KAYNTS D. Domain structure and phase transformations in chalcogenide ferroelectrics-semiconductors and ferroelastics

The thesis is dedicated to investigations of domain structure of the ferroelectrics Sn2P2S6, SbSI, as well as pure and mixed ferroelastic crystals Cu6PS5Br(I,Cl) of argyrodite family. Real domain structure of these crystals was visualized using chemical etching, AFM and optical methods. A possibility of existence of charged domain boundaries, inclined to the polar vector, in addition to the neutral ones (parallel to the polar vector) was demonstrated. . . A real structure of the Cu6PS5Br crystals is described as a natural super-lattice based on the cubic pra-phase with fully occupied copper positions, as compared with the real occupancy 1/3. The cubic super-lattice -43с exists for iodine-enriched compositions with х ? 0,5. For clarification of the temperatures and characteristics of the phase transformations and crystal structures in these studied crystals, the X-ray and calorimetric analyses in Cu6PS5Br1-хIх samples were carried out in a wide temperatures range. A "temperature-concentration" phase diagram of the Cu6PS5Br1-хIх mixed crystals is proposed basind on the the domain structure observation, as well as the temperature dependences of the physical properties and X-ray scattering.