α- and βNa2UO4: Structural and Thermochemical Relationships

The crystal structures of α- and β-Na2UO4 have been determined by neutron powder diffraction. In α-Na2UO4 edge-shared UO6 octahedra extend in chains along the c axis. In β-Na2UO4 the UO6 octahedra share corners with each other to form two-dimensional perovskite layers. The octahedra are tilted around both the b and c axes of the unit cell, α- and β-Na2NpO4 probably have the same structures. The enthalpy of transition of α-Na2UO4 to β-Na2UO4 has been determined both by DTA measurements and by calorimetric measurements. The unusually high value obtained can be explained from a structural point of view.