X-RAY CRYSTAL-STRUCTURES OF HIGH-TEMPERATURE, INCOMMENSURATE, AND FERROELECTRIC MODIFICATIONS OF RB2COCL4

The X-ray crystal structures of three phase modifications of Rb2CoCl4- high-temperature, incommensurate, and ferroelectric-are determined for the first time. A split-atom model was proposed for the structure of the high-temperature phase at 295 K. The structural changes that accom pany two phase transitions are discussed. The anomalous behavior of th ermal parameters is the ferroelectric phase transition is revealed. Va riations of the modulation vector q, the unit-cell parameters, and the satellite-reflection intensities with temperature were studied for th e modulated phases. The calculated, value of spontaneous polarization in the ferroelectric phase (0.12 mu C/cm(2)) is typical of the beta-K2 SO4-type ferroelectrics.