The structure refinement was performed using the TOPAS-Academic program.12 The medium-low temperature XRD data were recorded using an Anton Paar TTK 450 temperature controlling unit with liquid nitrogen flow cooling. All UV excitation/emission spectra and the luminescence decay curves were measured on an Edinburgh FLS 920 combined fluorescence steady state and lifetime spectrometer using a 450 W Xe900 lamp and a 150 W F900 lamp with a pulse width of 1 ns as the excitation source, respectively. A temperature controlling system made by Cryogenic Transportation, LLC (CTI), was employed to measure the luminescence and decay responses of samples under different temperatures. The internal quantum efficiency (IQE) of samples was measured on an Absolute PL Quantum Yield Measurement System of HAMAMATSU PHOTONICS. The vacuum ultraviolet (VUV) excitation and corresponding emission spectra were measured at the VUV spectroscopy experimental station on beamline 4B8 of Beijing Synchrotron Radiation Facility (BSRF). The signal was detected by a Hamamatsu H8259-01 photon counting unit and corrected by the excitation intensity of sodium salicylate (o-C6H4OHCOONa) measured simultaneously in the same condition. Ce-L3 edge X-ray absorption near-edge structure (XANES) analysis was carried out in the fluorescence mode at RT on the beamline 1W2B of BSRF.13 2.2. Computational Details. The Ce-doped LSCSO crystal was modeled using a 2 × 1 × 1 supercell containing 128 atoms, in which a Sr2+ (or Ca2+) ion was substituted with a Ce3+, and a nearest-neighbor (NN) Ca2+ (or Sr2+) ion was replaced by a Li+ for charge compensation. This corresponds to the chemical formula Li4+xSr1−xCa1−xCex(SiO4)2 (x = 0.125), which is consistent with experiments. The two single substitutions have opposite effective charges and thus are expected to be close to each other. In consideration of the size of the supercell, the nearest Ce3+−Ce3+ distance in the periodic supercells is around 9.9 Å, which is large enough to neglect their mutual influence in the study of localized electronic states of individual Ce3+ ions. The atomic coordinates and lattice parameters of the supercells

2. METHODOLOGY 2.1. Experimental Details. A series of powder samples with Ce3+ singly doped in LSCSO was prepared by a high temperature solid-state reaction route using raw materials Li2CO3 (A.R.), SrCO3 (A.R.), CaCO3 (A.R.), SiO2 (99.99%), and CeO2 (99.99%). According to the chemical formulas Li 4+x Sr 1−x Ca 1−x Ce x (SiO 4 ) 2 (abbreviated as LSCSO:xCe hereafter) (x = 0−0.3), the stoichiometric amount of raw materials was ground thoroughly in an agate mortar and then heated to 1198 K in 3 h and kept at this temperature for reaction about 4 h in a CO reducing atmosphere arising from the incomplete combustion of thermal carbon. The samples were gradually cooled B

DOI: 10.1021/acs.inorgchem.7b02561 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

trivalent charge state and the influence of Ce4+ on spectroscopic properties of Ce3+ can be neglected.24 The orthorhombic structure of LSCSO was also optimized by hybrid DFT calculations. As indicated in the footnotes of Table S1 of the SI, the calculated lattice parameters agree well with the experimental data, with the deviations less than 0.14%. For the LSCSO supercells doped with a NN CeSr−LiCa or CeCa−LiSr substitution, the DFT total energies for the optimized structures indicate that the former substitution is more stable (by 334 meV) than the latter, in agreement with the above experimental results. Figure 2 displays the optimized

