Ukrainian Journal of Physical Optics 

¹Girnyk I., ²Kaynts D., ¹Krupych O., ¹Martunyuk-Lototska I., ¹Vlokh R.

¹Institute of Physical Optics, 23 Dragomanov Str., 70005 L’viv, Ukraine
²Uzhgorod National University, 46 Pidhirna Str, 88000 Uzhgorod, Ukraine

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On the basis of studies of the domain structure and the temperature dependences of thermal expansion and the ultrasonic velocity, the x,T-phase diagram of Cu6PS5IxBr1-x mixed crystals is obtained. It is shown that all the compounds possess the first-order superionic (SI) phase transitions (PTs). Their temperatures are determined with the aid of thermal expansion measurements. The mixed crystals with the concentration x=0-1 possess the second-order structural PTs into a ferroelectric-ferroelastic (FF) phase. In the composition range x=0-0.75, the temperature of the structural PT slowly decreases and the line of TC crosses the line of TS approximately at x=0.75 (at heating) and at x=0.9 (at cooling). Due to a wide temperature hysteresis of TS in the range of dx=0.75-0.9, the sequence of PTs should depend on the particular temperature scan process – cooling or heating. In this range of the composition the polycritical point exists, in which the crossed curves of PTs separate the following phases: paraelectric-paraelastic-SI (the symmetry-43m), paraelectric-paraelastic-nonSI (-43m), ferroelectric-ferroelastic-SI (m) and ferroelectric-ferroelastic-nonSI (m). The coordinates of the polycritical point are determined as (x;T)=(0.76;217K).

Key words: ferroelastics, superionics, domain structure, thermal expansion, ultrasonic velocity
PACS: 42.70.Nq , 42.25.Lc, 77.90.+k, 81.30.-t

doi 10.3116/16091833/4/3/144/2003