RAMAN INVESTIGATIONS OF ORIENTATIONAL ORDERING IN NISIF6-CENTER-DOT-6H(2)O, NISIF6-CENTER-DOT-6D(2)O, AND ZNSIF6-CENTER-DOT-6H(2)O CRYSTALS

The results of investigations of polarized Raman spectra in ZnSiF6 . 6 H(2)O and NiSiF6 . 6D(2)O single crystals in the temperature range 2-3 00 K are presented. It is found that the crystals are orientationally ordered at low temperatures. The orientational disordering is describe d by a model in which the rotational motion of the SiF62- complex is d etermined by a two-well asymmetric potential function. The parameters of the model are determined from the temperature behavior of the Raman spectral lines. The calculated values of concentration for two orient ational positions of the SiF62- complex at room temperature are in goo d agreement with the x-ray data. (C) 1997 American Institute of Physic s. [S1063-777X(97)00912-2].