were optimized by periodic density functional theory (DFT) calculations using a hybrid functional in the PBE0 scheme,14 as implemented in VASP.15,16 The fraction of Hartree−Fock (HF) exchange in the PBE0 functional was increased to 32% in order to match the calculated band gap with the experimental one (see later). The Li (1s22s1), Sr (4p65s2), Ca (3p64s2), Si (3s23p2), O (2s22p4), and Ce (5s25p64f15d16s2) were treated as valence electrons, and their interactions with the respective cores were described by the projected augmented wave (PAW) method.17 The geometry optimizations were performed until the total energies and the Hellmann−Feynman forces on the atoms were converged to 10−6 eV and 0.01 eV Å−1, respectively. Due to the large size of the supercells and the high computational cost of hybrid DFT with plane-wave basis sets, only one k-point Γ was used to sample the Brillouin zone, with the cutoff energy of the plane-wave basis set to 530 eV. On the basis of the DFT-optimized supercell geometries, the cerium-centered (CeSrO10)17− and (CeCaO6)13− clusters were constructed, with their immediate surroundings within the spheres of a radius 10.0 Å represented by several hundred of ab initio model potentials (AIMPs).18 The remaining crystalline environment was simulated by tens of thousands of point charges at lattice sites, which were generated by the method of Lepetit.19 Wave-function-based CASSCF/CASPT2 calculations with the spin−orbit effect were carried out to obtain the 4f1 and 5d1 energy levels of Ce3+, using the program MOLCAS.20 In the CASSCF calculations, a [4f, 5d, 6s] complete active space was adopted. In the CASPT2 calculations, the bonding, static, and dynamic correlation effects of the Ce3+ 5s, 5p, 4f, and 5d electrons and the O2− 2s, 2p electrons were taken into account. The details of the basis sets can be found in refs 21 and 22. In addition, extra basis sets (10s7p)/[1s1p] and (4s)/[1s] were added to the Si and Li atoms closest to the embedded clusters in order to improve the cluster-host orthogonality.

Figure 2. DFT-optimized local coordination structures of Ce at the Sr and Ca sites. The relaxed (unrelaxed) Ce−O distances (Å) are indicated.

local structures of Ce3+ located at the Sr and Ca sites, with the values of selected relaxed (unrelaxed) bond lengths indicated. The distortion induced by the CeSr (CeCa) substitution reduces the site symmetry from Cs (Ci) to C1, and causes a decrease (an increase) in the average bond length by 0.089 Å (0.012 Å), consistent with relative ionic sizes of the host and dopant ions. 3.2. Photoluminescence Properties of Ce3+ in LSCSO. Figure 3a,b displays the emission spectra of Ce3+ in LSCSO:0.01Ce measured under 370 and 290 nm excitations at low temperatures, respectively. The two excitation wavelengths correspond to the wavelengths within the two lowest energy excitation bands (A′ and A) as shown in Figure 3d,c. Under 370 nm excitation, an emission band centered at 420 nm is observed (Figure 3a) and can be deconvoluted into two Gaussian bands with maxima at 2.70 and 3.00 eV (the inset). These two bands are attributed approximately to the transitions from the lowest 5d emitting levels to the 2F7/2 and 2F5/2 spin− orbit multiplets of a given type of Ce3+ centers. In the following, the Ce3+ centers responsible for this emission will be denoted as Ce1 centers. On the other hand, under 290 nm excitation, an emission band with a maximum at 350 nm is observed. In the inset of Figure 3b, this emission is deconvoluted into four Gaussian bands peaking at 2.69, 3.00, 3.39, and 3.68 eV. The two former Gaussian bands at lower energies are assigned to the emissions from Ce1 centers due to the similar peak energies to those indicated in the inset of Figure 3a. The latter two Gaussian bands at higher energies are attributed to another set of Ce3+, which will be denoted as Ce2 centers hereafter. These results also imply that, under 290 nm excitation, energy transfer (ET) occurs from Ce2 to Ce1 centers in the studied sample. The mechanism of ET is electric dipole-electric dipole (ED-ED) since the involved transitions are both ED-allowed. Figure 3c,d presents the low temperature excitation spectra of LSCSO:0.01Ce3+ by monitoring the emissions at 335 and

3. RESULTS AND DISCUSSION 3.1. Structure Characterization. Figure 1a shows the results of Rietveld refinement of the high-quality powder XRD data of LSCSO. The values of refinement parameters, as given in the legend of the figure, indicate that the sample is of single phase without any impurity or second phase. The compound crystallizes in an orthorhombic structure with the space group Pbcm. The refined lattice parameters and atomic positions are listed in Table S1 of the Supporting Information (SI). The Sr− O and Ca−O bond lengths in the coordination polyhedra are given in Table S2 of the SI, with the average values of 2.7602 and 2.3863 Å, respectively. The nearest-neighbor (NN) Sr−Sr, Sr−Ca, and Ca−Ca distances are 6.6466, 4.7394, and 4.9649 Å, respectively. Given the similarity in ionic radii of Sr2+ (1.36 Å, CN = 10), Ca2+ (1.0 Å, CN = 6), Ce3+ (1.25 Å, CN = 10; 1.01 Å, CN = 6),23 the dopant Ce3+ are expected to be located at Sr or Ca sites in Ce-doped LSCSO. Figure 1b shows that the refined lattice parameters (a, b, c) decrease gradually with increasing Ce content. Since the ionic radius of Ce3+ is smaller (larger) than that of Sr2+(Ca2+), this result indicates that the number of Ce3+ at Sr sites is larger than that at Ca sites. Figure 1c shows the refined ratio, R(CeSr/CeCa), of Ce occupancies at the Sr and Ca sites as a function of the doping concentration, which indicates again the preference of the CeSr substitution over the CeCa one in a wide concentration range. Figure 1d plots the dependence of the lattice parameters of LSCSO on temperature, showing that the lattice expands with increasing temperature in the range of 160−660 K with the orthorhombic structure preserved. Moreover, Ce-L3 edge XANES spectra of representative samples LSCSO:xCe (x = 0.03, 0.07, 0.13, 0.25) were measured and are shown in Figure S1 of the SI. The results show that, in the samples, most cerium exist in the C

