A.N. Golovanov

Electronic structure of lead telluride-based alloys, doped with vanadium

The crystal structure, composition, galvanomagnetic properties in low magnetic fields (4.2 K ≤ T ≤ 300 K, B ≤ 0.07 T), and the Shubnikov–de Haas effect (T = 4.2 K, B ≤ 7 T) are studied in Pb1−x−ySnxVyTe (x = 0, 0.05–0.18) alloys synthesized by the Bridgman technique with variable vanadium impurity concentrations. It is shown that increasing the vanadium content leads to the formation of regions enriched in vanadium and of microscopic inclusions of compounds with compositions close to V3Te4. In Pb1−yVyTe stabilization of the Fermi level by a deep vanadium level, an insulator–metal transition, and a rise in the free electron concentration are observed as the vanadium content is increased. The variation in the free charge carrier concentration with increasing vanadium concentration in Pb1−yVyTe and Pb1−x−ySnxVyTe (x = 0.05–0.18) alloys is compared. Possible models for rearrangement of the electronic structure in Pb1−x−ySnxVyTe alloys with vanadium doping are discussed.

The first vanadate-carbonate, K2Mn3(VO4)2(CO3): crystal structure and physical properties.

Mixed potassium-manganese vanadate-carbonate, K(2)Mn(3)(VO(4))(2)(CO(3)), represents a novel structure type; it has been synthesized hydrothermally from the system MnCl(2)-K(2)CO(3)-V(2)O(5)-H(2)O. Its hexagonal crystal structure was determined by single-crystal X-ray diffraction with a = 5.201(1) Å, c = 22.406(3) Å, space group P6(3)/m, Z = 2, ρ(c) = 3.371 g/cm(3), and R = 0.022. The layered structure of the compound can be described as a combination of honeycomb-type modules of [MnO(6)] octahedra and [VO(4)] tetrahedra, alternating in the [001] direction with layers of [MnCO(3)] built by [MnO(5)] trigonal bipyramids and [CO(3)] planar triangles, sharing oxygen vertices. The K(+) ions are placed along channels of the framework, elongated in the [100], [010], and [110] directions. The title compound exhibits rich physical properties reflected in a phase transition of presumably Jahn-Teller origin at T(3) = 80-100 K as well as two successive magnetic phase transitions at T(2) = 3 K and T(1) = 2 K into a weakly ferromagnetic ground state, as evidenced in magnetization, specific heat, and X-band electron spin resonance measurements. A negative Weiss temperature Θ = -114 K and strongly reduced effective magnetic moment μ(eff)(2) ~ 70 μ(B)(2) per formula unit suggest that antiferromagnetic exchange interactions dominate in the system. Divalent manganese is present in a high-spin state, S = 5/2, in the octahedral environment and a low-spin state, S = ½, in the trigonal-bipyramidal coordination.

Vanadium deep impurity level in diluted magnetic semiconductors Pb1 − x − ySnxVyTe

The crystal structure, Sn and V distribution over the length of single-crystal ingots, and galvanomagnetic effects in low magnetic fields (4.2 K ≤ T ≤ 300 K, B ≤ 0.07 T) in Pb1−x−ySnxVyTe alloys (x = 0.05−0.21, y ≤ 0.015) are studied. It is shown that all the samples are single-phase, while the Sn and V concentrations exponentially increase from the beginning to the end of the ingots. Upon doping with V, a decrease in the concentration of free holes and a metal-insulator transition are found. They are related to the appearance of a deep impurity level of V in the band gap, electron redistribution between the level and the valence band, and pinning of the Fermi-level to the impurity level. The shift rate of the V level relative to the conduction band bottom is determined and a diagram of the reconstruction of the electronic structure of the Pb1 − x − ySnxVyTe alloy upon varying the host composition is suggested.

Galvanomagnetic properties and electronic structure of Pb1?x?ySnxVyTe under pressure

We study the galvanomagnetic properties in weak magnetic fields (4.2 ? T ? 300 K, B ? 0.07? T) and the Shubnikov-de Haas effect (T = 4.2 K, B ? 7 T) in single crystal Pb1?x?ySnxVyTe alloys under variation of alloy composition (x = 0.05?0.20, y ? 0.01) and hydrostatic compression up to 15 kbar. The increase of vanadium impurity content leads to the p-n-conversion and to a transition in the insulating phase due to the pinning of Fermi level by the donor-type deep vanadium impurity level situated under the bottom of the conduction band. We found that pressure induces a decrease of the activation energy of the vanadium level, the n-p-inversion of the conductivity type at low temperatures and an insulator?metal transition. In the metallic phase, a sharp increase of the Hall mobility (up to 3???105?cm2? V?1? s?1) and appearance of Shubnikov-de Haas oscillations are observed at helium temperature. The pressure and temperature coefficients of vanadium deep level energy are determined, and the diagram of the electronic structure rearrangement for Pb1?x?ySnxVyTe under pressure is proposed.

Magnetic Properties of Diluted Magnetic Semiconductors Pb1-xVxTe

The structural and magnetic properties as well as the electron paramagnetic resonance in Pb1-xVxTe (х0.7 at.%) solid solutions have been investigated. It was found that the magnetic field and the temperature dependences of magnetization have a paramagnetic character, connected obviously with the paramagnetic contribution of vanadium impurity isolated ions. Electron paramagnetic resonance spectra were measured and the temperature dependence of the g-factor in the temperature range 85-200 K was obtained.

