Abstract
The recent and general theory of the origin of incommensurate structures in insulators is applied for the first time to a cooperative Jahn-Teller system. As expected from the general theory, two modes of deformation are found in K2PbCu(NO2)6 which, on symmetry grounds, can interact to lower the energy of the incommensurate phase but which cannot interact at the symmetry point in k-space defining the commensurate structure. In this salt the operative pair of modes are the equivalent x2-y2 and 3z2-r2 types of distortion of the octahedral complex around the copper atom. The role of rotations, displacements and acoustic modes is also considered. On this basis a structure for the incommensurate phase is proposed.

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