Crystal, electronic structures and photoluminescence properties of rare-earth doped LiSi2N3

The crystal and electronic structures, and luminescence properties of Eu²⁺, Ce³⁺ and Tb³⁺ activated LiSi₂N₃ are reported. LiSi₂N₃ is an insulator with an indirect band gap of about 5.0 eV (experimental value ∼6.4 eV) and the Li 2s, 2p states are positioned on the top of the valence band close to the Fermi level and the bottom of the conduction band. The solubility of Eu²⁺ is significantly higher than Ce³⁺ and Tb³⁺ in LiSi₂N₃ which may be strongly related to the valence difference between Li⁺ and rare-earth ions. LiSi₂N₃:Eu²⁺ shows yellow emission at about 580 nm due to the 4f⁶5d¹→4f⁷ transition of Eu²⁺. Double substitution is found to be the effective ways to improve the luminescence efficiency of LiSi₂N₃:Eu²⁺, especially for the partial replacement of (LiSi)⁵⁺ with (CaAl)⁵⁺, which gives red emission at 620 nm, showing highly promising applications in white LEDs. LiSi₂N₃:Ce³⁺ emits blue light at about 450 nm arising from the 5d¹→4f¹5d⁰ transition of Ce³⁺ upon excitation at 320 nm. LiSi₂N₃:Tb³⁺ gives strong green line emission with a maximum peak at about 542 nm attributed to the ⁵D₄→⁷F_J (J=3-6) transition of Tb³⁺, which is caused by highly efficient energy transfer from the LiSi₂N₃ host to the Tb³⁺ ions.     

Electronic structure and photocatalytic properties of ABi2Ta2O9 (A=Ca, Sr, Ba)

A new series of layered perovskite photocatalysts, ABi₂Ta₂O₉ (A=Ca, Sr, Ba), were synthesized by the conventional solid-state reaction method and the crystal structures were characterized by powder X-ray diffraction. The results showed that the structure of ABi₂Ta₂O₉ (A=Ca, Sr) is orthorhombic, while that of BaBi₂Ta₂O₉ is tetragonal. First-principles calculations of the electronic band structures and density of states (DOS) revealed that the conduction bands of these photocatalysts are mainly attributable to the Ta 5d+Bi 6p+O 2p orbitals, while their valence bands are composed of hybridization with O 2p+Ta 5d+Bi 6s orbitals. Photocatalytic activities for water splitting were investigated under UV light irradiation and indicated that these photocatalysts are highly active even without co-catalysts. The formation rate of H₂ evolution from an aqueous methanol solution is about 2.26 mmol h^-1 for the photocatalyst SrBi₂Ta₂O₉, which is much higher than that of CaBi₂Ta₂O₉ and BaBi₂Ta₂O₉. The photocatalytic properties are discussed in close connection with the crystal structure and the electronic structure in details.     

A re-investigation of the crystal structure and luminescence of BaCa2MgSi2O8:Eu

Barium calcium magnesium silicate (BaCa₂MgSi₂O₈), a compound whose space group was obtained via X-ray diffraction data, was re-investigated using neutron diffraction techniques. A combined powder X-ray and neutron Rietveld method revealed that BaCa₂MgSi₂O₈ crystallizes in the trigonal space group P3? (Z=1, a=5.42708(5) Å, c=6.79455(7) Å, V=173.310(4) Å³; R_p/R_wp=5.52%/7.63%), instead of the previously believed space group P3?m1. The difference in the two structures arises from the displacement of the O2 atom. Blue emission from Ba_0.98Eu_0.02Ca₂MgSi₂O₈ under 325-nm excitation is ascribed to the 4f⁶5d¹→4f⁷ transitions of Eu²⁺ ions at Ba sites and Ca sites. Site assignment of Eu²⁺ ions in the titled compound was performed by analysis of emission spectra at temperatures in the range of 4.2-300 K.     

