HIGH-TEMPERATURE PHASE-TRANSITION IN NA2UI6 BY NEUTRON POWDER DIFFRACTION

The behaviour of Na2UI6 has been studied as a function of temperature by neutron powder diffraction. The neutron diffraction experiments car ried out at 650 and 700 K, revealed a gradual rearrangement of the occ upancy of the sodium atoms which tend - as the temperature is raised - to occupy the sites at the 3(b) and 6(c) positions of the R (3) over bar space group. At 750 K i.e. slightly below the melting point a stru ctural phase transition to a simple body-centered cubic lattice has be en observed.