X-ray diffraction study on K_2ZnCl_4 down to10 k

摘要

K_2ZnCl_4 belongs to the A_2BX_4 structure family exhibiting a sequence of transitions from a normal to an incommensurate and to a commensurate phase. This sequence is repeated at lower temperatures a second time with the modulation direction perpendicular to the high temperature modulation. The idealised, basic structure at high temperatures is shown in Fig. 1a (a_0 = 7.30 A; B_0=12.54 A; c_0=8.96 AK SG=Pmcn R=8.7percent [5]). This model does not account for the dynamics in the structure. In fact, a split model had to be introduced in order to obtain a satisfactory agreement with the observed data (R=4.1percent [5]). Fig 1b exhibits the two equilibrium positions, uncorrelated thermal oscillations occur between them. The realistic model for the N-phase is a disordered one.