Raman scattering studies of the GeS2-Ga2S3-CsCl glassy system

Room temperature Raman spectra of samples on four serials within GeS₂-Ga₂S₃-CsCl glassy system have been investigated systematically. Based on the analysis of the local coordination surroundings of Cs⁺ ions, the similarities and changes of Raman spectra for glass Ga₂S₃-2CsCl and bridged molecular GaCl₃ were explained successfully. With a profound consideration of the effect of Cs⁺ ions on mixed anion units (GaS_4−xCl_x) and bridged units (Ga₂S_6−xCl_x) and the corresponding micro-structural model, the Raman spectral evolution of the samples within GeS₂-Ga₂S₃-CsCl glassy system was reasonably elucidated.     

Raman and infrared spectra of CdIn2S4 and ZnIn2S4

Four one-phonon Raman lines have been found in CdIn₂S₄ (ZnIn₂S₄) spinels at 92 (72) cm^-1, 186 (184) cm^-1, 246 (253) cm^-1, and 367 (372) cm^-1 for incident photon energies well below the energy gap E_G ～ 2.4 (2.2) eV at 300 K. For photon energies close to E_G, the 367 cm^-1 mode underwent a resonant enhancement in CdIn₂S₄ and four infrared active but Raman forbidden F_1u modes appeared in the CdIn₂S₄ and ZnIn₂S₄ Raman spectra: TO modes at 226 (221) cm^-1 and 309 (312) cm^-1, and LO modes at 274 (272) cm^-1 and 340 (342) cm^-1.     

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.     

Electric and magnetic properties of the system between Cr3S4 and VCr2S4

The electrical resistivity and magnetization of V_xCr_3?xS₄ (x = 0.0 - 1.0) have been measured. Weak ferromagnetic moments are observed at low temperatures for x ≠ 0.0. Curie temperatures are determined from magnetization curves and resistivity anomalies.     

Spectroscopic analysis and magnetic susceptibility of CuO-TeO2-V2O5 glasses

This article reports on the structure of the glassy system xCuO-65TeO₂-(35−x)V₂O₅, 5≤x≤10 mol% which was studied using infrared (IR) and Raman spectroscopy methods as well as magnetic susceptibility measurements. IR and Raman spectroscopy analysis reveals the presence of four main absorption bands attributed to [TeO₃], [TeO₄], [VO₄], and [VO₅] structure units. It suggests that Cu²⁺ ions occupy the available open spaces of the Te-O network without straining the bonds too much. Increasing the concentration of Cu²⁺ ions beyond 5 mol% results in the modification of the glass by straining and locally distorting the surrounding of the Te-O network. The magnetic susceptibility of these materials was investigated in the temperature range of 5-200 K revealing the paramagnetic behavior described by the Curie-Weiss law and indicating the presence of weak antiferromagnetic exchange interactions between Cu ions. The magnetic entropy change of the glasses was determined based on the temperature and magnetic field dependence of magnetization.     

NMR of 69Ga, 71Ga,and 51V in V3−xFex Ga alloys

We have studied at room temperature the intensities and the Knight shifts of the nuclear magnetic resonances of ⁶⁹Ga, ⁷¹Ga, and ⁵¹V in V_3?xFe_xGa for Fe concentrations ranging from x = 0.04 to x = 0.6. The results interpreted in terms of a redistribution of the conduction electrons and a paramagnetism due to the Fe impurities.     

Measurement of the cation partitionings in FexV3−xS4 (x = 1.0 and 2.0) by powder neutron diffraction

Full profile analyses of powder neutron diffraction data have provided direct measures of the cation partitionings in the ternary sulfides Fe_xV_3−xS₄, x = 1.0 and 2.0. In Fe₂VS₄, the cation sites of type I in the metal-deficient layers are completely populated by iron atoms, giving a structural formula Fe^IFe^IIV^IIS₄. In FeV₂S₄, in contrast to earlier results, the partitioning of the iron atoms is found to be incomplete, the corresponding structural formula being Fe_0.72^IV_0.28^IFe_0.25^IIV_1.75^IIS₄. In both materials, the diffraction data suggest that the distributions of iron and vanadium atoms in the metal-rich layers are non-random. The present structural results are considered in relation to those from earlier experiments on this system.     

