Yellow InCl as a distorted rock-salt lattice

Abstract

Because of the subgroup - group relationship which exists between the space group of yellow InCl and that of rock salt , a Landau expansion of the free energy in invariants of the symmetry group of higher order can be performed and a mechanism for distortion arises. The special feature of this hypothetical phase transition is that three lattice modes of different symmetry type and of almost equal force constant bring about the deformation. These three modes, which are apparently nearly degenerate, effect longitudinal displacements of rows of ions, with different amplitudes for the cations and the anions. When these are described by an order parameter, an expansion of the free energy up to the fourth power and containing third-order terms is possible. The cooperation of pseudo-degenerate modes of different symmetry is a hitherto unknown distortion mechanism. Yellow InCl is further characterized by bilinear quantities describing ferrodistortive chirality and electric `' octupoles and antiferrodistortive spiral arrangements. These bilinear quantities are related to fourth-order Landau invariants. Yellow InCl has an electric octupole lattice. In addition, on the basis of the analogy with optical rotatory power, new mechanisms of this effect are predicted.