Ionic conductivity of crystalline and glassy Mg4.5Na7(P2O7)4

In our work single crystals of Mg_4.5Na₇(P₂O₇)₄ were prepared, pulverized, pressed into pellets and sintered in order to measure the electrical conductivity of polycrystalline specimens. The conductivity was also measured on glassy specimens obtained by the melting of previously prepared crystals. The electrical conductivities at 25°C with values of the order of 10⁻¹⁶ Ω⁻¹ cm⁻¹ for polycrystalline samples and a value of the order of 10⁻¹⁴ Ω⁻¹ cm⁻¹ for glass, show that the glassy phase of Mg_4.5Na₇(P₂ because of its greater molar volume and loosely packed structure, is a better matrix for ionic motion.     

Ferroelectric phase transitions in NH4HSeO4 crystals

Results are presented of measurements of spontaneous polarization, electric permittivity and electrical conductance of NH₄HSeO₄ crystals in three principal crystallographic directions from liquid nitrogen temperature to 320 K. NH₄HSeO₄ crystals exhibit ferroelectric properties in the a-axis direction between 106–250 K, whereas in the b-direction they are pyroelectrie over the entire temperature range. The calculated activation energy (1.42 eV) is indicative of semiconducting behavior.     

Single crystal synthesis and electrical properties of CdSnO3, Cd2SnO4, In2TeO6 and Cdln2O4

Single crystals of CdSnO₃, Cd₂SnO₄, ln₂TeO₆ and CdIn₂O₄ were grown by either flux or high pressure methods. High electrical conductivity resulted from chemical substitution or oxygen deficiency. Crystallographic and conductivity data are given.     

Proton conduction in (NH4)3 H(SO4)2 single crystals

D.C. electrical conductivity, DTA and coulometric studies on (NH₄)₃ H(SO₄)₂ single crystals are made. Conductivity is markedly anisotropic with maximum along c¹ direction. A sudden jump in the conductivity plot along c¹ direction at 413 K is supported by a large endothermic peak in DTA, confirming the presence of transition at this temperature. The values of activation energy calculated from conductivity measurements indicated that the charge carriers are protons. This was further confirmed by coulometric experiment where the gas evolved was hydrogen, as established by a gas chromatograph and the volume of H₂ released agreed with that expected from electrolysis. The mechanism of protonic conduction in this crystal is discussed.     

Magnetic and transport properties of YbRhIn5 and YbIrIn5 single crystals

Single crystals of YbRhIn₅ and YbIrIn₅ have been grown by flux method. The crystals were characterized by means of X-ray diffraction, magnetic and electrical transport measurements. Both compounds were found to be weak diamagnets with metallic character of the electrical conductivity and the Seebeck coefficient.     

Influence of Se-deficiencies on the properties of In-doped CdCr2Se4 single crystals

Lattice constant, Curie temperature, and electrical conductivity of CdCr₂Se₄:In single crystals have been measured after heat treatments of the crystals in Se atmosphere and under streaming hydrogen. By these treatments, the concentration of the Se vacancies and of the charge carrier concentration is altered drastically. The lattice constant as well as the magnetic ordering temperature have been found not to be affected by these heat treatments.Since the Se vacancies act as doubly changed donors, the electrical conductivity is strongly dependent on the concentration of the Se vacancies. A resistivity anomaly and large magnetoresistance are observed only in crystals with considerable Se deficiency. From these results it is concluded that the magnetoresistance is caused by hopping conduction between donor sites partly emptied by compensating A-site vacancy acceptors. Large magnetoresistance is found in samples with considerable Se deficit because only in this case the conduction at lower temperatures is dominated by the impurity band.     

