Ferroelectrics, 298: 361–366, 2004 Copyright C  Taylor & Francis Inc. ISSN: 0015-0193 print / 1563-5112 online DOI: 10.1080/00150190490423877 Phase Transitions and Disordering Effects in CuInP 2 S(Se) 6 Layered Ferrielectrics YU. M. VYSOCHANSKII L. BELEY S. PERECHINSKII M. GURZAN O. MOLNAR O. MYKAJLO V. TOVT V. STEPHANOVICH Institute for Solid State Physics and Chemistry Uzhgorod University Uzhgorod, 88000, Ukraine By X-ray diffraction structure determination, together with Raman scattering, optical birefringence and low frequency dielectric measurements, ferrielectric phase transitions in regions of trigonal and monoclinic phases on a temperature—concentration diagram of CuInP 2 (Se x S 1-x ) 6 mixed crystals are investigated. The phase transitions temperatures rapidly decrease and related anomalies smear at deviation from pure compounds. In a middle interval of concentrations a dipole ordering obviously has peculiar character. Keywords Phase transitions; ferrielectrics; mixed crystals Introduction For layered ferrielectrics CuInP 2 Se 6 two consecutive phase transitions were observed: sec- ond order at T i ≈ 249 K (P-31c at T > T i ) and first order at T c ≈ 235 K (P31c at T < T c ) [1, 2]. For CuInP 2 S 6 first order ferrielectric transition (C2/c → Cc) occurs at T c ≈ 315 K [3, 4]. The phase transitions have clearly order/disorder character. A spontaneous polariza- tion in the ferrielectric phases appears normally to the structure layers due to two opposite contributions—copper ordering and indium displacement. In the case of CuInP 2 (Se x S 1-x ) 6 mixed crystals on the concentration—temperature diagram the monoclinic and trigonal phases are divided by a morphotropic phase boundary. At partial substitution in the CuInP 2 S 6 compound sulfur by selenium a temperature of ferrielectric phase transition rapidly decreases. A substitution of selenium by sulfur atoms in the CuInP 2 Se 6 compound a value of T i more slowly reduces. The ferrielectric ordering obviously is not observed in a middle interval of concentrations, namely in a region 0.2 < x < 0.8 [5, 6]. In order to specify a phase diagram of CuInP 2 (Se x S 1-x ) 6 mixed crystals the X-ray diffraction and optical (birefringence, Raman scattering) investigations were carried out and compared with low frequency dielectric data. Received August 17, 2003. 361