A. P. Vokhmyanin

Enhanced power factor and high-pressure effects in (Bi,Sb)2(Te,Se)3 thermoelectrics

We investigated the effects of applied high pressure on thermoelectric, electric, structural, and optical properties of single-crystalline thermoelectrics, Bi2Te3, BixSb2−xTe3 (x = 0.4, 0.5, 0.6), and Bi2Te2.73Se0.27 with the high thermoelectric performance. We established that moderate pressure of about 2–4 GPa can greatly enhance the thermoelectric power factor of all of them. X-ray diffraction and Raman studies on Bi2Te3 and Bi0.5Sb1.5Te3 found anomalies at similar pressures, indicating a link between crystal structure deformation and physical properties. We speculate about possible mechanisms of the power factor enhancement and suppose that pressure/stress tuning can be an effective tool for the optimization of the thermoelectric performance.

Magnetic phase transitions in TbNi5 single crystal : Bulk properties and neutron diffraction studies

Magnetic susceptibility, heat capacity, and neutron diffraction measurements have been carried out in order to study the unusual magnetic ordering of TbNi5 using a single crystal over the temperature region of 2–30 K. Two spontaneous magnetic transitions have been observed: one is a second-order transition from a paramagnetic state to an incommensurate structure (Tp = 24 K), and the other, a first-order transition from the incommensurate structure to a lock-in phase (Tfc = 10 K). We also found an irreversible magnetic field-induced transition from the modulated structure to the ferromagnetic collinear one.

Magnetic structure of Er5Si3 at T ≥ 20 K

The symmetry analysis of magnetic structures of the Er5Si3 compound, possible at temperatures of 20 and 25 K, has been performed to interpret the experimental elastic neutron scattering data obtained at these temperatures. It was shown that the smallest dispersion factor Rm at these temperatures corresponds to the modulated magnetic structure in which the magnetic moments of Er atoms are directed along the a3 axis of the unit cell and form an antiferromagnetic longitudinal spin wave characterized by the wave vector k = μb3, where μ is approximately 0.264 and 0.274 at 20 and 25 K, respectively. The analysis of the temperature behavior of the magnetic reflection intensities demonstrated that the Er5Si3 compound is paramagnetic at 30 K.

Magnetic structure of Er5Ge3 at 4.2 K

A symmetry analysis of the possible magnetic structures of Er5Ge3 in the ground state is performed using the results of measurements of elastic magnetic neutron scattering at 4.2 K. It is shown that the minimum discrepancy factor Rm≈9.5% corresponds to a modulated collinear magnetic structure in which the magnetic moments of erbium atoms are oriented along the a3 axis of the unit cell of the crystal structure and induce an antiferromagnetic longitudinal spin wave (AFLSW). The magnetic structure is characterized by the wave vector k=2π(0, 0, µ /a3) (where µ≈0.293) and the modulation period λ≈3.413a3. The magnetic ordering temperature TN≈38 K is determined from the temperature dependence of the intensity of magnetic reflections.

Magnetic structure of the Nd 5 Ge 3 compound

Neutron diffraction measurements of the Nd5Ge3 intermetallic compound with a hexagonal structure (space group P63/mcm) have been performed at temperatures of ∼10 and 293 K. The basis functions of irreducible representations of the space group D6h3 (P63/mcm), which are calculated as a result of the symmetry analysis of possible magnetic structures with the wave vector k = μb1, are used to facilitate the search for a real model of the magnetic structure of the compound.

Effects of magnetic anisotropy and exchange in Tm 2 Fe 17

Neutron diffraction experiments have been carried out to study the magnetocrystalline anisotropy of two (2b and 2d) Tm sublattices and four (4f, 6g, 12j, and 12k) Fe sublattices in ferrimagnetic compound Tm2Fe17 (space group P63/mmc). We have determined the temperature dependence of the magnitude and orientation of magnetization for each of the thulium and iron sublattices in the range (10–300) K. A spontaneous rotation (at about 90 K) of the Tm and Fe sublattice magnetizations from the c-axis to the basal plane is accompanied by a drastic change in the magnetization magnitude, signifying a large magnetization anisotropy. Both Tm sublattices exhibit an easy-axis type of the magnetocrystalline anisotropy. The Fe sublattices manifest both the uniaxial and planar anisotropy types. The sublattice formed by Fe atoms at the 4f position reveals the largest planar anisotropy constant. The Fe atoms at the 12j position show a uniaxial anisotropy. We find that the inelastic neutron scattering spectra measured below and above the spin-reorientation transition are remarkably different.