H. Duda

Dielectric and magnetic permittivities of three new ceramic tungstates MPr2W2O10 (M = Cd, Co, Mn)

Broadband dielectric spectroscopy measurements revealed an anomalously large relative permittivity value (ε r = 884) for MnPr2W2O10, a smaller value (ε r = 156) for CoPr2W2O10 and the smallest value (ε r = 22) for CdPr2W2O10 at low frequency (ν = 0.1 Hz) and above room temperature in the insulating and paramagnetic state. Below 273 K, the relative permittivity (ε r ∼ 24) did not depend significantly on frequency for all the tungstates under study. Electrical resistivity, thermoelectric power, electron paramagnetic resonance, magnetic susceptibility and magnetization provided experimental evidence that the studies tungstates were paramagnetic insulators with low n-type conduction. Only in the case of MnPr2W2O10 was a ferrimagnetic order below 45 K observed. These effects are discussed within the framework of Maxwell–Wagner polarization, chemical covalent bonds and porosity mechanism.

Paramagnetic Behaviour in RE2W2O9 Tungstates (RE = Pr, Nd, Sm – Gd)

Magnetic susceptibility measurements revealed a disordered state of magnetic moments above 4.2 K for all compounds under study, and a weak response to the magnetic field and the temperature for Sm2W2O9 and Eu2W2O9 tungstates. The temperature independent component of magnetic susceptibility has a positive value for RE2W2O9 (RE = Pr, Nd and Gd) indicating a domination of van Vleck contribution. Only for Gd2W2O9 the magnetization is a universal function of µ0H/T, characteristic for the superparamagnetism.

Electrical and Magnetic Properties of CuEu2W2O10 and Cu3Eu2W4O18

Magnetic susceptibility measurements showed both a weak response to magnetic field and a lack of the Curie-Weiss region for CuEu2W2O10 and Cu3Eu2W4O18 tungstates characteristic for the multiplet widths comparable to thermal energy. Magnetization measurements displayed the linear temperature dependence with the lower magnetic moment for Cu3Eu2W4O18 in comparison with CuEu2W2O10, indicating that the effect of the electric charges associated with the surrounding ligands can change the multiplet width of individual states. It is affecting the electrical properties of examined tungstates which reveal the insulating state and low relative permittivity εr ~ 29 in case of CuEu2W2O10 and the thermally activated p-type electrical conduction for Cu3Eu2W4O18 with the activation energy of 1.11 eV and the large value of εr ~ 217 above the room temperature.

Electrical resistivity dip in Sb x V y Mo z O t phases

Electrical resistivity dips have been discovered in the temperature range 100–500 K both in the SbVO4.96 matrix and the Sb x V y Mo z O t phases for 10 mol% solubility of MoO3 in SbVO5. As the Sb content increases and simultaneously the V content decreases, the value of the resistivity at the dip, ρ d, decreases and shifts the dip to higher temperatures. The magnetic measurements showed a spontaneous magnetization and parasitic magnetism of the solid solutions under study. Characteristic for parasitic magnetism is a small value of the magnetic moment, here 0.014 μ B/f.u. at 4.2 K and at a magnetic field of 14 T as well as a small value of the mass susceptibility, here 10−5 cm3/g. The value of the Néel temperature, T N ≤ 8 K, and the Curie–Weiss temperature, θ CW ≤ −208 K, indicate a collinear antiferromagnetic (AFM) order. We suggest that neither the magnetism nor the Mo-content can be correlated with the resistivity anomalies. Therefore, these effects may rather be interpreted in terms of a small-polaron gas in the resistivity dip area. Alternatively, they could mark a lattice/electronic entropy-driven incomplete metal–insulator transition.

Electrical Resistivity Anomalies in SbxVyMozOt Phases

Electrical resistivity dips in the temperature range 269-287 K and n-type conductivity below 415 K for solid solutions of MoO3 in SbVO5 with general formula SbxVyMozOt are observed. The electrical resistivity anomalies are interpreted as due to conduction of small-polarons, generated here as electrons together with distortions of their associated defective oxygen lattice and/or alternatively based on the crossover of electronic or polaronic states.

The electrical conductivity of the strongly defective HgCr 2 Se 4 single crystals

The effect of vacancies on the electrical conductivity in HgCr 2 Se 4 spinels is considered. For this purpose the X-ray studies, the electrical conductivity and the thermopower measurements and the calculation of the vacancy model were used. The above investigations showed that: 1) the single crystals under study are p-type semiconductors, 2) the as grown and vacuum annealed single crystals exhibit Arrhenius plot of the electrical conductivity, 3) the copper doping single crystals reveal the jump of the electrical conductivity near Curie temperature, and 4) the strong defectiveness of the spinel structure makes the magnon excitations impossible below the Curie temperature.

Magnetic Properties of CdMoO4:Dy3+ Single Crystal

X-ray diffraction measurement at 298 K of CdMoO4:Dy3+ showed that the molybdenum ions are tetrahedral coordinated and Cd/Dy – dodecahedral coordinated. The Dy3+ ions are randomly distributed in the unit cell, substituting the Cd2+ ones. The temperature dependence of ac and dc magnetic susceptibility showed a lack of the Curie-Weiss behaviour and a weak response to the magnetic field. The magnetization isotherms, M(H), showed a paramagnetic-diamagnetic transition at 17 K for 〈100〉 direction and at 35 K for 〈001〉 one in the magnetic field of 70 kOe. As the temperature increased this transition was moving toward smaller magnetic fields.

Influence of Ce substitution on the critical properties of 3D-Heisenberg Cd x Ce y Cr2Se4 ferromagnets

The critical behaviour of single-crystalline Cd x Ce y Cr2Se4 spinels investigated around the paramagnetic–ferromagnetic phase transition revealed an increase in the critical exponents from β = 0.359, γ = 1.186, δ = 4.404 at T C = 131.95 K for Cd0.96Ce0.03Cr2Se4 to β = 0.382, γ = 1.286, δ = 4.487 at T C = 133.73 K for Cd0.83Ce0.13Cr2Se4. These values are close to those predicted by the renormalization-group calculations for a three-dimensional isotropic short-range Heisenberg ferromagnet, although their increase with the Ce substitution suggests a shift towards the mean-field estimates, characteristic of isotropic long-range exchange interactions. These results are being considered in relation to the strong lowering of magnetic moment from 5.67 µB/f.u. for y = 0.03 to 3.0 µB/f.u. for y = 0.13.

Transport processes in the insulating solid solutions of Cr2V4-xMoxO13+0.5x

The measurement of the electrical resistivity, the Seebeck effect and the UV/VIS absorption were carried out in the solid solutions under study. It was observed that: 1) with the increase of temperature the activation energy changes rapidly for all samples, 2) at about 700 K a phase transition from insulating to semiconducting state takes place, 3) the shift of the absorption edge to higher wavelengths is explained by the d-d electron transitions between the chromium ions with the valence state 3+ and 2+.

Influence of Co Moment on Magnetic Properties of Co2Sm2W3O14 Tungstate

The Co2Sm2W3O14 compound crystallizes in the orthorhombic system, and melts congruently at 1443 K. The magnetic measurements showed that Co2Sm2W3O14 is a paramagnet in the temperature range 4.2-225 K showing both the residual magnetic interactions since the paramagnetic Curie-Weiss temperature   0 and the uncompensated temperature independent contributions of magnetic susceptibility since 0  0. The Brillouin fit of the Landé factor revealed an increase of the orbital contribution to the total magnetic moment of the compound what seems to be responsible for its hard and spontaneous magnetization at low temperatures.