Structural,elastic and thermodynamic properties under pressure and temperature effects of MgIn2S4 and CdIn2S4

A density functional-based method is used to investigate the structural, elastic and thermodynamic properties of the cubic spinel semiconductors MgIn₂S₄ and CdIn₂S₄ at different pressures and temperatures. Computed ground structural parameters are in good agreement with the available experimental data. Single-crystal elastic parameters are calculated for pressure up to 10 GPa and temperature up to 1200 K. The obtained elastic constants values satisfy the requirement of mechanical stability, indicating that MgIn₂S₄ and CdIn₂S₄ compounds could be stable in the investigated pressure range. Isotropic elastic parameters for ideal polycrystalline MgIn₂S₄ and CdIn₂S₄ aggregates are computed in the framework of the Voigt–Reuss–Hill approximation. Pressure and thermal effects on some macroscopic properties such as lattice constant, volume expansion coefficient and heat capacities are predicted using the quasi-harmonic Debye model in which the lattice vibrations are taken into account.     

EPR powder spectra of Co(II) and Cr(III) in MgIn2S4 spinel

The EPR powder spectra of high spin Co(II) and Cr(III) diluted in partially inverse spinel matrix MgIn₂S₄ are reported. While the spectrum of Co(II) in octahedral sites is orthorhombic the Cr(III) one is axial. Evidence is given that the EPR spectra cannot be directly associated with the cation distribution of the matrix.     

New ferrimagnetic spinel compositions in the system MCr2S4−xSex where M=Fe,Co, Mn

Phase relationship between ferrimagnetic semiconducting spinels of the type MCr₂S₄ and defect NiAs compositions of the type MCr₂Se₄ (where M=Mn, Fe, Co) are established. It was found that the amount of Se which can be substituted, with retention of the spinel structure, decreases from MnCr₂S_4?xSe_x (x=0 to x=2) to FeCr₂S_4?xSe_x (x=0 to x=1·25) to CoCr₂S_4?xSe_x (x=0 to x=1). This phenomena is accounted for on the basis of sulfide spinel stability and unit cell size. The Curie temperatures of the spinel compositions decrease slowly with increasing Se content. This is presumably caused by weakening of the spinel A-B superexchange interaction.     

Structural determination of CoIn2S4 single crystal spinel

Single crystals of the spinel CoIn₂S₄ were grown by iodine vapor transport. The structural determination leads to Fd3m space group with Co²⁺ ions distribution between O_h (63%) and T_d (37%) sites of the spinel structure.     

Electrochemical luminescence of Mg1−xCaxIn2O4:Er electrodes

Improvement for electrochemical luminescence (ECL) property of MgIn₂O₄ is attempted by partial exchange of Mg²⁺ ion in MgIn₂O₄ to Ca²⁺ ion. Mg_1−xCa_xIn₂O₄ solid solution was obtained in the region 0<x<0.4. Efficiency for ECL per unit current for Mg_1−xCa_xIn₂O₄:Er³⁺ increased with the increase in the ratio of Ca²⁺ ion, and showed a peak at x=0.25 and then decreased steeply. ECL efficiency for other rare-earth ion-(RE:Sm, Eu, Ho) doped Mg_1−xCa_xIn₂O₄ also increased comparing with those for MgIn₂O₄:RE. This Ca²⁺ addition effect on the ECL efficiency seems to be caused by the improvement of the efficiency for the impact activation.     

Heat capacity and thermal expansion of CdCr2Se4 and CdCr2S4

The thermodynamic properties of the spinel ferromagnetic compounds CdCr₂Se₄ and CdCr₂S₄ have been investigated by making heat capacity and thermal expansion measurements on single crystals. For both compounds, the ferromagnetic transition is marked by λ-type thermal anomalies, and the results provide a pressure dependence of the transition temperatures that is in agreement with direct measurements. Below the transition, CdCr₂S₄ shows an anomalous heat-capacity contribution and negative thermal expansion, which are in contrast to the conventional behavior found in CdCr₂Se₄.     

Spin-glass like behaviour in a concentrated chromium spinel: Zn0.5Cd0.5Cr2S4

Low field d.c. and a.c. susceptibility measurements are reported for the non magnetically diluted spinel Zn_0.5Cd_0.5Cr₂S₄. A spin-glass like behaviour is observed at low temperature (T_F = 15.4Katv = 198 Hz).This is the result of the presence of competing interactions (ferromagnetic between nearest-neighbours and antiferromagnetic between higher order neighbours) and of the disorder of their distribution due to the substitution between non-magnetic zinc and cadmium ions in the tetrahedral sites of the spinel lattice.     