The electronic structures of vanadate salts: Cation substitution as a tool for band gap manipulation

The electronic structures of six ternary metal oxides containing isolated vanadate ions, Ba₃(VO₄)₂, Pb₃(VO₄)₂, YVO₄, BiVO₄, CeVO₄ and Ag₃VO₄ were studied using diffuse reflectance spectroscopy and electronic structure calculations. While the electronic structure near the Fermi level originates largely from the molecular orbitals of the vanadate ion, both experiment and theory show that the cation can strongly influence these electronic states. The observation that Ba₃(VO₄)₂ and YVO₄ have similar band gaps, both 3.8 eV, shows that cations with a noble gas configuration have little impact on the electronic structure. Band structure calculations support this hypothesis. In Pb₃(VO₄)₂ and BiVO₄ the band gap is reduced by 0.9-1.0 eV through interactions of (a) the filled cation 6s orbitals with nonbonding O 2p states at the top of the valence band, and (b) overlap of empty 6p orbitals with antibonding V 3d-O 2p states at the bottom of the conduction band. In Ag₃VO₄ mixing between filled Ag 4d and O 2p states destabilizes states at the top of the valence band leading to a large decrease in the band gap (E_g=2.2 eV). In CeVO₄ excitations from partially filled 4f orbitals into the conduction band lower the effective band gap to 1.8 eV. In the Ce_1−xBi_xVO₄ (0≤x≤0.5) and Ce_1−xY_xVO₄ (x=0.1, 0.2) solid solutions the band gap narrows slightly when Bi³⁺ or Y³⁺ are introduced. The nonlinear response of the band gap to changes in composition is a result of the localized nature of the Ce 4f orbitals.     

The effect of replacement of Sr by Ca on the structural and luminescence properties of the red-emitting Sr2Si5N8:Eu LED conversion phosphor

The influence of the replacement of Sr by Ca on structural and luminescence properties of Eu²⁺-doped Sr₂Si₅N₈ is reported. The Rietveld refinement of the powder X-ray diffraction data shows that the Ca²⁺ ion preferentially occupies the larger Sr site in Sr₂Si₅N₈:Eu²⁺. Although the excitation spectrum is hardly modified, the position of the emission band of Eu²⁺ can be tailored through partial replacement of Sr by Ca in Sr₂Si₅N₈:Eu²⁺, resulting in red-emission shifting from 620 to 643 nm. Furthermore, (Sr, Ca)₂Si₅N₈:Eu²⁺ shows high potential as a conversion phosphor for white-light LED applications due to similar absorption, conversion efficiency and thermal quenching behaviour for 465 nm excitation after the introduction of the Ca ion.     

Solution precursor synthesis and magnetic properties of Eu1−xCaxTiO3

Europium titanate, EuTiO₃, is a paraelectric/antiferromagnetic cubic perovskite with T_N=5.5 K. It is predicted that compressive strain could induce simultaneous ferroelectricity and ferromagnetism in this material, leading to multiferroic behavior. As an alternative to epitaxial strain, we explored lattice contraction via chemical substitution of Eu²⁺ with the smaller Ca²⁺ cation as a mechanism to tune the magnetic properties of EuTiO₃. A modified sol-gel process was used to form homogeneously mixed precursors containing Eu³⁺, Ca²⁺, and Ti⁴⁺, and reductive annealing was used to transform these precursors into crystalline powders of Eu_1−xCa_xTiO₃ with x=0.00, 0.05, 0.10, 0.15, 0.25, 0.35, 0.50, 0.55, 0.60, 0.65, 0.80, and 1.00. Powder XRD data indicated that a continuous Eu_1−xCa_xTiO₃ solid solution was readily accessible, and the lattice constants agreed well with those predicted by Vegard's law. SEM imaging and EDS element mapping indicated a homogeneous distribution of Eu, Ca, and Ti throughout the polycrystalline sample, and the actual Eu:Ca ratio agreed well with the nominal stoichiometry. Measurements of magnetic susceptibility vs. temperature indicated antiferromagnetic ordering in samples with x≤0.60, with T_N decreasing from 5.4 K in EuTiO₃ to 2.6 K in Eu_0.40Ca_0.60TiO₃. No antiferromagnetic ordering above 1.8 K was detected in samples with x>0.60.     

