Structural phase transitions in hexagonal ABCl3Jahn-Teller crystals: I. Face-sharing coupling and ground-state configuration

The structural phase transitions in hexagonal ABCl 3 Jahn-Teller crystals (A=Rb, Cs; B=Cu, Cr) consisting of the chains of face-sharing octahedra of BCl 6 are discussed. The authors use two Jahn-Teller active distortive modes Q 2 and Q 3 . The phase transitions are caused by an interaction between distorted octahedra along the chain and between the chains. Stressing the face-sharing coupling, they introduce a pseudo-spin which describes the differences between the distortive phase angles of neighbouring octahedra in the Q 3 -Q 2 plane. The phase of distortion turns by an angle of 2 pi /3 or -2 pi /3 with respect to that of the reference octahedron. Relating the pseudo-spin arrangements found in the low-temperature phases to the spin structures of a linear-chain Ising model with up to third-nearest-neighbour interactions, they investigate the ground-state configurations. Microscopic models of the intermediate beta -phases of RbCrCl 3 and RbCuCl 3 are proposed. The transition pictures consistent with the observed crystal structures are described.