ОСОБЛИВОСТІ КРИСТАЛІЧНОЇ СТРУКТУРИ ТА МЕХАНІЗМ ІОННОЇ ПРОВІДНОСТІ У ТВЕРДИХ РОЗЧИНАХ СИСТЕМИ Cu6PS5I-Cu6AsS5I

Abstract

Проведено рентгеноструктурні дослідження, які показали, що у системі Cu6PS5I-Cu6AsS5I утворюється неперервний ряд твердих розчинів Cu6(P1-xAsx)S5I, які кристалізуються в гранецентрованiй кубiчнiй комiрцi F43m. Встановлено, що концентраційна зміна параметра гратки відбувається з відхиленням від закону Вегарда. Проаналізовано особливості кристалічної будови та вивчено механізм іонного транспорту в твердих розчинах системи Cu6PS5ICu6AsS5I. Ключові слова: Тверді розчини, дифрактограма, параметр гратки, іонна провідність

Purpose. This work is aimed at the X-ray investigation, study of the compositional behavior of lattice parameter, main features of crystal structure as well as the mechanism of the ionic conductivity in Cu6(P1-xAsx)S5I solid solutions. Methods. Alloys of Cu6PS5I-Cu6AsS5I system were synthesized by the method of solid-phase reactions. X-ray studies were carried out by using diffractometer DRON 4-07 (emission CuKα, angle scan step 2Θ - 0.02 deg, exposition - 1 s). Refinement of P+5↔As+5 substitution mechanism and lattice atomic coordinates were carried out on the basis of the refined models of original structures by means of Rietveld method. The model calculation and refinement were carried out using EXPO 2014 software package, visualization was performed using VESTA 3.4.4 software. Results. X-ray structural studies of solid solutions were conducted in the system Cu6PS5I-Cu6AsS5I. The number and nature of the reflexes on the diffractograms of the alloys of the Cu6PS5I-Cu6AsS5I system indicate that a continuous series of solid solutions, which crystallize in a face-centered cubic lattice F43m, are formed in the system. After studying the compositional behavior of the lattice parameter, it was established that it occurs with a small positive deviation from the Vegard law, which indicates a complex structural rearrangement of the anionic framework that affects the cationic sublattice. The peculiarities of the structure of the investigated solid solutions, the basis of the anionic framework of which are [P(As)S4] tetrahedra, are studied. In the middle and on the sides of the tetrahedra [S3I] and [SI4] coordinated Cu atoms responsible for ionic conductivity. It was established that during the formation of solid solutions tetrahedra [P(As)S4] retain their symmetry. The lengths of the P(As)-S bonds, the S-S distance and the volumes of tetrahedra for Cu6(P1-xAsx)S5I solid solutions were determined. The dependence of [P(As)S4] tetrahedra volume on the composition of solid solutions is anomalous, with similar changes observed for lattice parameters. The possible explanation for this behavior may be the difference in the values of P and As electronegativities and the P+5 and As+5ion radii. On the basis of structural investigations, the mechanism of ion transport in solid solutions Cu6(P1-xAsx)S5I is explained. The determinants of ionic conductivity are Cu atoms, which are in positions Cu1(24g) with triangular coordination of S1S2S1 atoms, which determines their mobility in the crystall lattice. At the same time, the Cu atoms at positions Cu2(48h), which correspond to lower values of the coefficients of filling the positions, contribute less to the ion transport due to placement in the centers of the tetrahedra (S1S2S1I), although there is a shift to the plane of the triangles S1I1S2 and the S2I edge, but it is insignificant. The compositional dependences of the site occupation factor (SOF) for Cu1 and the maximum distance between the mobile positions Cu1 are obtained, which are consistent with the concentration dependences of the electrical conductivity and the activation energy of Cu6(P1-xAsx)S5I solid solutions. The analysis of the SOF compositional dependency shows that in the process of forming solid solutions, the mobility of Cu1 position decreases, which is evident from the growth of SOF, with the exception of Cu6(P0.4As0.6)S5I solid solution. On the other hand, there is a decrease in the maximum distances between the moving positions ((Cu6(P0.8As0.2)S5I), and their further growth in the process of formation of solid solutions, which is a limiting factor for the migration of cations in the crystal, and correlates with an increase in the activation energy of the ion conductivity. Conclusions. The synthesis of alloys of the system Cu6PS5I-Cu6AsS5I was carried out by the method of solid-phase reactions within the modified methodology. X-ray diffraction studies indicated the formation of a continuous series of solid solutions in the Cu6PS5I-Cu6AsS5I system. It was determined that the compositional variation of the lattice parameter occurs with a small positive deviation from the Vegard law. It was established that during the formation of solid solutions [P(As)S4] tetrahedra retain their symmetry. The lengths of the P(As)-S bonds, the distance S-S and the volumes of tetrahedra for Cu6(P1-xAsx)S5I solid solutions were determined. On the basis of structural investigations, the mechanism of ion transport in Cu6(P1-xAsx)S5I solid solutions is explained, namely, the determinants of ionic conductivity are Cu atoms, which are in positions Cu1(24g) with triangular coordination of S1S2S1 atoms, which determines their mobility in the crystall lattice. The compositional dependences of the site occupation factor (SOF) for Cu1 and the maximum distance between the mobile positions Cu1 are obtained, which are consistent with the compositional dependences of the electrical conductivity and the activation energy of Cu6(P1-xAsx)S5I solid solutions. Keywords:. Solid solutions, diffractogram, lattice parameter, ionic conductivity.