Photoluminescence of new red phosphor SrZnO2:Eu

SrZnO₂:Eu³⁺ has been synthesized by solid-state reaction and its photoluminescence in ultraviolet (UV)-vacuum ultraviolet (VUV) range was investigated. The broad bands around 254 nm are assigned to CT band of Eu³⁺-O²⁻. With the increasing of Eu³⁺ concentration, Eu³⁺ could occupy different sites, which leads to the broadening of CT band. A sharp band is observed in the region of 110-130 nm, which is related to the host absorption. The phosphors emit red luminescence centered at about 616 nm due to Eu³⁺⁵D₀→⁷F₂ both under 254 and 147 nm, but none of Eu²⁺ blue emission can be observed.     

Eu luminescence—a structural probe in BiCa4(PO4)3O, an apatite related phosphate

Eu³⁺ luminescence is studied in apatite-related phosphate BiCa₄(PO₄)₃O. Compositions of the formula Bi_1−xEu_xCa₄(PO₄)₃O [x=0.05, 0.1, 0.3, 0.5, 0.8 and 1.0] are synthesized and they are isostructural with parent BiCa₄(PO₄)₃O. Room temperature photoluminescence shows the various transitions ⁵D₀→⁷F_J(=0,1,2) of Eu³⁺. The emission results of compositions with different Eu³⁺ content show the difference in site occupancy of Eu³⁺ in Bi_1−xEu_xCa₄(PO₄)₃O. The intense ⁵D₀-⁷F₀ line at 574 nm for higher Eu³⁺ content is attributed to the presence of strongly covalent Eu-O bond that is possible by substituting Bi³⁺ in the Ca(2) site. This shows the preferential occupancy of Bi³⁺ in Ca(2) site and this has been attributed to the 6s² lone pair electrons of Bi³⁺. This is further confirmed by comparing the emission results with La_0.95Eu_0.05Ca₄(PO₄)₃O.     

A novel red emitting phosphor Ca2SnO4: Eu

A novel red emitting phosphor, Eu³⁺-doped Ca₂SnO₄, was prepared by the solid-state reaction. X-ray powder diffraction (XRD) analysis confirmed the formation of Ca₂SnO₄: Eu³⁺. Field-emission scanning electron-microscopy (FE-SEM) observation indicated a narrow size-distribution of about 500 nm for the particles with spherical shape. Photoluminescence measurements indicated that the phosphor exhibits bright red emission at about 615 nm under UV excitation. The excellent luminescence properties make it possible as a good candidate for plasma display panels (PDP) application. Splitting of the ⁵D₀-⁷F_J transitions of Ca₂SnO₄: Eu³⁺ suggests that the Eu³⁺ ions occupied two nonequivalent sites in the crystallite. The luminescence lifetime measurement showed a bi-exponential decay, providing other evidence for the existence of two different environments for Eu³⁺ ions.     

Reduction of Eu to Eu in MAl2Si2O8 (M=Ca, Sr, Ba) in air condition

In general, the reduction of Eu³⁺ to Eu²⁺ in solids needs an annealing process in a reducing atmosphere. In this paper, it is of great interest and importance to find that the reduction of Eu³⁺ to Eu²⁺ can be realized in a series of alkaline-earth metal aluminum silicates MAl₂Si₂O₈ (M=Ca, Sr, Ba) just in air condition. The Eu²⁺-doped MAl₂Si₂O₈ (M=Ca, Sr, Ba) powder samples were prepared in air atmosphere by Pechini-type sol-gel process. It was found that the strong band emissions of 4f⁶5d¹-4f⁷ from Eu²⁺ were observed at 417, 404 and 373 nm in air-annealed CaAl₂Si₂O₈, SrAl₂Si₂O₈ and BaAl₂Si₂O₈, respectively, under ultraviolet excitation although the Eu³⁺ precursors were employed. In addition, under low-voltage electron beam excitation, Eu²⁺-doped MAl₂Si₂O₈ also shows strong blue or ultraviolet emission corresponding to 4f⁶5d¹-4f⁷ transition. The reduction mechanism from Eu³⁺ to Eu²⁺ in these compounds has been discussed in detail.     

