MICROSCOPIC DISTORTION AND ORDER-PARAMETER IN LANGBEINITE K2CD2(SO4)(3)

The structural data on the orthorhombic phase of K2Cd2(SO4)(3) at four different temperatures of Abrahams et al. (1978) [J. Chem. Phys., 68, 1926] is reanalyzed in terms of frozen symmetry modes. The eigenvecto r of the primary distortion present in the orthorhombic phase is deriv ed. The temperature variation of the amplitude of the order parameter is determined and compared with those of the amplitude of the frozen s econdary distortion and the amplitude of the orthorhombic strain. In c ontrast with some previous literature, it is shown that, within experi mental accuracy, all these magnitudes exhibit the correlation expected from a conventional Landau model.