YELLOW INCL AS A DISTORTED ROCK-SALT LATTICE

Because of the subgroup-group relationship which exists between the sp ace group of yellow InCl (P2(1)3, Z = 32) and that of rock salt (Fm (3 ) over bar m, Z = 1), a Landau expansion of the free energy in invaria nts of the symmetry group of higher order can be performed and a mecha nism for distortion arises. The special feature of this hypothetical p hase 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 longitudi nal displacements of rows of ions, with different amplitudes for the c ations 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 m odes of different symmetry is a hitherto unknown distortion mechanism. Yellow InCl is further characterized by bilinear quantities describin g ferrodistortive chirality (A(1u)) and electric 'A(2u)' octupoles and antiferrodistortive spiral (A(2g)) arrangements. These bilinear quant ities are related to fourth-order Landau invariants. Yellow InCl has a n electric octupole lattice. In addition, on the basis of the analogy with optical rotatory power, new mechanisms of this effect are predict ed.