Competing structural phase sequences in (C3H7NH3)2CuCl4: reconstructive phase transitions and the low-temperature incommensurate phase

Abstract

The structural phase sequence of (C3H7NH3)2CuCl4 (abbreviated to C3CuCl) has been determined between 20 and 300 K by optical and x-ray methods. The Jahn-Teller compound undergoes two types of reconstructive phase transition between antiferrodistortive (AF) and ferrodistortive (F) structures: the first may be characterized by a spontaneous 90 degrees switching of the long Jahn-Teller axes of the octahedra within every second layer ('octahedron-axes switching' (OAS)), and the second by a spontaneous reorientation of the hydrogen-bonding pattern ('hydrogen-bond switching' (HBS)) in the whole crystal. With these mechanisms, the highly unusual sequence of the phases delta (Pbca, Z=4), epsilon (P21/n11, Z=2), zeta (IC(P21/n11)), and eta (P21/c11, Z=4) is delta -180 K(OAS) to zeta -132 K(HBS) to eta , delta from 192 K(OAS)- epsilon from 180 K- zeta from 141 K(HBS)- eta . On cooling, the OAS transition at 180 K leads directly to the incommensurate (IC) zeta phase with a modulation wavevector qzeta =0.58t2*+0.08t3*. On heating, the modulation disappears at the same temperature and, as a new phase, the symmetrical epsilon phase occurs; at 192 K, the compound switches back to the room-temperature delta phase. The structure of the low-temperature eta phase, generated by the HBS mechanism, may be considered a distorted version of the twin structure of the delta phase. No symmetry relation exists between the IC zeta phase and the delta and eta phases. We find that the complex transition sequence of C3CuCl is in fact the result of two interfering phase sequences, namely those of the AF and F states, respectively. X-ray data on the high-temperature IC gamma phase are also presented.