PHASE FORMATION BASED ON TlInP2Se6 COMPOUND
Authors: Barchiy I.E. , Fedorchuk A.O. , Pogodin A.I. , Filep M.J. , Malakhovska T.A. , Stercho I.P. , Chundak S.Yu. , Bushtin A.V. 
Published: July 2021

Special attention of scientists in the field of inorganic synthesis is paid to the M2P2X6 type chalcogenide compounds (X - S, Se), which have a number of promising properties (ferroelectric, piezoelectric, electro-optical, thermoelectric) for functional materials. According to the directions of "chemical design", the improvement of the properties of M2P2X6 compounds is possible by heterovalent substitution in the composition of two M2+ metal ions, which form a cationic sublattice of crystal structure by two different ions M1+ and M23+. Also, the formation of limited solid solutions based on complex compounds M1M2P2X6 contributes to obtaining of materials with the same structure and different chemical composition, which allows enables purposefully change their properties.
The TlInP2Se6 quaternary compound with melts congruently at 875 K is formed in the Tl2Se–In2Se3–“P2Se4” ternary system. Physical-chemical analysis of the interaction on the In2Se3–TlInP2Se6, TlInP2Se6–Tl4P2Se6, TlInSe2–TlInP2Se6, TlInP2Se6–In4(P2Se6)3 sections showed the formation of limiting solid solutions (up to 7 mol.%) based on the TlInP2Se6 compound.
To study the formation mechanisms of solid solutions, alloys of solid solutions (TlInP2Se6)0.95(Tl4P2Se6)0.05, (TlInP2Se6)0.95(In4(P2Se6)3)0.05, (TlInP2Se6)0.95(Tl2Se)0.05, (TlInP2Se6)0.95(TlInSe2)0.05, (TlInP2Se6)0.95(In2Se3)0.05 were synthesized and investigated by X-ray phase and X-ray diffraction analyzes (DRON 4-13 diffractometer, Guinier Huber G670 X-ray chamber, WinCSD program). Crystal structure of TlInP2Se6 compound (SG P-1 (2), a=6.4488(7), b=7.5420(9), c=12.166(2) Å, a=100.72(0), b=93.63(0), g=113.32(0), V=527.7 Å3) is form anionic sublattice by layers of atom groups [P2Se6]4– in the two fused tetrahedrons shape. Cation atoms occupy positions between layers of atoms of anionic groups. In3+ cations are located at the boundary between tetrahedral and octahedral cavities (in the same plane with the centers of the anionic groups [P2Se6]4–), Tl+ cations are slightly shifted toward the octahedral cavities. The volume of the unit cell (V) is a slight decrease during the transition from the basic matrix structure of TlInP2Se6 to solid solutions (from 527.7 Å3 to 523.9 Å3), which indicates the formation of defects in the basic structure. The study of the formation mechanisms of solid solutions based on TlInP2Se6 quaternary compound from the standpoint of crystal chemical formulas showed the formation of solid solutions by the mechanism of substitution (cationic sublattice) and subtraction (anionic sublattice), which leads to the formation of defects in the crystal structure, and respectively should lead to changes in optical properties.
Experimental studies of the diffusion reflection spectra R=f(λ) showed that the compound TlInP2Se6 and solid solutions belong to indirect semiconductors. During the transition from pure compound to solid solutions the phonons energy conductivity Ephonon decreases (0.082-0.068 eV), the band gap value Eg increases (2.13-2.15 eV).