Synthesis, band structure, and optical properties of Ba2ZnV2O8

A novel compound Ba₂ZnV₂O₈ has been synthesized in high temperature solution reaction and its crystal structure has been characterized by means of single crystal X-ray diffraction analysis. It crystallizes in monoclinic system and belongs to space group P2₁/c with a=7.9050(16), b=16.149(3), , β=90.49(3). It builds up from 1-D branchy chains of [ZnV₂O₈⁴⁻]_∞, and the Ba²⁺ cations are located in the space among these chains. The IR spectrum, ultraviolet-visible diffuse reflection integral spectrum and fluorescent spectra of this compound have been investigated. The calculated results of energy band structure by the density functional theory method show that the solid-state compound of Ba₂ZnV₂O₈ is an insulator with direct band gap of 3.48 eV. The calculated total and partial density of states indicate that the top valence bands are contributions from the mixings of O-2p, V-3d, and Zn-3d states and low conduction bands mostly originate from unoccupied antibonding states between the V-3d and O-2p states. The V-O bonds are mostly covalence characters and Zn-O bonds are mostly ionic interactions, and the ionic interaction strength is stronger between the Ba-O than between the Zn-O. The refractive index of n_x, n_y, and n_z is estimated to be 1.7453, 1.7469, and 1.7126, respectively, at wavelength of 1060 nm for Ba₂ZnV₂O₈ crystal.     

Subnitride chemistry: A first-principles study of the NaBa3N, Na5Ba3N, and Na16Ba6N phases

An ab initio study on the electronic structure of the subnitrides NaBa₃N, Na₅Ba₃N, and Na₁₆Ba₆N is performed for the first time. The NaBa₃N and Na₅Ba₃N phases consist of infinite ¹_∞[NBa_6/2] strands composed of face-sharing NBa₆ octahedra surrounded by a “sea” of sodium atoms. The Na₁₆Ba₆N phase consist of discrete [NBa₆] octahedra arranged in a body-cubic fashion, surrounded by a “sea” of sodium atoms. Our calculations suggest that the title subnitrides are metals. Analysis of the electronic structure shows partial interaction of N(2s) with Ba(5p) electrons in the lower energy region for NaBa₃N and Na₅Ba₃N. However, no dispersion is observed for the N(2s) and Ba(5p) bands in the cubic phase Na₁₆Ba₆N. The metallic band below the Fermi level shows a strong mixing of N(2p), Ba(6s), Ba(5d), Ba(6p), Na(3s) and Na(3p) orbitals. The metallic character in these nitrides stems from delocalized electrons corresponding to hybridized 5d^l6s^m6pⁿ barium orbitals which interact with hybridized 3sⁿ3p^m sodium orbitals. Analysis of the electron density and electronic structure in these nitrides shows two different regions: a metallic matrix corresponding to the sodium atoms and the regions around them and heteropolar bonding between nitrogen and barium within the infinite ¹_∞[NBa_6/2] strands of the NaBa₃N and Na₅Ba₃N phases, and within the isolated [NBa₆] octahedra of the Na₁₆Ba₆N phase. The nitrogen atoms inside the strands and octahedra are negatively charged, the anionic character of nitrogens being larger in the isolated octahedra of the cubic phase Na₁₆Ba₆N, due to the lack of electron delocalization along one direction as opposed to the other phases. The sodium and barium atoms appear to be slightly negatively and positively charged, the latter to a larger extent. From the computed Ba-N overlap populations as well as the analysis of the contour maps of differences between total density and superposition of atomic densities, we suggest partial covalent bonding between nitrogen and barium atoms along the infinite ¹_∞[NBa_6/2] strands and within isolated [NBa₆] octahedra.     

Crystal structure of BaMg2Si2O7 and Eu luminescence

Crystal structure of BaMg₂Si₂O₇ was determined and refined by a combined powder X-ray and neutron Rietveld method (monoclinic, C2/c, no. 15, Z=8, a=7.24553(8) Å, b=12.71376(14) Å, c=13.74813(15) Å, β=90.2107(8)°, V=1266.44(2) Å³; R_p/R_wp=3.38%/4.77%). The structure contains a single crystallographic type of Ba atom coordinated to eight O atoms with C₁ (1) site symmetry. Under 325-nm excitation Ba_0.98Eu_0.02Mg₂Si₂O₇ exhibits an asymmetric emission band around 402 nm. The asymmetric shape of the emission band is likely associated with a small electron-phonon coupling in BaMg₂Si₂O₇. The integrated intensity of the emission band was observed to remain constant over the temperature range 4.2-300 K.     

