The union of temperature-dependent phase transition and relating structural transformation via modification of chemical compositions is of fundamental importance for the discovery of new materials or their functional properties optimization. Herein, the synthesis temperature and Eu2+-doping content induced phase selection and variations of the local structures in nepheline, low-carnegieite and high-carnegieite types of NaAlSiO4 polymorphs were studied in detail. The luminescence of Eu2+ in low-carnegieite and nepheline phases shows blue (460 nm) and yellow (540 nm) broad-band emissions, respectively, under near-ultraviolet excitation. The photoluminescence evolution can be triggered by the different synthesis temperatures in relation to the Eu2+-doping concentration, as corroborated by density functional theory calculations on the local coordination structures and corresponding mechanical stabilities in terms of the Debye temperature. The fabricated white light-emitting diode device with high color rendering index demonstrates that the multicolor phosphors from one system provides a new gateway for the photoluminescence tuning.

The formation energies and electronic properties of intrinsic defects of SnSe, including two vacancies (VSn and VSe), two interstitials (Sni and Sei) and two antisites (SnSe and SeSn), are investigated by using density functional theory (DFT) calculations. The results indicate that, due to a relatively low formation energy as well as a desirable ultra-shallow transition energy level, VSn can act as an effective source for p-type conduction under both Sn- and Se-rich conditions, which implies that SnSe is a native p-type semiconductor. On the other hand, a native n-type conduction is unlikely to be realized due to the absence of effective intrinsic sources. In addition, all the three types of intrinsic defects are not capable of inducing magnetism.

•Site occupation of Ce3+ in Sr4Ca4La2(PO4)6O2 was studied by experiments.•Three main types of Ce centers were identified based on low temperature spectra.•The Ce centers on La(6 h) sites was dominant in number supported by calculations.Cerium-doped oxyapatite phosphors, Sr4Ca4La2(PO4)6O2: Ce3+, are prepared by a traditional solid-state reaction method. X-ray diffraction (XRD) refinement reveals that the hexagonal Sr4Ca4La2(PO4)6O2 structure is characterized by a random distribution of Sr and Ca atoms on the nine-coordinated cationic 4f sites and of Sr, Ca, and La atoms on the seven-coordinated cationic 6 h sites. Photoluminescence properties of Ce-doped samples are then studied with excitation energies in the vacuum-ultraviolet (VUV) to ultraviolet (UV) range at low temperature. Three main types of occupation sites for Ce3+ are identified based on analysis of emission and excitation spectra and of luminescence decay behaviors. The Ce3+ occupation on the seven-coordinated La (6 h) site is found to be dominant, which is supported by wave function-based CASSCF/CASPT2 embedded cluster calculations on Ce3+ 4f → 5d transition energies at the spin-orbit level. The role of the coordinated oxygen ion that is not bonded with P5+ in the 5d centroid shift of CeLa(6h)3+ is emphasized. The thermal stability and doping concentration dependence of the 5d luminescence are also investigated and discussed in association with the coordination structures of Ce3+.

Incorporation of Si–N into Ce-doped Y3Al5O12 (YAG:Ce) has previously been shown to give distinct lower-energy emission but with stronger thermal quenching than the typical yellow YAG:Ce emission. Here, we investigate geometric and electronic structures of Ce and Si–N co-doped YAG with first-principles methods, to gain microscopic insight into effects of Si–N addition on electronic structures and optical properties of Ce3+. Hybrid density functional theory (DFT) calculations reveal that the Si–N prefers to be substituted for the tetrahedral Al(tet)–O sites with a random distribution, among which the nearest-neighbor (NN) SiAl(tet)–NO substitutions with the NO coordinated to Ce3+ result in a slight upward shift of the 4f1 ground-state level with respect to the host valence band. Wave function-based CASSCF/CASPT2 calculations at the spin–orbit level show that the NN SiAl(tet)–NO substitutions induce a redshift of the lowest energy Ce3+ 4f(1) → 5d(1) transition, in agreement with experimental observations. The redshift originates from an increase in the 5d crystal field splitting and a decrease in the 5d centroid energy of Ce3+ in comparable magnitude. Combining these results, we find that the energy separation between the lowest Ce3+ 5d(1) level and the host conduction band minimum (CBM) remains largely unchanged upon the NN SiAl(tet)–NO substitution, thus excluding the thermal ionization of the 5d electron as the underlying mechanism for the temperature quenching of the lower-energy Ce3+ emission. This finding also suggests that the thermally activated crossover from the 5d(1) to the 4f1 states could be responsible for the luminescence quenching, which is also consistent with present calculated results.

