On the Jahn-Teller-induced helical deformations in CsCuCl3

The presence of helical deformations, which have been found earlier in the low-temperature structure of CsCuCl 3 , implies that there are odd deformations of the octahedra, together with the usual even modes due to the E(X) epsilon Jahn-Teller effect. An explanation of the dipole moments of the octahedra is given by means of shifts of lattice planes, with magnitudes about as large as the Jahn-Teller deformations. The origins of the third- and fourth-order anharmonic terms in the expansion of the free energy according to Landau can be indicated better than previously. The third-order terms arise mainly from the Jahn-Teller effect of the octahedra and from the shifts of the lattice planes. The fourth-order term is connected with the helical structure. In addition the implication of the Lifshitz term is given and is explicitly discussed. The connection with the theory of Heine and McConnell (1984) for incommensurate structures is indicated.