Low-temperature anomalies of dielectric permittivity of Sn2P2S6 crystals were investigated. It is shown that these phenomena have a relaxation character and the observed anomalies could be related to the small hole polarons dynamics with donor-acceptor compensation processes in lattice with tin and sulfur vacancies. To confirm it, we measured the dielectric properties of tin-enriched and sulfur-enriched crystals. It is shown that deviation from stoichiometry leads to a significant change in the low-temperature anomalies of dielectric losses.

Structural changes of Er- or Yb-ion doped LiNbO₃ (LN) nanocrystals were studied in relation to the high-energy ball milling process. The evolution of the size of the particles and the formation of different phases were followed by dynamic light scattering and X-ray diffraction measurements, while the electronic transitions of rare-earth (RE) ions were investigated by absorption spectroscopy in the infrared spectral range. During the milling process, RE ions left the crystal lattice and an RE₂O₃ phase appeared to an increasing extent next to the LN. The change in the absorption spectra and the phases formed during the grinding process were found to be very similar for both investigated RE ions and were independent of their original concentration in the starting crystal samples. The extent of the RE loss was found to be 90% after 100 min of wet grinding.

The results of the study of the temperature dependence of the dielectric permittivity of Cd2P2S6 crystals are presented. It is shown that in the temperature range 80–420 K there are three regions in which hysteresis phenomena are observed. At high temperatures (T > 320 K) there is an activation change of the dielectric losses associated with the increased conductivity of Cd2P2S6 crystals. The transition from heating to cooling leads to a decrease in losses, which is manifested as a hysteresis in the dielectric constant. A jump-like change in dielectric permittivity is found at 228 K in the cooling mode and 265 K in the heating mode, which is due to the first-order phase transition in these crystals. The third hysteresis region appears at low temperatures of 80–90 K. When the temperature is cycled in this interval, the classical hysteresis behavior is observed with a decrease in both amplitude and temperature range with each cycle. The cause of this behavior will be the subject of further investigation.