First-principles investigations are performed on the stabilities and electronic and optical properties of SnSe2(1–x)S2x (x = 0.0625, 0.25, 0.5, 0.625, 0.8125, and 1.0) monolayer alloys by using density functional theory calculations. It is found that, above a critical temperature of 702 K, the mixing of SnSe2 and SnS2 is likely to form random alloys. The calculated negative substitution energy of S at the Se site of SnSe2 suggests an alternative strategy for the synthesis of the alloys, i.e., by the substitution of S for Se in SnSe2 monolayers. It is also shown that, due to the lattice mismatch and the pronounced charge transfer between SnSe2 and SnS2, the band-gap values of the alloys deviate strongly from the concentration-averaged values of the constituents. Moreover, the dielectric functions of the alloys are determined to be anisotropic, with optical properties along the xy plane being more susceptible to the S content than those along the z direction, and the alloying enhances the absorption strength in the visible spectral region. We hope that these insights will be useful for future applications of SnSe2(1–x)S2x alloys.

It was recently reported that Ce-doped Ca6BaP4O17 displayed blue-green emission under excitation in the near-ultraviolet (UV) region and that luminescence intensities can be greatly improved by codoping with Si. Here, a combination of hybrid density functional theory (DFT) and wave function-based CASSCF/CASPT2 calculations at the spin–orbit level has been performed on geometric and electronic structures of the material to gain insights into effects of Si codoping on its optical properties. It is found that the observed luminescence arises from 4f–5d transitions of Ce3+ occupying the two crystallograhically distinct Ca1 and Ca2 sites of the host compound with comparable probabilities, with the energy of the lowest 4f → 5d transition of CeCa1 being slightly higher than that of Ceca2. The codopant Si prefers to substitute for the nearest-neighbor (NN) P1 atom over the NN P2 atom around Ce3+, and this preference induces a blueshift of the lowest-energy 4f → 5d transition, consistent with experimental observations. The blueshift originates from a reduction in 5d crystal field splitting of Ce3+ associated mainly with electronic effects of the NN SiP1 substitution, while the contribution from the change in 5d centroid energy is negligible. On the basis of calculated results, the energy-level diagram for the 4f ground states and the lowest 5d states of all trivalent and divalent lanthanide ions on the Ca2+ sites of Ca6BaP4O17 is constructed and discussed in connection with experimental findings.

•Site occupancy of Ce3+ in β-Ca2SiO4 was studied by spectral and ab initio methods.•Two distinct Ce3+ sites were revealed by experimental spectral measurements.•The two Ce3+ sites were identified as Ce3+ on Ca1 and Ca2 sites of β-Ca2SiO4.•The Ce3+ ions prefer to occupy Ca2 sites over Ca1 sites in β-Ca2SiO4.•The change of the lowest 4f→5d transition energy from CeCa1 to CeCa2 was analyzed.Low-temperature photoluminescence properties of the β-Ca2(1−x)CexNaxSiO4 (x = 0.0005) phosphor synthesized by a solid-state reaction method are investigated with excitation energies in the vacuum ultraviolet (VUV) to ultraviolet (UV) range. Two distinct types of emission and excitation spectra are observed, which are attributed to 4f–5d transitions of two different sets of Ce3+ centers. On the basis of density functional theory (DFT) calculations within the supercell model and wave function-based CASSCF/CASPT2 embedded cluster calculations, the two sets of Ce3+ centers are ascribed to the Ce3+ located on the seven-coordinated Ca1 and eight-coordinated Ca2 sites, respectively. Furthermore, from the observed relative spectral intensities, DFT total energy calculations, and comparison of experimental and calculated 4f → 5d transition energies, it is concluded that the occupation of Ce3+ on the Ca2 site is more energetically favorable than the occupation on the Ca1 site. Finally, the redshift of the lowest 4f → 5d transition of Ce3+ on the Ca2 site relative to that on the Ca1 site is discussed in terms of the changes of the 5d centroid energy and crystal-field splitting with the local coordination structure.

Low-temperature photoluminescence properties of Sr1–2xCexNaxAl2O4 (x = 0.001) synthesized by a solid-state reaction method are measured with excitation energies in the vacuum ultraviolet (VUV) to ultraviolet (UV) range. Two distinct activator centers with different emission and excitation intensities are observed and attributed to Ce3+ occupying the Sr1 and Sr2 sites of SrAl2O4 with different probabilities. Hybrid density functional theory (DFT) calculations within the supercell model are then carried out to optimize the local structures of Ce3+ located at the two Sr sites of SrAl2O4, on which wave function-based CASSCF/CASPT2 embedded cluster calculations with the spin–orbit effect are performed to derive the Ce3+ 4f1 and 5d1 energy levels. On the basis of the observed relative spectral intensities, the calculated DFT total energies, and the comparison between experimental and calculated 4f → 5d transition energies, we conclude that, in SrAl2O4:Ce3+, the dopant Ce3+ prefers to occupy the slightly smaller Sr2 site, rather than the larger Sr1 site as proposed earlier. Furthermore, by using an established linear relationship between the lowest 4f → 5d transition energies of Ce3+ and Eu2+ located at the same site of a given compound, we find that, in SrAl2O4:Eu2+, the dominant green emission observed at room temperature arises from Eu2+ located at the Sr2 site of SrAl2O4.

