Papers by Author: Tadeusz Groń

Abstract: New scheelite type Cd_1-3xDy_2x[]_xMoO₄ solid solution, where 0.0098 ≤ x ≤ 0.2 and [] denotes the cationic vacancies, was investigated by using UV-vis-NIR and dielectric spectroscopies at room temperature and in the temperature range 77-400 K, respectively. These studies showed the optical band gap energy E_g above 3.4 eV and relative low dielectric permittivity: ε_r ~ 6.0, 9.5, 7.0 and 8.2 for the x parameter 0.0098, 0.0839, 0.1667 and 0.2000, respectively. ε_r slightly decreases with increasing frequency and slightly increases at higher temperatures. All samples showed the maximum permittivity in the temperature range of 250-350 K. The loss tangent exhibited similar behaviour and its maximum value did not exceed 0.25. These results are discussed in a context of the shallow trap levels and the Maxwell-Wagner model.

Abstract: 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.

Abstract: Magnetization, M, and susceptibility, , measurements of Cu_xCr_yTi_zSe₄ (z = 0.24 and 0.35) showed both strong lowering of magnetic moment in comparison with CuCr₂Se₄ matrix and zero-field-cooling (ZFC) – field-cooling (FC) susceptibility splitting characteristic for the spin-glass (SG) behaviour. Isothermal magnetization curves, M(H), easy saturate and large both the Curie (TC ~ 250 K) and Curie-Weiss (θCW ~ 280 K) temperatures indicate the ferromagnetic order which coexists with the SG state. The critical behaviour of polycrystalline Cu_xCr_yTi_zSe₄ spinels investigated around the paramagnetic–ferromagnetic (PM-FM) phase transition revealed that the values of critical exponents are close to those predicted by the mean field model for long-range interactions only for Cu_1.02Cr_1.77Ti_0.24Se₄. It means that the larger spin frustration the more difficulty is it to describe the critical properties with the aid of the theoretical models.

Abstract: 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.

Abstract: 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.

Abstract: The complex ac dynamic magnetic susceptibility was used to study the magnetic state in the polycrystalline Cd_0.88Cr_1.96V_0.31Se₄ spinel. The temperature dependences of the zero field in-phase (real part) and out-of-phase (imaginary part) components of fundamental susceptibility showed the mictomagnetic behavior at low temperature and a weak Hopkinson-like effect close to the ordering temperature. These effects are also evidenced by the non-vanishing both the real and imaginary components of the 2^nd and 3^rd harmonics in the region of magnetic order.

Abstract: A fitting procedure of the Curie-Weiss law, eliminating the temperature independent contribution (0) from the experimental susceptibility data, was used for determination of the magnetic parameters, i.e. a Curie constant, a Curie-Weiss temperature and an effective magnetic moment, because the theoretical considerations showed that the Curie-Weiss law is invalid for 0  0, even small. This method revealed for the HgCo(NCS)4 paramagnet, commonly accepted as a magnetic susceptibility standard, 0 = 2.43710-6 cm3/g, while for the ZnCr2Se4 antiferromagnet, known as a matrix of various diluted systems, – 0 = -2.56610-6 cm3/g, for comparison.

Abstract: A high spin (HS) – low spin (LS) transition has been discovered in the Co[Cr0.5Ga1.5]S4 spinel, where the Co ions occupy tetrahedral sites and the Cr and Ga ions octahedral ones. The latter are diluted by the non-magnetic Ga-ions. Application of magnetic fields revealed ferrimagnetic order with a Curie temperature TC = 126 K, a strong suppression of the magnetic susceptibility () and a slight shift of TC to lower temperatures. A cusp at 15.6 K on the ac (T) curve suggests a spin frustration of the re-entrant type. Characteristic for the HS-LS transition is an inflection point at 150 K on the 1/(T) curve, a lack of the Curie-Weiss region above TC and a small value of the magnetization, 4.76 emu/g at 4.4 K and at a magnetic field of 57.5 kOe.

Abstract: 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.

Abstract: 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.