ABSTRACT
Sulfides and selenides of copper and silver have been intensively studied, particularly as potentially efficient thermoelectrics. Ag3CuS2 (jalpaite) is a related material. However very little is known about its physical properties. It has been found that the compound undergoes several structural phase transitions, having the tetrahedral structural modification I41/amd at room temperature. In this work, its band structure, phonon spectrum and thermoelectric properties were studied theoretically and experimentally. Seebeck coefficient, electrical conductivity and thermal conductivity were measured in a broad temperature range from room temperature to 600 K. These are the first experimental data on transport properties of jalpaite. Ab initio calculations of the band structure and Seebeck coefficient were carried out taking into account energy dependence of the relaxation time typical for the scattering of charge carriers by phonons. The results of the calculations qualitatively agree with the experiment and yield large values of the Seebeck coefficient characteristic for lightly doped semiconductor. The influence of intrinsic defects (vacancies) on the transport properties was studied. It was shown that the formation of silver vacancies is the most probable and leads to an increase of hole concentration. Using the temperature dependent effective potential method, the phonon spectrum and thermal conductivity at room temperature were calculated. The measurements yield low lattice thermal conductivity value of 0.5 W/(m K) at 300 K, which is associated with the complex crystal structure of the material. The calculated room temperature values of the lattice thermal conductivity were also small (0.14-0.2 W/(m K)).
ABSTRACT
Monosilicides of transition metals crystallizing in a B20 (FeSi-type) structure (space group P2 1 3, #198) possess a wide range of specific properties. Among them are semiconductors, metals, and paramagnetic, diamagnetic, and ferromagnetic compounds. Some of them were studied as promising thermoelectric materials. Recently, B20 monosilicides have attracted attention as a new class of topological semimetals with topological charge greater than unity. In the present work, we analyze the electronic structures of B20-type monosilicides of the fourth, fifth, and sixth periods of the Periodic Table in order to reveal their common features and peculiarities. To make this analysis more consistent, we performed a density-functional study of the electronic structures of the monosilicides in a unified manner. We reviewed the results of previous calculations and the available experimental data, comparing them with our results. The band structures of ReSi and TcSi not found in the literature were calculated and analyzed as well. The topological properties of these materials and of some isostructural germanides and stannides were investigated. Analysis reveals the current understanding of electronic structures and properties of this compound group.
ABSTRACT
The behavior of the resistivity and thermopower of the Gd0.2Sr0.8FeO3-δ ferrite samples with a perovskite structure and the sample stability in an inert gas atmosphere in the temperature range of 300â»800 K have been examined. It has been established that, in the investigated temperature range, the thermoelectric properties in the heatingâcooling mode are stabilized at δ ≥ 0.21. It is shown that the temperature dependencies of the resistivity obtained at different δ values obey the activation law up to the temperatures corresponding to the intense oxygen removal from a sample. The semiconductorâsemiconductor electronic transitions accompanied by a decrease in the activation energy have been observed with increasing temperature. It is demonstrated that the maximum thermoelectric power factor of 0.1 µW/(cm·K²) corresponds to a temperature of T = 800 K.