Energy band structure and Frenkel excitons in PbGa2S4

Optical reflection spectra are measured and calculated in PbGa₂S₄ crystals in the region of resonances related to excitons with large oscillator strength and binding energy (Frenkel excitons). The splitting of the upper valence band in the center of the Brillouin zone due to crystal field (Δ_cf) and spin orbit (Δ_so) interaction are determined. Optical reflection spectra are measured and calculated according to Kramers-Kronig relations in the region of 3-6 eV in Е⊥с and Е∥с polarizations, and the optical constants n, k, ε₁ and ε₂ are determined. The observed electronic transitions in PbGa₂S₄ crystals are discussed in the frame of theoretical energy band structure calculation for thiogallate crystals.     

Electronic states of CdIn2S4 and of other II–III2–VI4 compounds: How sensitive are they to the crystal structure?

The valence band states of the spinel semiconductor CdIn₂S₄ have been studied by UV photoemission spectroscopy. Contrary to what structural considerations would suggest the measured spectra closely resemble those of defect-zincblend CdIn₂Se₄ and of other II–III₂–VI₄ compounds. The likelihood of structural effects in the electronic states of this family is discussed in light of the above results.     

X-ray photoelectron spectra of the spinel systems CdCrxIn2−xS4

XRS spectra of CdCr_xIn_2?xS₄ and ZnCr_xAl_2?xS₄ spinel solid solutions are reported. A nearly constant electron density is found over all atomic sites in both series in the whole range of concentration x; this situation is related to covalency. Some interesting features in the Cr3s multiplet splitting are discussed.     

Raman scattering study of pressure-induced phase transitions in AIIB2IIIC4VI defect chalcopyrites and spinels

A^IIB₂^IIIC₄^VI defect chalcopyrites (DC) and spinels were investigated by Raman scattering spectroscopy under hydrostatic pressure up to 20 GPa. All these compounds were found to undergo a phase transition to a Raman inactive defect NaCl-type structure. The phase transition is reversible for spinels and irreversible for DC. From the analysis of the pressure behavior of Raman-active modes, it was concluded that the phase transition from spinel to NaCl-type structure is direct in MnIn₂S₄ and CdIn₂S₄, while it occurs via an intermediate LiVO₂-type NaCl superstructure in MgIn₂S₄. The observed differences in the pressures and the paths of the pressure-induced phase transitions in A^IIB₂^IIIC₄^VI compounds are discussed.     

The Phosphorescence Excitation Spectrum of Jet-Cooled Xanthione in the Region of the T1←S0 and S1←S0 Absorption Transitions

The ³A₂(nπ*)→¹A₁X (T₁→S₀) phosphorescence excitation spectrum of jet-cooled xanthione was investigated in the region 14 920-17 600 cm⁻¹. The structure observed is shown to be due to the T₁←S₀ absorption and an assignment in terms of the vibronic structure of that band is proposed. A previous assignment of the S₁←S₀ origin is considered in detail and the transition involved is shown to be most probably due to absorption of a vibronic triplet state T_1z,ν7. An alternative but tentative assignment of the S_1,0←S_0,0 transition is suggested. Comparison is made with previous spectroscopic and theoretical work on the molecule and its congeners, 4H-pyran-4-thione and 4H-1-benzopyran-4-thione.     

Optical reflectivity of CdCr2Se4 single crystals in the visible and ultra-violet spectral region

The specular reflectivity of CdCr₂Se₄ single crystals has been measured within the spectral energy interval E = 0.5 ? 12.0 eV at room temperature. The spectral dependence of the real and imaginary part of the dielectric constant has been obtained from a Kramers-Kronig analysis of the experimental data. The observed optical transitions and the origin of the red-shifting band are discussed with respect to the semiempirical ionic model of the spinel structure, based on the recently reported increase of the structure parameter u of the spinel during cooling (Göbel, J. Magn. Magn. Mat. 3 (1976) 143). Contradicting results of the optical measurements in the region of the red-shifting edge may be attributed to the sensitivity of the parameter u on the internal stress.     

Ordered local distortions in cubic FeCr2S4

Mössbauer spectra of ⁵⁷Fe in FeCr₂S₄ and Co_0.02Cr₂S₄ were taken at various temperatures. The temperature dependence of H_h? and eQV_zz in Co_0.98Fe_0.02Cr₂S₄ can be explained as the combined effect of exchange, spin-spin, and spin-orbit interaction. In FeCr₂S₄ at low temperatures H_h? is not reduced and the EFG is no longer axially symmetric. This behaviour is explained on the basis of a magnetically induced quadrupole splitting influenced by negatively coupled local distortions ordered in an antiparallel way in two sublattices, which coincide with the two Bravais lattices of tetrahedral sites in the spinel lattice. The ‘antiferrodistortive’ transition occurs between 13 and 23 K.     