DOI: 10.1021/acs.inorgchem.7b02561 Inorg. Chem. XXXX, XXX, XXX−XXX

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the smaller size of the Ca site (CN = 6) than that of the Sr site (CN = 10), we may tentatively assign Ce1 (Ce2) centers to Ce3+ located at Ca (Sr) sites, since a smaller coordination polyhedron generally causes a larger 5d crystal field splitting. To confirm the above assignment and further elucidate the relationship between spectroscopic properties and local structures, wave-function-based CASSCF/CASPT2 calculations at the spin−orbit level were performed on Ce-centered embedded clusters. These clusters were constructed from the DFT-optimized supercells containing the NN CeSr−LiSr and CeCa−LiCa substitutions. The calculated 4f1−7 and 5d1−5 energy levels split from 4f1 and 5d1 configurations of Ce3+ are listed in Table S3 of the SI. From the energy level data, the calculated 4f1 → 5di (i = 1−5) transition energies of Ce3+ at the Sr and Ca sites are schematically plotted in Figure 3e,f, respectively. The relative transition intensities within each cluster were evaluated using the wave functions and energies at the spin−orbit level.26 A comparison of Figure 3c−f reveals immediately that the Ce1 and Ce2 centers are formed by Ce3+ located at the Ca and Sr sites, respectively. Specifically, the bands A−E are ascribed to 4f1 → 5di (i = 1−5) transitions of CeSr, respectively. The bands A′−C′ are attributed to 4f1 → 5di (i = 1−3) transitions of CeCa, respectively, and the band D′ has contributions from 4f1 → 5d4,5 transitions of CeCa. Thus, the calculated transition energies provide a direct proof that the Ce1 (Ce2) centers with lower- (higher-) energy emission are due to Ce3+ located at the six (ten)-coordinated Ca (Sr) sites in LSCSO. From the energy level data in Table S3 of the SI, it is also interesting to see that, from CeSr to CeCa, the 5d centroid energy (ΔEced) is only decreased by 130 cm−1. This closeness in ΔEced is the consequence of two nearly canceling contributions. According to the Judd−Morrison model,27,28 ΔEced is reduced by the shortening of the average Ce−O distance, but at the same time, it is enhanced by a decrease in the coordination number (from 10 to 6). On the other hand, the 5d crystal field splitting (ΔEcfs) is increased by 19717 cm−1, as a result of the size reduction of the coordination polyhedron. This effect dominates the red shift of the lowest 4f1 → 5d1 transition from CeSr to CeCa. Figure 4a depicts the decay curves of Ce3+ emissions in LSCSO:0.01Ce at 3 K. The decay curves (a and b) of CeSr 335 nm and CeCa 455 nm emissions were measured under excitation at 290 and 370 nm, respectively, and can be well fitted by monoexponential functions with luminescence lifetimes of τa = 19.72 ns and τb = 40.26 ns. On the other hand, the decay curve (c) of CeCa 455 nm emission under 290 nm excitation shows an initial rise, indicating again the occurrence of ET from CeSr to CeCa since only CeSr centers can be excited by 290 nm radiation (see Figure 3c). Thus, the luminescence lifetime τb for CeCa is equal to the radiative lifetime τbr, while τa for CeSr has an additional contribution from the ET rate (kET), i.e., 1/τa = 1/τar + kET. To evaluate the value of kET, the following expression29 was used to analyze the decay curve c

Figure 3. (a, b) The emission spectra of Ce3+ in LSCSO:0.01Ce under 370 and 290 nm excitations at low temperatures and the insets give the Gaussian deconvolution results. (c, d) The low temperature excitation spectra of the sample by monitoring 335 and 455 nm emissions. (e, f) The calculated 4f1 → 5di (i = 1−5) transition energies of Ce3+ at the Sr and Ca sites.