Structural phase transitions in the kagome lattice based materials Cs2−xRbxSnCu3F12 (x = 0, 0.5, 1.0, 1.5)

The solid solution Cs2−xRbxSnCu3F12 (x = 0, 0.5, 1.0, 1.5) has been investigated crystallographically between 100 and 300 K using synchrotron X-ray powder diffraction and, in the case of x = 0, neutron powder diffraction. For Cs2SnCu3F12 (x = 0), there is a structural transition from the previously reported room temperature rhombohedral symmetry (Rm) to monoclinic (P21/n) symmetry at 170 K. This transformation is repeated for the x = 0.5 composition, but with an increased transition temperature of 250 K. For x = 1.0 the monoclinic phase is found at 300 K, suggesting that the transition temperature is increased even further. For x = 1.5 a different behaviour, more akin to that previously reported for Rb2SnCu3F12, is found: a single phase transition between rhombohedral symmetry (R) and triclinic symmetry (P) is found at 280 K. In agreement with previous single crystal studies, Cs2SnCu3F12 powder exhibits strong antiferromagnetic interactions (Θ ~ −268 K) and long-range magnetic order at TN ~ 19.3 K. The finite magnetic moment observed for T < TN might be explained by a Dzyaloshinskii–Moriya interaction, due to the lowering of symmetry from rhombohedral to monoclinic, which was not suggested in the earlier single crystal study.

Copper rubidium diphosphate, Rb2Cu3(P2O7)2: synthesis, crystal structure, thermodynamic and resonant properties

A new compound, Rb2Cu3(P2O7)2, has been obtained from the melt in the Rb–Cu–P–O system. Its monoclinic crystal structure was determined by single-crystal X-ray diffraction: space group P21/c, Z = 2, a = 7.7119(8) A, b = 10.5245(9) A, c = 7.8034(9) A, β = 103.862(5)° at 293 K, R = 0.030. The copper ions show coordination number (CN) 6 (4+2, distorted tetragonal bipyramidal). Trimers of [CuO6] polyhedra sharing cis-edges form together with diphosphate groups of two tetrahedra [P2O7] a microporous 3D framework with channels open along the c direction. The rubidium ions positioned in the channels show CN 10. The new phase is isotypic to Cs2Cu3(P2O7)2. The regular changes in cell dimensions in the row Cs2Cu3(P2O7)2 → Rb2Cu3(P2O7)2 are caused by the compression of channel volumes due to decrease of the Cu–O–P angles in the framework windows. An electron spin resonance study indicates appearance of short range magnetic correlations below ∼120 K, long range magnetic order takes place at TN = 9.2 K as follows from magnetization and specific heat measurements. First principles calculations of the magnetic exchanges indicate that the effective Cu–Cu hopping interactions corresponding to super–super-exchange paths involving P atoms are much stronger than those within the edge-sharing Cu2–Cu1–Cu2 trimer units.

The magnetocaloric effect and magnetic transitions in hydride compounds: GdNiH3.2 and TbNiH3.4

The paper presents the investigation of GdNiH3.2 and TbNiH3.4 hydrides magnetic transitions and magnetocaloric properties. The isothermal magnetization data in the fields up to 5T are obtained for GdNi and TbNi compounds and their hydrides and the values of magnetic entropy change are calculated. The maximum values of magnetic entropy change ΔSM in GdNiH3.2 and TbNiH3.4 are extremely large. It is shown that the hydrogenation shifts ΔSM(T) maximum to lower temperatures.

Insulator-Metal Transition in Diluted Magnetic Semiconductor Pb1-x-ySnxVyTe under Pressure

The galvanomagnetic properties in weak magnetic fields (4.2T300 K, B0.07 T) as well as Shubnikov-de Haas effect (T=4.2 K, B7 T) in the single crystal Pb1-x-ySnxVyTe (x=0.20, y0.01) under hydrostatic compression up to 15 kbar have been investigated. It is shown that under pressure the decrease of activation energy of vanadium deep level, n-p-inversion of the conductivity type at low temperatures and insulator-metal transition take place. In the metallic phase sharp increase of the Hall mobility and appearance of Shubnikov-de Haas oscillations at helium temperature are observed. The pressure coefficient of vanadium level energy is determined and the diagram of the electronic structure rearrangement for Pb1-x-ySnxVyTe under pressure is proposed.

Rearrangement of electronic structure of Pb1-x-ySnxVyTe under pressure

The galvanomagnetic properties in weak magnetic fields (4.2≤T≤300 K, B≤0.07 T) and the Shubnikov-de Haas effect (T = 4.2 K, B≤7 T) in the single crystal Pb1-x-ySnxVyTe (x = 0.05-0.20, y≤0.01) alloys at atmospheric pressure and under hydrostatic compression up to 15 kbar have been investigated. We found that the increase of the vanadium impurity content leads to the p-n-conversion, transition to the insulating phase and to the pinning of Fermi level by the deep impurity level, lying under the bottom of the conduction band. Under pressure a decrease of the activation energy of the vanadium level, the n-p-inversion of the conductivity type at low temperatures and an insulator-metal transition occur. The pressure and the temperature coefficients of vanadium deep level energy are determined and the diagram of the electronic structure rearrangement for Pb1-x-ySnxVyTe under pressure is proposed.