Dielectric properties and phase transitions of TlIn(S$_{\rm{1-x}}$Se$_{\rm{x}}$)$_2$ polycrystals under pressure

TlIn(S$_{\rm{1-x}}$Se$_{\rm{x}}$)$_2$ polycrystals belong to class of complex ferroics of different dimensionality and are interesting to be studied since they are characterized by the existence of an incommensurate phase and polycritical phenomena.. At atmospheric pressure, anomalies in the temperature interval 160--220 K are observed in the temperature dependences of dielectric constant \textit{$\varepsilon $(T)} in polycrystals TlIn(S$_{\rm{1-x}}$Se$_{\rm{x}}$)$_2$, corresponding to phase transitions into incommensurate and ferroelectric phases. In the pressure range 530 MPa $\le p <$ 660 MPa, a qualitative change of the temperature dependences of dielectric constant is observed, related to polycritical phenomena. Based on the studies of the temperature dependences of the dielectric permittivity at different hydrostatic pressures, ($p$, $T)$ phase diagrams of the TlIn(S$_{\rm{1-x}}$Se$_{\rm{x}}$)$_2$ polycrystals (0.01 $\le \quad x \quad \le $ 0.07) were built.