455 nm, respectively. Six distinct excitation bands are observed in the VUV-UV excitation spectrum of the 335 nm emission, with maxima at around 286, 264, 248, 234, 215, and 167 nm (labeled as A−F in Figure 3c, respectively), as derived by fitting the spectrum with Gaussian bands; see Figure S2 of the SI. Since the 335 nm emission arises primarily from Ce2 centers, the five excitation bands A−E are ascribed to 4f → 5d transitions of Ce2. The band F at 167 nm is assigned to the host excitonic absorption, from which the host band gap is estimated to be 8.02 eV after taking into account the electron− hole binding energy of the exciton.25 When monitoring the emission at 455 nm, the UV-vis excitation spectrum shows three bands at 357 (band A′), 337 (band B′), and 322 nm (band C′), as derived by Gaussian fitting (Figure S2 of the SI). Meanwhile, the VUV-UV excitation spectrum exhibits a profile similar to that (Figure 3c) obtained when monitoring the 335 nm emission, except for a broad band (D′) emerging at 185 nm. Since the 455 nm emission originates mainly from Ce1 centers, the excitation bands A′−C′ in Figure 3d are attributed to 4f → 5d transitions of Ce1 centers, and the excitation bands A−E of Ce2 centers are observed as a result of ET from Ce2 to Ce1. Moreover, from the energy differences between the lowest-energy excitation band maxima and the higher-energy emission band maxima (derived from Gaussian fitting), the Stokes shift (ΔESS) of the 5d → 4f emission is determined to be 0.47 and 0.66 eV for the Ce1 and Ce2 centers, respectively. The preceding spectral analysis suggests that two distinct types of Ce centers exist in LSCSO:0.01Ce3+, in which the 5d emitting levels of Ce1 centers are lower in energy than those of Ce2 centers, and ET occurs from Ce2 to Ce1 when Ce2 is excited, e.g., by radiation at 290 nm. This is understandable in view of the presence of the Sr and Ca sites in the sample for which Ce3+ may substitute and the short NN Sr-Ca distance of 4.7394 Å which enables an efficient ET. Moreover, considering

I(t ) = Na exp( −t /τb) +

NbkET [exp( −t /τb) 1/τa − 1/τb

− exp( −t /τa)]

where Na and Nb are two parameters representing the initial population densities in the excited states of CeSr and CeCa, respectively. The fitting is shown in Figure 4a, and the derived value of kET is 14.09 (μs)−1. Thus, the radiative lifetime of CeSr 335 nm emission is 27.29 ns, which is smaller than that of CeSr D

DOI: 10.1021/acs.inorgchem.7b02561 Inorg. Chem. XXXX, XXX, XXX−XXX

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the ED-ED mechanism of the ET and the close separation of the Sr and Ca sites where the dopant Ce3+ are located. Figure 4b,c shows the luminescence decay curves of CeSr emissions at 335 nm (λex = 290 nm) and CeCa emissions at 455 nm (λex = 370 nm) in LSCSO:0.01Ce at temperatures from 77 to 500 K. For each group of emissions, all decay curves exhibit similar monoexponential behaviors with increasing temperature. The derived lifetimes are plotted in Figure 4d, showing that almost no luminescence quenching was observed at temperatures up to 500 K. Figure 5a gives representative quantum efficiency emission spectra of LSCSO:0.01Ce under 290 nm excitation at 298−473 K. The temperature dependence of the excitation light absorption efficiency (ELAE) at 290 nm is depicted in Figure 5b. Here, the ELAE value of the studied sample is determined by ELAE =