The Ce3+ ions occupying the two crystallographically distinct Y3+ sites both with C1 point group symmetry in the X2-Y2SiO5 (X2-YSO) crystal are discriminated by their spectroscopic properties calculated with ab initio approaches and phenomenological model analyses. Density functional theory (DFT) calculations with the supercell approach are performed to obtain the local structures of Ce3+, based on which the wave function-based embedded cluster calculations at the CASSCF/CASPT2 level are carried out to derive the 4f → 5d transition energies. From the ab initio calculated energy levels and wave functions, the crystal-field parameters (CFPs) and the anisotropic g-factor tensors of Ce3+ are extracted. The theoretical results agree well with available experimental data. The structural and spectroscopic properties for the two types of Ce3+ ions in X2-YSO are thus distinguished in terms of the calculated local atomic structures, 4f → 5d transition energies, and spectral parameters.

The magnetic properties of the Gd12O18 cluster cut from the bulk Gd2O3 crystal are investigated using the spin-polarized density functional theory within the broken-symmetry approach. Our work reveals that in the ground state of the cluster the antiferromagnetic coupling between adjacent Gd (4f7) spins is preferred energetically. This result is in contrast to a recent prediction made by Pedersen and Ojamäe (Pedersen, H.; Ojamäe, L. Nano Lett.2006, 6, 2004) but is consistent with recent experimental observations. The optimized structures of the cluster in the lowest-energy broken-symmetry state and the highest-spin ferromagnetic state are almost identical. The latter state is 71.5 cm−1 higher in energy than the former one, giving a value of about −0.24 cm−1 for the magnetic coupling constant, which is comparable to that estimated from experiments on the bulk crystal. The relative energies of various 4f7 spin patterns of the cluster are calculated, and certain characteristics of the cluster in the lowest-energy broken-symmetry state are discussed.

The red phosphor Sr2Si5N8:Eu2+ exhibits a significant emission redshift with increasing Eu concentration, but the reason remains controversial. Here, we investigate energetic, mechanical, and electronic properties of Sr2Si5N8:Eu2+ by using density-functional theory (DFT) approaches with the periodic supercell model. Total-energy calculations for Sr2−xEuxSi5N8 (x=0.028, 0.125, 0.5) supercells reveal that Eu2+ ions occupy the two distinct Sr1 and Sr2 sites with almost equal preference, with a nearly even distribution of Eu2+ on the two Sr sites, irrespective of the doping concentration. Calculations for the Debye temperature and electronic properties show that, with increasing EuSr1 or EuSr2 content, the structural rigidity decreases gradually and the occupied 5d state in the excited Eu2+(4f65d1) ion becomes more delocalized, which may result in an enlarged Stokes shift of the 5d→4f emission and thus its redshift as observed experimentally.

Incorporation of Si–N into Ce-doped Y3Al5O12 (YAG:Ce) has previously been shown to give distinct lower-energy emission but with stronger thermal quenching than the typical yellow YAG:Ce emission. Here, we investigate geometric and electronic structures of Ce and Si–N co-doped YAG with first-principles methods, to gain microscopic insight into effects of Si–N addition on electronic structures and optical properties of Ce3+. Hybrid density functional theory (DFT) calculations reveal that the Si–N prefers to be substituted for the tetrahedral Al(tet)–O sites with a random distribution, among which the nearest-neighbor (NN) SiAl(tet)–NO substitutions with the NO coordinated to Ce3+ result in a slight upward shift of the 4f1 ground-state level with respect to the host valence band. Wave function-based CASSCF/CASPT2 calculations at the spin–orbit level show that the NN SiAl(tet)–NO substitutions induce a redshift of the lowest energy Ce3+ 4f(1) → 5d(1) transition, in agreement with experimental observations. The redshift originates from an increase in the 5d crystal field splitting and a decrease in the 5d centroid energy of Ce3+ in comparable magnitude. Combining these results, we find that the energy separation between the lowest Ce3+ 5d(1) level and the host conduction band minimum (CBM) remains largely unchanged upon the NN SiAl(tet)–NO substitution, thus excluding the thermal ionization of the 5d electron as the underlying mechanism for the temperature quenching of the lower-energy Ce3+ emission. This finding also suggests that the thermally activated crossover from the 5d(1) to the 4f1 states could be responsible for the luminescence quenching, which is also consistent with present calculated results.