Effect of Bi2O3 on EPR, optical transmission and DC conductivity of vanadyl doped alkali bismuth borate glasses

Glasses with composition xBi₂O₃·(30−x)M₂O·70B₂O₃ (M=Li, Na) containing 2 mol% V₂O₅ have been prepared over the range 0≤x≤15 (x is in mol%). The electron paramagnetic resonance spectra of VO²⁺ of these glasses have been recorded in the X-band (≈9.3 GHz) at room temperature (RT≈300 K). Spin Hamiltonian parameters, g_∥, g_⊥, A_∥, A_⊥, dipolar hyperfine coupling parameter, P, and Fermi contact interaction parameter, K, have been calculated. The molecular orbital coefficients, α² and γ², have been calculated by recording the optical transmission spectra. In xBi₂O₃·(30−x)Li₂O·70B₂O₃ glasses there is decrease in the tetragonality of the V⁴⁺O₆ complex for x up to 6 mol% whereas for x≥6 mol%, tetragonality increases. In xBi₂O₃·(30−x)Na₂O·70B₂O₃ glasses there is increase in the tetragonality of the V⁴⁺O₆ complex with increasing x. The 3d_xy orbit expands with increase in Bi₂O₃:M₂O ratio. Values of the theoretical optical basicity, Λ_th, have also been reported. The DC conductivity increases with increase in temperature. The order of conductivity is 10⁻⁵ ohm⁻¹ m⁻¹ at low temperature and 10⁻³ ohm⁻¹ m⁻¹ at high temperature. The DC conductivity decreases and the activation energy increases with increase in Bi₂O₃:M₂O ratio.     

Influence of hydrogen absorption on magnetic properties of Zr(Fe1−xVx)2 ternaries

Measurements of magnetization, susceptibility and Mössbauer effect were made on Zr(Fe_1?xV_x)₂ ternaries and their hydrides. Absorbed hydrogen leads to a large increase (20–30%) in volume without a change in the crystal structure. Ferromagnetism in the Fe-rich region is enhanced by hydrogen absorption, whereas hydrogenation leads to suppression of superconductivity in the V-rich range. The Fe moments in the Zr(Fe_1?xV_x)₂ hydrides are remarkably larger than those in the corresponding host compounds. The Fe moment in the β-ZrFe₂ hydride extrapolated reaches to 2.9μ_B, which exceeds the saturation value in bcc Fe. The hyperfine fields of ⁵⁷Fe in both Zr(Fe_0.8V_0.2)₂ and the hydride distribute widely, indicating that the Fe moments are very sensitive to the local environment arround the Fe atoms. Arguments are presented that it is possible to interprete the Fe moment increase by hydrogenation in terms of a decrease in occupancy of the 3d-band state due to electron transfers from Fe to hydrogen and/or vanadium.     

Cr-doping effect on the structural, magnetic, transport properties and Raman spectroscopy of La(2+x)/3Sr(1−x)/3Mn1−xCrxO3 perovskites

A series of the double-doping samples La_(2+x)/3Sr_(1−4x)/3Mn_1−xCr_xO₃ (0?x?0.25) with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 have been fabricated. The structural, magnetic, transport properties and Raman spectroscopy have been investigated, and no apparent crystal structure change is introduced by Cr doping up to x=0.25. But the Curie temperature T_C and metal-insulator transition temperature T_MI are strongly affected by Cr substitution. The room temperature Raman spectra start exhibiting some new features following the increasing concentration of Cr substitutions. Moreover, it is worth noting that the frequency of the A_1g phonon mode can also be well correlated with the A-site mismatch effect (σ²), which is influenced mainly by the variety of the Sr content.     