∫ λ·{E(λ) − R(λ)} dλ ∫ λ · E (λ ) d λ

where E(λ)/hν and R(λ)/hν are the photon number in the spectrum of excitation light, reflectance of the phosphor, respectively. With the increase of temperature, the value of absorption efficiency remains almost constant at 61.1−62%. This indicates that the sample possesses steady light absorption efficiency over a wide temperature range. Figure 5c shows the variation of IQE of the sample with temperature under excitation at 290 nm. The data are normalized with respect to the IQE value (39.6%) at RT. With increasing temperature, the relative IQE of Ce3+ emission decreases slightly and, at 473 K, has a value of 85% of that at RT. This result further demonstrates that the Ce3+ luminescence in the studied sample has a high thermal stability which is beneficial in high-power lighting applications. Figure 5d presents the concentration dependence of IQE under 290 nm excitation at RT. It shows that, when x = 0.07, the maximum of the IQE values is found to

Figure 4. (a−c) Decay curves of Ce3+ emissions in LSCSO:0.01Ce at 3 K and 77−500 K, and (d) the temperature dependence of decay times of CeSr and CeCa emissions.

455 nm emission, which is consistent with that the radiative lifetime is proportional to the emission wavelength.30 The ET efficiency, as defined by η = kET·τa, is predicted to be ∼28%, which is quite high when considering the low level (x = 0.01) of the doping concentration in the sample. This could be due to

Figure 5. (a) The representative quantum efficiency emission spectra of LSCSO:0.01Ce sample under 290 nm excitation at different temperatures. (b) The temperature-dependent ELAE of the sample (λex = 290 nm) and (c) the variation of relative IQE values of the sample. (d) The concentration dependence of the luminescence IQE under 290 nm excitation at RT. E

DOI: 10.1021/acs.inorgchem.7b02561 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. (a) The band structure of LSCSO calculated with the DFT-PBE method. (b) Total and partial DOSs for LSCSO obtained with the DFTPBE032 method. The Fermi level is set at zero energy. (c, d) Total and partial DOSs for LSCSO:Ce calculated with the DFT-PBE032 method. The occupied Ce3+ 4f states are indicated by the dashed lines, and their energy positions with respect to the host VBM are indicated.

be ∼50%, which should be improved through further optimization of synthesis conditions. 3.3. Electronic Properties of Ce3+ in LSCSO. The band structure of undoped LSCSO was first calculated using DFT with the pure PBE functional (Figure 6a). The calculations predicted a direct band gap of 4.87 eV, with the conduction band minimum (CBM) and the valence band maximum (VBM) located at the high symmetry k-point of Γ. As expected, this PBE band gap underestimates the experimental value (8.02 eV) which, however, can be reached by employing hybrid DFT with a modified PBE0 functional that contains 32% HF exchange (denoted as PBE032 hereafter). The calculated band gap with PBE032 is 8.05 eV which is also larger than the value of 7.30 eV obtained with the standard PBE0 functional that contains 25% HF exchange. With PBE032, the calculated total density of states (TDOS) and partial density of states (PDOS) projected onto Li, Sr, Ca, Si, and O contributions are shown in Figure 6b. The top of the valence band is dominated by the O p orbitals with a dispersion of 5.85 eV, and can be divided into three portions (VB1−3) as indicated in the figure. Interestingly, the highest VB1 just below the Fermi level (EF) is mainly contributed by interactions of Li s and p orbitals with O p orbitals. The Si p orbitals mainly contribute to the middle VB2, whereas the Si s orbitals are mainly responsible for the lowest VB3. The bottom of the CB is composed of several discrete peaks, where the CBM is constituted by a small DOS peak at

8.05 eV that consists mainly of s orbitals of O and Li. The other discrete DOS peaks have additional contributions mainly from Ca d and O p orbitals, and above these, the CB consists mainly of d orbitals of Ca and Sr. Finally, we note that, with the PBE and PBE0 functionals, the calculated orbital characters of the VB and CB are basically the same as those obtained with PBE032, albeit with smaller band gaps. The calculated TDOS and PDOS for Ce-doped LSCSO supercells with the DFT-PBE032 method are depicted in Figure 6c,d. The incorporation of Ce in LSCSO results in the formation of an occupied 4f electronic state deep inside the band gap (indicated by the dashed lines in the figure), corresponding to a lone 4f electron of Ce3+. The occupied 4f state is located at 1.39 eV for CeSr (Figure 6c) and 1.79 eV for CeCa (Figure 6d) above the host VBM, which reflects its dependence on the local coordination environment. It should be noted that these energy positions as derived from single particle Kohn−Sham (KS) eigenvalues are, however, not rigorous. They can be only used for qualitative comparisons with spectroscopic data. This problem can be partly solved by the calculation of the charge transition energy levels. The Ceinduced charge transition level ε(Ce3+/4+) with reference to the host VBM is defined as the Fermi level at which the defect formation energies of Ce3+ and Ce4+ are equal to each other. It can be calculated by using the expression31 F

