TEMPERATURE-DEPENDENCE OF THE CRYSTAL-STRUCTURE AND DYNAMIC DISORDER OF CADMIUM IN CADMIUM PYRONIOBATES [CD2NB2O7 AND CD2TA2O7]

The crystal structure of Cd2Nb2O7 was determined at RT, 360K, 440K and 525K. Precise single crystal structure analysis reveals considerable anisotropy of cadmium temperature factors which may be explained by the dynamic disorder of cadmium atoms displaced at random by about 0.1 angstrom from the central position. We conclude that the instability of cadmium position results in local symmetry breaking and is a driving force of phase transitions observed in cadmium pyroniobate. The crystal structure of cadmium pyrotantalate was redetermined at room temperature. The anisotropy of atomic displacements of cadmium atoms and instability of cadmium position in this compound is still stronger then in the cadmium pyroniobate.