The development of methods for the research and synthesis of solid phases by the scientific school of Ya. A Ugai. Review
The scientific school founded by Yakov A. Ugai has existed at Voronezh State University for over fifty years. One of its focus areas has been the development of physics and chemistry for obtaining solid phases in systems with volatile components. This determined the necessity to develop methods for the investigation of vapour pressure (tensimetric methods). This article only focuses on some of the works by the VSU staff dedicated to the study and construction of P-T-x diagrams. This review analyses phase equilibria and the nature of the intermediate phases in the A<sup>IV</sup> - B<sup>V</sup>, A<sup>IV</sup> - B<sup>V</sup> - C<sup>V</sup>, and A<sup>III</sup> - B<sup>VI</sup> systems. Owing to the special nature of the cation-cation and anion-anion bonds, these compounds have highly specific properties that make them promising materials (2D materials in particular). The article presents an overview of works devoted to the construction of P-T-x diagrams and the investigation of defect formation processes in binary and ternary systems based on A<sup>IV</sup>B<sup>V</sup> compounds. It should be emphasised that the known techniques needed updating due to the high values of vapour pressure. This allowed conducting experiments at pressures of about 35-40 atmospheres. The study of the A<sup>III</sup> - B<sup>VI</sup> systems, on the contrary, is complicated by low values of vapour pressure over indium and gallium chalcogenides and the complex composition of the vapour. For such systems the auxiliary component method was developed. The possibilities of its application are wide and are not limited to A<sup>III</sup>B<sup>VI</sup> compounds. A new method for nonstoichiometry regulation was developed and applied using non-destructive selective chemical transport reactions (i.e. with the participation of an auxiliary component). This method is based on the introduction or removal of one of the sample components by means of a selective chemical transport reaction. In conclusion, the development of methods for the research and synthesis of intermediate phases with variable compositions (properties) was analysed based on the example of the discussed systems.