Specific features of the formation of the ferroelectric phase in polytypes of TlGaSe$_2$ crystals

The polytypism of layered crystals of thallium gallium diselenide TlGaSe$_2$ has been found to substantially affect the temperature of phase transformations and the mechanism of formation of the polar state in these ferroelectrics. In particular, it is shown that the phase transition observed in the C-TlGaSe$_2$ polytype is an improper ferroelectric phase transition occurring at a temperature $T_c\approx$ 108 K, whereas the phase transition observed in the 2C-TlGaSe$_2$ polytype is a proper ferroelectric phase transition occurring at a higher temperature $T_c\approx$ 111 K. It is concluded that the elucidation of the polytype of a particular sample is a necessary stage of investigation of the TlGaSe$_2$ crystals.