DOI: 10.1021/acs.inorgchem.7b02561 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ε(Ce3 + /4 +) = Etot(Ce3 +) − Etot(Ce 4 +) − εVBM

where Etot is the total energy of the optimized LSCSO supercell containing Ce3+ or Ce4+, and εVBM is the VBM energy of the Ce4+-doped supercell and is aligned with that of the perfect one by the macroscopic averaging approach.32 The predicted value of ε(Ce3+/4+) is 2.61 eV for CeSr and 3.39 eV for CeCa, which are larger than those determined from KS eigenvalues by 1.22 and 1.60 eV, respectively. Given the value of ε(Ce3+/4+), the energy difference (ΔE5d1) between the lowest Ce3+ 5d1 level and the host CBM can be estimated by ΔE5d1 = Egap − [ε(Ce3 + /4 +) + ΔE(4f1 → 5d1) − (1/2)ΔESS]

Figure 7. Host referred binding energies (HRBE) for the 4f ground states and the lowest energy 5d1 states of Ln3+ and Ln2+ ions located at the Sr and Ca sites of LSCSO, with the values of selected energy differences indicated.

where ΔE(4f1 → 5d1) is the lowest 4f1 → 5d1 transition energy of Ce3+. From the experimental values of Egap (8.02 eV), ΔE (4f1 → 5d1) (4.33 eV for CeSr and 3.47 eV for CeCa), and ΔESS (0.66 eV for CeSr and 0.47 eV for CeCa), the value of ΔE5d1 is estimated to be 1.41 eV for CeSr and 1.40 eV for CeCa. Then, by using the crude relationship, T0.5 = 680 × ΔE5d1,33 the thermal quenching temperatures (T0.5) for CeSr and CeCa are found to be 959 and 952 K, respectively. These results are in qualitative agreement with the experimental results (see Figure 4d) that thermal quenching was not yet observed for CeSr and CeCa luminescence when the temperature was raised up to 500 K. 3.4. Energy Level Diagram of Ln3+ and Ln2+ in LSCSO. In view of the good agreement between calculated and experimental 4f → 5d transition energies (Figure 3c−f), we may approximate the ΔEced of Ce3+ by those derived from the calculated 5d energy level data (Table S3 of the SI). Thus, the 5d centroid shifts (ΔEc) with respect to the free ion value (6.35 eV) are estimated to be 1.35 and 1.34 eV for CeSr and CeCa, respectively. Then, by using the relationship U (6) = 5.44 + 2.834e−ΔEc /2.2 eV,11 the energy differences between the 4f ground states of Eu2+ and Eu3+ are derived with the values of U(6) being 6.97 and 6.98 eV for the location at the Sr and Ca sites, respectively. These are important quantities, from which the 4f ground level of Eu3+ can be derived from that of Eu2+ or vice versa. To proceed, we have performed spectral measurements of Eu3+ in LSCSO at low temperatures (Figure S3 of the SI). Two distinct excitation bands with maxima at 280 nm (4.43 eV) and 240 nm (5.17 eV) are assigned to the charge transfer transitions (CTs) of Eu3+ at the Sr and Ca sites, respectively, on the basis of the observation that a larger crystal field strength usually leads to a higher CT transition energy.34 These CT transition energies of Eu3+ can be taken as the energy differences between the 4f ground levels of Eu2+ and the host VBM.35 Then, with the above values of U(6), the ground state position of Eu3+ is placed at −2.54 eV when located at the Sr site and −1.81 eV when located at the Ca site. With the availability of the 4f ground level positions of Eu3+ and Eu2+, we may derive the 4f ground level positions for the other Ln3+ and Ln2+ ions by utilizing the characteristic energy differences between Ln3+ or Ln2+ ions that are largely independent with the host material.36 The results are schematically displayed by the double zigzag curves (solid symbol lines) in Figure 7, where the host VBM was set as zero energy and the host CBM was drawn at 8.02 eV as estimated experimentally. It is interesting to note that the 4f ground states of Ce3+ at the Sr and Ca sites are located at 2.70 and 3.43 eV above the host VBM, respectively, in very good