Band structure calculations on the layered compounds FeGa2S4 and NiGa2S4

For the compounds FeGa₂S₄ and NiGa₂S₄ band structure calculations have been performed by the ab initio plane wave pseudo-potential method. The valence charge density distribution points to an ionic type of chemical bonding between the transition metal atoms and the ligand atoms. Two models for the pseudo-potentials are used to calculate the band structures: (a) only s and p electrons and (b) also the d-shells of the transition metal atoms are included in the pseudo-potentials. The differences between these two cases of band structures are discussed. Energy gap formation peculiarities are analysed for both crystals. Zak's elementary energy band concept is demonstrated for the energy spectra of the considered crystals.     

Inelastic light scattering and photoluminescence in the mixed valence systems Eu3O4, Eu3S4 and Sm3S4

The thermally activated valence fluctuations of Eu₃S₄ and Sm₃S₄ have been studied using light scattering and photoluminescence techniques and are compared with measurements on the non-fluctuating compound Eu₃O₄, Eu₃S₄ exhibits an anomalous vibrational mode which is associated with the frequency factor of hopping, ν₂=1.3×10¹³ sec^?1. In Sm₃S₄ electronic Raman scattering is observed within the ⁷F₉ multiplet of the Sm²⁺ ion. An anomalous frequency shift of the5d-4f photoluminescence emission band in Eu₃S₄ is related to the temperature dependent fluctuation rate which passes through the reference time scale of the photoluminescence. Intra-4f photoluminescence has also been observed in Eu₃S₄ and Sm₃S₄.     

Metal-insulator transition in the spinel-type Cu(Ir1−xCrx)2S4 system

A normal thiospinel CuIr₂S₄ exhibits a temperature-induced metal-insulator (M-I) transition around 230 K with structural transformation, showing hysteresis on heating and cooling. On the other hand, CuCr₂S₄ has the same normal spinel structure without the structural transformation. CuCr₂S₄ has been found to be metallic and ferromagnetic with the Curie temperature T_c~377 K. In order to see the effect of substituting Cr for Ir on the M-I transition, we have carried out a systematic experimental study of electrical and magnetic properties of Cu(Ir_1−xCr_x)₂S₄. The M-I transition temperature shifts to lower temperature with increasing Cr-concentration x and this transition is not detected above x~0.05. The ferromagnetic transition temperature decreases as x is decreased and the transition does not occur below x~0.20.     

Detection of partial inverse spinel structure in CdIn2S4 by fourier transform spectroscopy

The lattice vibrational spectrum of a single crystal of CdIn₂S₄ has been recorded at low temperature (4.2 K) by using Fourier transform spectroscopy. The absorption line corresponding to the T_1u IR active mode has been studied in detail. It has been observed that this line is split into two closely spaced lines located at 69.9 and 71.1 cm^?1. This splitting is explained on the basis of the existence of a partially inverse spinel structure.     

Influence of vacancies on the electrical properties of the ZnCr2−xNixSe4 spinels

The effect of vacancies on the electronic transport in the ZnCr_2−xNi_xSe₄ paramagnetic single crystals is considered. For this purpose, the structure refinements, the high temperature electrical conductivity and the thermoelectric power measurements as well as the calculations of the vacancy model parameters were used. The electrical measurements have been done in the temperature range from 290 to 520 K for single crystals with x=0.001, 0.05, and 0.065. The above investigations provide evidence for polaron conduction in defective spinel materials. In particular: (1) at high temperatures a linear dependence between thermopower S and the electrical resistivity (ln ρ), a characteristic of small polarons, was observed, (2) an origin of small polarons in this case could be associated with a crystalline distortion, which is characterized by larger values of the anion parameter than u=0.375, which describes an ideal spinel structure, and (3) a large defectiveness, which is identified by the large value of the vacancy parameter of about β=11.5%. These effects are explained in terms of a polaron mechanism of the electrical conductivity including structural defects.     

Electron band structure of α-ZnIn2S4 and related polytypes

Pseudopotential calculations have been carried out for the α, β and γ polytypic forms of the layer semiconductor ZnIn₂S₄, respectively, corresponding to space groups C⁵_3v, C¹_3v and D³_3d. The required form factors are consistent with those used in our previous calculations for ZnS and CdIn₂S₄. The band structure of the α phase, the only one up to now for which optical data are available, compares quite satisfyingly with very recent photoemission and reflectivity experimental data. The computed band structures of the β and γ phases are very alike; on the contrary, interesting differences exist between these structures and the α phase which could easily be verified by experimental investigations.