INFLUENCE OF POLYTYPY ON PHASE TRANSITIONS IN TlGaSe2 CRYSTALS

It is shown that, in the C-polytype of TlGaSe2 crystals, the structural PT, which occurs at a temperature of T ~ 108 K, is accompanied by an abrupt change in the value of the parameter c (∆c ~ 0.004 Å). For the 2C-TlGaSe2 polytype, the c (T) dependence shows only one clearly pronounced maximum at the temperature Tc2. Moreover, at temperatures that are T1 ~ 30 K higher and T2~ 5 K below Tc2, the Curie-Weiss law is fulfilled. The absence of a fourfold increase in the parameter from the unit cell for 2C-TlGaSe2 samples in the temperature range T = 100-300 K indicates that spontaneous polarization is the order parameter of the ferroelectric PT