ALPHA-NA2UO4 AND BETA-NA2UO4 - STRUCTURAL AND THERMOCHEMICAL RELATIONSHIPS

The crystal structures of alpha- and beta-Na2UO4, have been determined by neutron powder diffraction. In alpha-Na2UO4 edge-shared UO6 octahe dra extend in chains along the c axis. In beta-Na2UO4 the UO6 octahedr a share corners with each other to form two-dimensional perovskite lay ers. The octahedra are tilted around both the b and c axes of the unit cell, alpha- and beta-Na2NpO4 probably have the same structures. The enthalpy of transition of alpha-Na2UO4 to beta-Na2UO4 has been determi ned both by DTA measurements and by calorimetric measurements. The unu sually high value obtained can be explained from a structural point of view. (C) 1995 Academic Press, Inc.