Structural phase transitions in hexagonal ABCl3Jahn-Teller crystals: II. Mean-field approximation

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Abstract
For pt.I see ibid., vol.19, p.4861 (1986). The structural phase transition in CsCuCl3, CsCrCl3, RbCrCl3 and RbCuCl3 are derived by the mean field approach and the phase transition pictures proposed in the preceding paper are confirmed. The important variables corresponding to the two types of order-parameters are the distortive modes Q2 and Q3 relevant for the Jahn-Teller distortion of the octahedron and the pseudo-spin sigma which describes the strong face-sharing coupling of neighbouring octahedra. The authors introduce the interchain coupling with respect to the Q-modes and the pseudo-spins. Combining this coupling and the pre-established short-range order of the pseudo-spins in the chain, they derive an effective interchain interaction, on the basis of which they investigate the phase transitions. Special attention is paid to the intermediate beta -phases which is considered to be Q- but not sigma -ordered. They investigate the effect of different kinds of strain, one of which is responsible, in particular, for the appearance of the beta -phase in RbCuCl3. They also discuss the influence of the warping term, which, if large, leads to the individual octahedron being elongated and formally to a model of interacting Potts spins each describing the three spatial orientations of an elongated octahedron.