Raman spectra and structure of As40−xS60Ix glasses

The Raman spectra of bulk As_40?xS₆₀I_x glasses with 0?x?20 have been studied. The obtained spectra can be successfully explained assuming the structure of these glasses to be composed of As₂S_3.45 polymer network matrix with dissolved AsBr₃ and S₈ molecules.     

Magnetic and electrical properties of BaVS3 and BaVxTi1−xS3

BaVS₃ has an hexagonal structure (space group P6₃/mmc), with chains of V⁴⁺ running along the C direction and largely separated from each other. Small single crystals of BaVS₃ prepared by flux method in BaCl₂ show evidence of 1D antiferromagnetic ordering below 70 K as previously observed on powder samples. The sulphur deficient compound gives ferromagnetic ordering below 16 K with an easy axis of magnetization along the C direction. BaV_xTi_(1?x)S₃ powder samples with 0 <x <1 of the same crystal symmetry as BaVS₃ have also been prepared and the effects of changing the population of d electrons along the V-Ti chains, from 0 electron per site, (BaTiS₃) to 1 electron per site, (BaVS₃) have been studied by X-ray, magnetic and resistivity measurements. A band model is suggested to explain these data.     

Inversion of conductivity type in Bi2Te3−xSx crystals

Transport parameters and optical properties of Bi₂Te_3?xS_x single-crystals with x=0–0.18 were studied. With increasing sulphur content the concentration of free current carriers decreases up to x=0.12, due to the interaction of S^x_Te defects with antisite defects Bi^′_Te, and then the p-type conductivity changes to the n-type. The optical gap of Bi₂Te_3?xS_x crystals increases with increasing S content. The obtained results led to the preparation of Bi₂Te₃-Bi₂Te_3?xS_xp–n junction by the heat treatment of p-type Bi₂Te₃ in S vapours.     

Synthesis of Gd2Ti2O7:Eu, V phosphors by sol-gel process and its luminescent properties

A red-emitting phosphor material, Gd₂Ti₂O₇:Eu³⁺, V⁴⁺, by added vanadium ions is synthesized using the sol-gel method. Phosphor characterization by high-resolution transmission electron microscopy shows that the phosphor possesses a good crystalline structure, while scanning electron microscopy reveals a uniform phosphor particle size in the range of 230-270 nm. X-ray photon electron spectrum analysis demonstrates that the V⁴⁺ ion promotes an electron dipole transition of Gd₂Ti₂O₇:Eu³⁺ phosphors, causing a new red-emitting phenomenon, and CIE value shifts to x=0.63, y=0.34 (a purer red region) from x=0.57, y=0.33 (CIE of Gd₂Ti₂O₇:Eu³⁺). The optimal composition of the novel red-emitting phosphor is about 26% of V⁴⁺ ions while the material is calcinated at 800  °C. The results of electroluminescent property of the material by field emission experiment by CNT-contained cathode agreed well with that of photoluminescent analysis.     

A XPS study of Fe1-χCuχCr2S4

The X-ray photoelectron spectroscopy is applied for a chalcogen chromite system Fe_{1 ? x}Cu_xCr₂S₄ to obtain microscopic information about valence states of the constituting atoms. The features of the Cu 2p spectra show that copper is always in a monovalent state in this system. This supports a model Fe³⁺_{1 ? x}Cu¹⁺_xCr³⁺₂S^1-_{2x ? 1}S^2-_{5 ? 2x} for a compositional range 0.5 < x < 1.     

Atomistic computer simulation studies of La1−xSrxVO3

Static computer simulation techniques have been employed for structural investigation of the La_1−xSr_xVO₃ series. Potential parameters for V³⁺-O²⁻ and V⁴⁺-O²⁻ have been derived which reproduces the crystal structures of end members with sufficient accuracy. Variations of lattice parameters and bond distances with Sr concentration have been studied. The calculated lattice parameters decrease with increase in the Sr concentration. A structural phase transition from orthorhombic to cubic is observed at 50% Sr doping level.     

