On the special points on the (P,T,x)-phase diagram for Sn2P2(Se x S1- x )6 crystals: acoustic studies of Sn2P2(Se0.28S0.72)6

On the basis of temperature dependences of the acoustic wave velocity and the acoustic attenuation in Sn2P2(Se0.28S0.72)(6) crystals under normal conditions and hydrostatic pressures, we show that increasing pressure causes these crystals to move away from the tricritical point and enter into the region of first-order phase transitions (PTs) in (P,T,x)-space. Still higher hydrostatic pressures lead to an increase in the attenuation of the acoustic wave with the velocity v(22) and broadening of its anomaly in the PT region, thus hinting at splitting of the PT, with the appearance of an intermediate incommensurate phase approximately at P=3.3kbar. We offer a (P,T,x)-phase diagram for the solid solutions Sn2P2(Se (x) S1- (x) )(6), which includes two special lines of PTs, tricritical and triple ones, which can intersect at the Lifshitz point at negative hydrostatic pressures. The variations of the surface of acoustic wave velocities occurring with changing temperature are obtained for Sn2P2(Se0.28S0.72)(6) crystals under the atmospheric pressure.