agreement with the values of 2.61 and 3.39 eV obtained from ab initio calculations of charge transition levels. By employing the red-shift model developed by Dorenbos,36 which states that the red shift of the first 4f1 → 5d1 transition energy with respect to the free ion value is the same for each Ln3+ (or Ln2+) ion situated at the same site of the same host, the lowest 5d1 level positions of Ln3+ ions can be derived from that of Ce3+. Then, by using the relationship between the red shifts of the 4f1 → 5d1 transition energies of Ln3+ and Ln2+ ions,37 the 5d1 levels positions of Ln2+ ions can be estimated. The results are shown by the dashed symbol lines in Figure 7. It shows that the 5d1 levels of Ln3+ at the Sr site have almost the same energy positions (higher by 0.03 eV) as those at the Ca site, whereas the 5d1 levels of Ln2+ at the Sr site are lower by 0.25 eV than those at the Ca site. From the energy-level diagram, the 4f1−5d1 transition energy of Tb3+ is predicted to be 5.66 and 4.90 eV at the Sr and Ca sites, respectively. Experimentally, the first spin-allowed 4f → 5d transition of LSCSO:Tb was observed at around 250 nm (4.96 eV)7 and, by comparing with the above theoretical values, this transition can be ascribed to Tb3+ located at the Ca site. Indeed, DFT-PBE032 calculations within the 2 × 1 × 1 supercell model indicates that the LSCSO:TbCa is more stable than LSCSO:TbSr by 269 meV, in agreement with the above assignment. It is also interesting to observe that the 5d1 levels of Eu2+ at the Sr and Ca sites are located at about 0.49 and 0.24 eV below the host CBM, enabling the observation of Eu2+ 5d → 4f emission at both sites, as shown in experiments.8,9 It also implies that the thermal stability of Eu2+ luminescence at the Sr site is better than that at the Ca site, which disagrees with the interpretation of the temperature dependence of Eu 2+ luminescence intensities.9 Actually, the observed slight increase of Eu2+ emission intensity at the Ca2+ site could be an enhancement of ET rate from Eu2+ at the Sr site with increasing temperature, in view of the large spectral overlap and the short NN Sr−Ca distance in LSCSO.

4. CONCLUSIONS We have investigated optical properties of Ce-doped LSCSO with a combined experimental and theoretical method. Under VUV-UV excitation, the material exhibits luminescence at 300− 500 nm with a high thermal quenching temperature (>500 K). The fine structures of the excitation spectra at low temperatures G

DOI: 10.1021/acs.inorgchem.7b02561 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