Magnetic resonance study of phases in FeVO4-Co3V2O8 system

Multicomponent vanadates Co_3+xFe_4−xV₆O₂₄ have been synthesized using the solid state reaction method from Co₃V₂O₈ and FeVO₄.oxides. The electron paramagnetic resonance/ferromagnetic resonance (EPR/FMR) spectra of 20 samples containing solid state phases formed in the FeVO₄-Co₃V₂O₈ system have been recorded at room temperature. The howardevansite structure (H-type phase) is produced, which corresponds to the Co_2.616Fe_4.256V₆O₂₄ formula while a homogeneity range of lyonsite (L-type phase) type structure could be described by the Co_3+1.5xFe_4−xV₆O₂₄ formula (0.476<x<1.667). Considering the values of g-factor and linewidth of each registered spectrum the existence of three types of magnetic centers was inferred and correlated with phases detected by XRD method.     

Crystallization kinetics of Li2O-PbO-V2O5 glasses

Lead vanadate glasses of the system 5Li₂O−(45−x) PbO−(50+x) V₂O₅, with x=0, 5, 10, and 15 mol% have been prepared and studied by differential scanning calorimetry (DSC). The crystallization kinetics of the glasses were investigated under non-isothermal conditions applying the formal theory of transformations for heterogeneous nucleation to the experimental data obtained by DSC using continuous-heating techniques. In addition, from dependence of the glass-transition temperature (T_g) on the heating rate, the activation energy for the glass transition was derived. Similarly the activation energy of the crystallization process was determined and the crystallization mechanism was characterized. The results reveal the increase of the activation energy for glass transition which was attributed to the increase in the rigidity, the cross-link density and the packing density of these glasses. The phases into which the glass crystallizes have been identified by X-ray diffraction. Diffractograms of the transformed material indicate the presence of microcrystallites of Li_0.30V₂O₅, Li_0.67O₅V₂, LiV₆O₁₅, Li₄O₄Pb, and O₇Pb₂V₂ in a remaining amorphous matrix.     

The suppression of structural phase transformation in LaVO3 and La1−xSrxVO3 thin films fabricated by pulsed laser deposition

Highly oriented (100) thin films of LaVO₃ and La_1−xSr_xVO₃ have been fabricated by pulsed laser deposition in a reducing atmosphere. The films show a transition from insulating to metallic behaviour in the composition region of x, 0.175<x<0.200. In the single crystals of the antiferromagnetic insulating phase, a first-order structural phase transition is observed few degrees below the magnetic transition, which manifests itself as a kink in the temperature dependence of resistivity. In the highly oriented thin films of LaVO₃ and La_1−xSr_xVO₃ fabricated on lattice matched substrates in this study, the structural phase transformation in the insulating phase has been suppressed. The electrical conduction is found to take place via hopping through localized states at low temperatures. The metallic compositions show a non-linear (T^1.5) behaviour in the temperature dependence of resistivity. V (2p) core level spectra of these films show a gradual change in the relative intensities of V³⁺ and V⁴⁺ ions as the value of x increases.     

Spin-glass-like behaviour of the mixed spinel systems MnxZn1-xCr2O4 and Mn0.75Mg0.25(Cr1-yVy)2O4

NMR and susceptibility measurements have been made on a randomly mixed insulating ferrimagnet and antiferromagnet, Mn_xZn_1-xCr₂O₄. The thermoremanence and the induced unidirectional anisotropy were observed for concentrations lower than x = 0.80, after field cooling. The compound Mn_0.75Mg_0.25Cr₂O₄ shows similar behaviour. When the latter is doped with V³⁺ at the B sites, its magnetic anisotropy increases strongly, but the change in the unidirectional anisotropy is smooth.