Synthesis, structure and properties of substituted cobalt chalcogenides Co7(Se,Te)8

Abstract

For the first time, polycrystalline samples of Co₇(Se_1–yTe_y)₈ were synthesized in the entire concentration range (0 ≤ y ≤ 1) and studied by X-ray diffraction, scanning electron microscopy, and by thermal expansion, specific heat capacity and electrical resistivity measurements. Depending on the concentration of tellurium, the solid-phase reaction method (at y < 0.9) and melting method (at y ≥ 0.9) were used to obtain single-phase samples. A change in the crystal structure (P3₁21 → P-3m1 → P6₃/mmc → P-3m1) due to the disordering of vacancies, significant anisotropy of the thermal atomic displacement, and anisotropic lattice expansion have been observed in this system when selenium is substituted with tellurium. According to specific heat measurements, an increase in tellurium concentration is accompanied by a decrease in the electronic specific heat coefficient, which indicates an increase in the metallicity of the system and is confirmed by electrical resistivity data. In substituted compounds using thermal expansion and temperature-dependent X-ray diffraction, spinodal decomposition of samples was detected upon heating. The second phase in dendritic form was observed using scanning electron microscopy on the surface of slowly cooled tellurium-rich sample.