Electrical investigations in the normal and incommensurate phases of [N(CH3)4]2ZnCl4 single crystals

The dielectric permittivity (?) of TMA-ZC single crystals was measured along the mean crystallographic axes a, b and c, in a temperature range from 273 to 340 K. The ?-T relationship exhibited peak values at T_i=296 K and T_c1=279 K for the three axes. These peaks are attributed to the contribution of discommensurations. The d.c. and a.c. electrical conductivity showed anomalous variation at the same transition temperatures, with a remarkable change in the value of the activation energy around the transition temperatures. The j-E characteristic indicates different types of electrical conduction. The mechanism of the phase transition and the electrical process were discussed on the basis of Shottky and Frenkel conduction mechanisms.     

High critical magnetic field superconductor La3S4

We report electrical conductivity, specific heat and magnetization measurements on La₃S₄ single crystals. The results show, that La₃S₄ is a strong coupling superconductor with a BCS coherence length of 132 Å. This extremely low value makes La₃S₄ an intrinsic high critical magnetic field superconductor with a Landau-Ginsburg parameter κ₀ of 20. For the temperature gradient of the upper critical magnetic field at the transition temperature we find values up to 35 kG/K.     

Transport properties of CdIn2S4 single crystals

Several transport properties have been studied on CdIn₂S₄ singlê crystals with different degrees of deviation from stoichiometry. The energy gap at 0 K was determined from the electrical measurements to be 2.2 eV. The anisotropy of the magnetoresistance effect was found and it was suggested that the minima of the conduction band were located at points along the [100] directions in k space. From an analysis of the mobility data it was found that the high resistivity of the samples is due to compensation of donors by acceptors introduced by excess sulphur. Several band parameters, the carrier scattering mechanisms and the impurity levels were determined. The thermal conductivity was measured from 4 K to 300 K and analysed by Callaway's formalism.     

Heat capacity and phase transitions in NH4LiSO4, Csx(NH4)1?xLiSO4, and RbLiSO4

The heat capacity of NH₄LiSO₄, RbLiSO₄ and Cs_x(NH₄)_1−x LiSO₄ crystals and its behavior over a broad temperature range including the phase transition regions were studied. The entropy changes corresponding to structural transformations in these crystals were found not to be characteristic of straightforward ordering of structural blocks. The results obtained are discussed in terms of phenomenological theory and model concepts. __________ Translated from Fizika Tverdogo Tela, Vol. 47, No. 4, 2005, pp. 696–704. Original Russian Text Copyright ? 2005 by Flerov, Kartashev, Grankina.     

Electron paramagnetic resonance of Mn IN (NH4)2Co2(SO4)3 and (NH4)2Ni2(SO4)3 single crystals

EPR studies are reported on single crystals of ammonium cobalt sulphate and ammonium nickel sulphate containing Mn²⁺ ions. In each case only one magnetic complex of Mn²⁺ ion is found. The resonance lines in the case of Mn²⁺ doped ammonium nickle sulphate are characterised by a strong angular dependence of line intensities. The resonance lines in both the cases are fitted to a spin-Hamiltonian corresponding to orthorhombic symmetry.     

The preparation and characterization of the cobalt skutterudites CoP3, CoAs3 and CoSb3

Single crystals of the cobalt skutterudites CoP₃, CoAs₃ and CoSb₃ have been prepared by the chemical vapor transport technique using chlorine as the transport agent. Chemical analysis and density measurements were used to determine accurately the stoichiometry of these crystals. Physical properties of chemically pure single crystals were obtained by X-ray diffraction, magnetic susceptibility and electrical measurements. CoP₃ is a diamagnetic semiconductor with an optical band gap of 0.45 eV. CoAs₃ and CoSb₃ are also diamagnetic and show an increase in their resistivity as a function of increasing temperature. In addition, they do not give any measurable absorption edge but absorb radiation at all frequencies from 0.4 to 4.3 eV.     