Preparation, crystal structure, and photoluminescence of Ca2SnO4:Eu, Y

Eu³⁺-doped Ca₂SnO₄ (solid solutions of Ca_2−xEu_2xSn_1−xO₄, 0?x?0.3) and Eu³⁺ and Y³⁺-codoped Ca₂SnO₄ (Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄) were prepared by solid-state reaction at 1400 °C in air. Rietveld analysis of the X-ray powder diffraction patterns revealed that Eu³⁺ replaced Ca²⁺ and Sn⁴⁺ in Eu³⁺-doped Ca₂SnO₄, and that Eu³⁺ replaced Ca²⁺ and Y³⁺ replaced Sn⁴⁺ in Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄. Red luminescence at 616 nm due to the electric dipole transition ⁵D_o→⁷F₂ was observed in the photoluminescence (PL) spectra of Ca_2−xEu_2xSn_1−xO₄ and Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄ at room temperature. The maximum PL intensity in the solid solutions of Ca_2−xEu_2xSn_1−xO₄ was obtained for x=0.1. The PL intensity of Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄ was 1.26 times greater than that of Ca_2−xEu_2xSn_1−xO₄ with x=0.1.     

Crystal structure and luminescence of Sr0.99Eu0.01AlSiN3

Strontium europium aluminum silicon nitride, Sr_0.99Eu_0.01AlSiN₃, was synthesized by heating a mixture of binary nitrides at 2173 K and a N₂ gas pressure of 190 MPa. Single crystals of Sr_0.99Eu_0.01AlSiN₃ approximately 30 μm were obtained. The structure was confirmed to be an isotypic structure of CaAlSiN₃ in the orthorhombic space group Cmc2₁, analyzed by single-crystal X-ray diffraction. The lattice parameters are a=9.843(3), b=5.7603(16), c=5.177(2) Å, cell volume=293.53(17) Å³. It shows an orange-red photoluminescence by 5d→4f transition of Eu²⁺ at around 610 nm under excitation ranging from ultraviolet to 525 nm. The photoluminescence intensity, temperature characteristics, and oxidative stability were comparable or superior to those of CaAlSiN₃:Eu²⁺ phosphor.     

Optical properties of CeBO3 and CeB3O6 compounds: first-principles calculations and experimental results

The cerium borates o-CeBO₃, m-CeBO₃ and CeB₃O₆ have been shown to be isostructural to their lanthanum derivatives. From diffuse reflectance, electron energy loss spectroscopy (EELS) and band structure calculations, it has been evidenced that a Ce³⁺ 4f-5d transition is responsible for weak absorption peaks around 3.5 eV while the O2p-Ce5d charge transfer gives rise to a strong absorption around 7 eV. Starting from self-consistent full potential LAPW calculations, the dielectric tensors of the three compounds were computed and compared to experimental data. It results in a satisfactory fit between the observed and the calculated extinction coefficient k and the index of refraction n.     

Synthesis, characterization, and photocatalytic properties of InVO4 nanoparticles

Nanosized InVO₄ with orthorhombic structure was successfully synthesized at a relatively low calcination temperature of 600 °C by using an amorphous heteronuclear complex as precursor. The photocatalytic activity of InVO₄ catalyst has been evaluated by the decomposition of formaldehyde (FAD) under UV light () and visible light irradiation (). The as-synthesized InVO₄ catalyst showed higher photocatalytic activity for the FAD decomposition compared to the sample prepared by the conventional solid-state reaction. The calculations of the electronic band structures indicated that the valence band was composed of the O 2p orbitals, whereas the conduction band was formed by the V 3d orbitals with a small contribution of the In 5s orbitals. The photocatalytic activity of the as-prepared sample is discussed on the basis of the electronic band structure and bulk material structure.     

