Abstract
Using X-ray diffraction methods it has been ascertained that the intermediate phase in Sn2P2Se6 crystals existing in the range 193 K to 220 K and preceding the ferroelectric transition at 193 K is an incommensurate phase. Modulated structure is characterized by the wave vector q(T) = (-1.146 δ(T)a*, 0, δ(T)c*), where δ(T) changes smoothly within 0.085 to 0.065 as temperature decreases and jumps to zero value at the ferroelectric transition. The transition from para- to ferroelectric phase is accompanied by a symmetry decrease from P21/c to Pc, the translational symmetry being unchanged. The incommensurate phase existence range manifests itself in the distinctive behaviour of both thermal expansion coefficients and reflection integrated intensities. In view of the results obtained some features of the behaviour of the mixed crystals like Sn2P2(S1-x Sex)6 and (Pb1-Y SnY)2P2Se61-6 are also discussed. It is concluded that the triple point in the Sn2P2S6-Sn2P2Se6 system phase diagram is a Lifshitz point. Furth...
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