In situ pressure-induced solid-state amorphization in Sm2(MoO4)3, Eu2(MoO4)3 and Gd2(MoO4)3 crystals: chemical decomposition scenario

The effect of pressure on the phase transformations in Sm₂(MoO₄)₃, Gd₂(MoO₄)₃ and Eu₂(MoO₄)₃ crystals has been studied in situ using synchrotron radiation. All three isostructural compounds undergo a structural phase transition at 2.2-2.8 GPa to a new phase, which is interpreted as a possible precursor of amorphization. Amorphization in these crystals occurs irreversibly over a wide pressure range, and its mechanism, interpreted as a chemical decomposition, is found to be weakly affected by the degree of hydrostaticity.     

Neodymium luminescence in chalcogenides of europium-gallium: EuGaS4 and EuGa2Se4

Photoluminescence of chalcogenides of europium-gallium, EuGa₂S₄ and EuGa₂Se₄, doped with neodymium is investigated. The positions of Stark levels are determined from the spectra. The symmetry of luminescence centres is shown to be lower than cubic and the existence of nonequivalent centers is established. At 77 K the decay time of luminescence from the excited levels of Nd³⁺ depends on the spin of the states. That indicates a slow relaxation rate in the crystals under investigation. It is probable that these crystals can be used as effective luminophores.     

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.     

Growth,structure and physical properties of gadolinium doped Sr2IrO4 single crystal

A series of Gd-doped Sr₂IrO₄ single crystals were grown using a flux method. Analysis of the temperature-dependent resistance of these crystals reveals that these samples show two-dimensional weak localization at 150 to 300 K, while three-dimensional variable range hopping (VRH) behavior is observed at temperatures lower than 150 K. Two localization lengths are observed in the VRH behavior, with a transition temperature of around 88 K. Correspondingly, temperature-dependent magnetization observations along the ab-plane reveal magnetic anomalies at both 150 and 85 K. This work demonstrates the correlation between the electrical and magnetic properties of 5d transition-metal compounds.     

Electrical properties of FeCr2S4 and CoCr2S4

Resistivity and magnetoresistance are measured as functions of temperature (77–450 K) on several FeCr₂S₄ and CoCr₂S₄ single crystals (as-grown, annealed in vacuum, annealed in S-atmosphere, and doped) and found to be very sensitive to these treatments. These results are discussed on the basis of the model consisting of donor level due to S-deficiencies in addition to Fe²⁺ and Cr²⁺ levels.     

Defects in Nd2CuO4 and Nd1.815Ce0.185CuO4 single crystals

We describe here four kinds of defects found in Nd₂CuO₄ and Nd_1.815Ce_0.185CuO₄ single crystals grown by the traveling solvent floating zone method. These defects include: (1) cracks, (2) inclusions, (3) gas bubbles and (4) uneven distribution of Ce⁴⁺. The formation mechanism of these defects and the measures of eliminating these defects are discussed. It is thought that the preparation of raw materials and stringent growth conditions are necessary to obtain high quality of Nd₂CuO₄ and Nd_1.815Ce_0.185CuO₄ single crystals.     

Effective ionic charges in CdGa2Se4 and CdGa2S4

Available experimental data (Raman and Infrared) on the optic phonon spectra in defect chalcopyrite crystals CdGa₂Se₄ and CdGa₂S₄ are used to determine the dynamical ionic effective charges. The results are compared with those found in other ternary compounds as spinels and chalcopyrites. An emperical correlation between the normalized Szigeti charges and the optical dielectric constant is discussed.     

Cation ordering and crystal structures in AGa2X4 compounds (CoGa2S4, CdGa2S4, CdGa2Se4, HgGa2Se4, HgGa2Te4)

Single crystals of some AGa₂X₄ compounds (CoGa₂S₄, CdGa₂S₄, CdGa₂Se₄, HgGa₂Se₄, HgGa₂Te₄) were prepared by chemical vapour deposition and flux method.The X-ray structural investigations indicated blende or defect chalcopyrite structures.A simple relationship is suggested between the c/a ratio and the cationic sublattice ordering.