Crystal structure of Eu-doped M3MgSi2O8 (M: Ba, Sr, Ca) compounds and their emission properties

Emission properties of Eu²⁺-doped M₃MgSi₂O₈ (M: Ba, Sr, Ca) are discussed in terms of the crystal structure. When Ba²⁺ ions account for over one third of M²⁺ ions, M₃MgSi₂O₈ crystallizes in glaserite-type trigonal structure, while Ba-free compounds crystallize in merwinite-type monoclinic structure. Under UV excitation, the Eu²⁺-doped glaserite-type compounds exhibit an intense blue emission assigned to 5d-4f electron transition at about 435 nm, regardless of the molar ratio of Ba²⁺, Sr²⁺ and Ca²⁺ ions. By contrast, the Eu²⁺-doped merwinite-type compounds show an emission color sensitive to the ratio. A detailed analysis of the emission spectra reveals that the emission chromaticity for the Eu²⁺-doped M₃MgSi₂O₈ is composed of two emission peaks reflecting two different sites accommodating M²⁺ ion.     

Syntheses, crystal and electronic structure, and some optical and transport properties of LnCuOTe (Ln=La, Ce, Nd)

Three new compounds, LaCuOTe, CeCuOTe, and NdCuOTe, have been synthesized from the respective rare-earth elements, CuO, and a KI flux at 1023 K. The compounds, which have the ZrSiCuAs structure type, are isostructural to LaCuOS, and crystallize in space group P4/nmm of the tetragonal system with two formula units in cells of dimensions at 153 K of , , for LaCuOTe; , , for CeCuOTe; and , , for NdCuOTe. The structure of LnCuOTe (Ln=La, Ce, Nd) is composed of alternating PbO-like [Ln₂O₂] and anti-PbO-like [Cu₂Te₂] layers stacked perpendicular to [0 0 1]. The experimental optical band gaps of LaCuOTe and NdCuOTe are 2.31 and 2.26 eV, respectively. At 298 K the electrical conductivity of LaCuOTe is 1.65 S/cm and the Hall mobility is +80.6 cm² V⁻¹ s⁻¹. The positive values of the Seebeck and Hall coefficients indicate p-type electrical conduction. First-principles theoretical calculations were performed on LaCuOQ (Q=S, Se, Te). In LaCuOTe, Cu 3d and Te 5p orbitals dominate the states near the valence band maximum; the states near the conduction band minimum are composed of Cu 4s, Te 5p, and La 5d orbitals. The larger dispersion of Cu 3d orbitals and the presence of Te 5p orbitals near the valence band maximum are responsible for the larger hole mobility of LaCuOTe compared to LaCuOS and LaCuOSe.     

Crystal structure of Ca12Al14O32Cl2 and luminescence properties of Ca12Al14O32Cl2:Eu

The crystal structure of Ca₁₂Al₁₄O₃₂Cl₂ was determined from laboratory X-ray powder diffraction data (CuKα₁) using the Rietveld method, with the anisotropic displacement parameters being assigned for all atoms. The crystal structure is cubic (space group , Z=2) with lattice dimensions a=1.200950(5) nm and V=1.73211(1) nm³. The reliability indices calculated from the Rietveld method were R_wp=8.48% (S=1.21), R_p=6.05%, R_B=1.27% and R_F=1.01%. The validity of the structural model was verified by the three-dimensional electron density distribution, the structural bias of which was reduced as much as possible using the maximum-entropy methods-based pattern fitting (MPF). The reliability indices calculated from the MPF were R_B=0.75% and R_F=0.56%. In the structural model there are one Ca site, two Al sites, two O sites and one Cl site. This compound is isomorphous with Ca₁₂Al_10.6Si_3.4O₃₂Cl_5.4. Europium-doped sample Ca₁₂Al₁₄O₃₂Cl₂:Eu²⁺ was prepared and the photoluminescence properties were presented. The excitation spectrum consisted of two wide bands, which were located at about 268 and 324 nm. The emission spectrum, when excited at 324 nm, resulted in indigo light with a peak at about 442 nm.