and Ce3+/Li+-Activated Ca1.65Sr0.35SiO4 Phosphors. Inorg. Chem. 2015, 54, 7684−7691. (5) Kalaji, A.; Mikami, M.; Cheetham, A. K. Ce3+-Activated γCa2SiO4 and Other Olivine-Type ABXO4 Phosphors for Solid-State Lighting. Chem. Mater. 2014, 26, 3966−3975. (6) Akella, A.; Keszler, D. A. The New Orthosilicate Li4SrCa(SiO4)2: Structure and Eu2+ Luminescence. Inorg. Chem. 1995, 34, 1308−1310. (7) Zhang, X. M.; Seo, H. J. Color Tunable and Thermally Stable Luminescence of Tb3+ Doped Li4SrCa(SiO4)2 Phosphors. Mater. Res. Bull. 2012, 47, 2012−2015. (8) Zhang, J.; Zhang, W.; He, Y.; Zhou, W.; Yu, L.; Lian, S.; Li, Z.; Gong, M. Site-Occupancy on the Luminescence Properties of a SinglePhase Li4SrCa(SiO4)2:Eu2+ Phosphor. Ceram. Int. 2014, 40, 9831− 9834. (9) Zhang, J.; Hua, Z.; Wen, S. Generation of Tunable-Emission in Li4Ca1‑xSr0.96+x(SiO4)2: 0.04Eu2+ Phosphors for LEDs Application. Opt. Mater. Express 2015, 5, 1704−1714. (10) Zhang, J.; Zhang, W.; Qiu, Z.; Zhou, W.; Yu, L.; Li, Z.; Lian, S. Li4SrCa(SiO4)2:Ce3+, a Highly Efficient Near-UV and Blue Emitting Orthosilicate Phosphor. J. Alloys Compd. 2015, 646, 315−320. (11) Dorenbos, P. Lanthanide 4f-Electron Binding Energies and the Nephelauxetic Effect in Wide Band Gap Compounds. J. Lumin. 2013, 136, 122−129. (12) Coelho, A. A. Topas Academic, version 4; Coelho Software: Brisbane, Australia, 2005. (13) Tao, Y.; Huang, Y.; Gao, Z.; Zhuang, H.; Zhou, A.; Tan, Y.; Li, D.; Sun, S. Developing VUV Spectroscopy for Protein Folding and Material Luminescence on Beamline 4B8 at the Beijing Synchrotron Radiation Facility. J. Synchrotron Radiat. 2009, 16, 857−863. (14) Perdew, J. P.; Ernzerhof, M.; Burke, K. Rationale for Mixing Exact Exchange with Density Functional Approximations. J. Chem. Phys. 1996, 105, 9982. (15) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (16) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (17) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (18) Barandiarán, Z.; Seijo, L. The Abinitio Model Potential Representation of the Crystalline Environment. Theoretical Study of the Local Distortion on NaCl:Cu+. J. Chem. Phys. 1988, 89, 5739− 5748. (19) Gellé, A.; Lepetit, M. Fast Calculation of the Electrostatic Potential in Ionic Crystals by Direct Summation Method. J. Chem. Phys. 2008, 128, 244716. (20) Karlström, G.; Lindh, R.; Malmqvist, P.-Å.; Roos, B. O.; Ryde, U.; Veryazov, V.; Widmark, P. O.; Cossi, M.; Schimmelpfennig, B.; Neogrady, P.; Seijo, L. Molcas: A Program Package for Computational Chemistry. Comput. Mater. Sci. 2003, 28, 222−239. (21) Seijo, L.; Barandiarán, Z.; Ordejón, B. Transferability of Core Potentials to f and d States of Lanthanide and Actinide Ions. Mol. Phys. 2003, 101, 73−80. (22) Barandiarán, Z.; Seijo, L. The Ab initio Model Potential Method. Cowan−Griffin Relativistic Core Potentials and Valence Basis Sets from Li (Z = 3) to La (Z = 57). Can. J. Chem. 1992, 70, 409−415. (23) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (24) Barandiarán, Z.; Meijerink, A.; Seijo, L. Configuration Coordinate Energy Level Diagrams of Intervalence and Metal-toMetal Charge Transfer States of Dopant Pairs in Solids. Phys. Chem. Chem. Phys. 2015, 17, 19874−19884. (25) Dorenbos, P. The Eu3+ Charge Transfer Energy and the Relation with the Band Gap of Compounds. J. Lumin. 2005, 111, 89− 104.

have been assigned to host-related excitonic absorptions and localized 4f → 5d transitions of Ce3+ at the Sr and Ca sites. These assignments were confirmed by wave-function-based ab initio calculations on Ce-centered embedded clusters at the spin−orbit level. Moreover, the energy location of Ce3+ 4f and 5d1 states within the host band gap was derived theoretically. The large energy separation (∼1.40 eV) between the 5d1 state of Ce3+ and the host CBM is consistent with the observed high thermal stability of the 5d luminescence. Finally, the energy positions for the 4f ground states and the lowest 5d states of all Ln3+ and Ln2+ ions with respect to the host valence and conduction bands have been derived and discussed in association with experimental findings. The present work demonstrates that a combination of experiments, ab initio calculations, and reliable theoretical models on Ce-doped materials can provide valuable insights into their luminescence properties and also background information to understand optical properties of other lanthanide ions in the same host.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02561. Refined lattice and atomic parameters (Table S1) and Sr−O and Ca−O bond lengths (Table S2) for LSCSO, calculated 4f1 and 5d1 energy levels (Table S3), Ce-L3 edge XANES spectra (Figure S1) and VUV-UV excitation spectra (Figure S2) for LSCSO:Ce, excitation and emission spectra of LSCSO:Eu3+ (Figure S3) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.N.). *E-mail: [email protected] (H.L.). ORCID

Rui Shi: 0000-0002-3120-0596 Litian Lin: 0000-0003-3241-2651 Lixin Ning: 0000-0003-2311-568X Hongbin Liang: 0000-0002-3972-2049 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is financially supported by the National Natural Science Foundation of China (21671201, 11574003, U1432249, and U1632101), and the Science and Technology Project of Guangdong Province (2017A010103034). L.N. acknowledges support from the Special and Excellent Research Fund of Anhui Normal University.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b02561 Inorg. Chem. XXXX, XXX, XXX−XXX