CRYSTAL-STRUCTURE OF THE HIGH-TEMPERATURE PHASE OF OXIDIZED PR2NIO4+DELTA

Powder neutron diffraction measurements have been carried out on oxidi sed Pr2NiO4+delta at high temperature under O-2 atmosphere. A structur al transition takes place at about 720 K from an orthorhombic symmetry to a tetragonal one. The high temperature structure has been refined in the P4(2)/ncm space group using a split-atom model for apical oxyge ns simulating their strong anharmonic motion. The interstitial oxygens occupy the sites (4e) and (8f) following a uniform distribution. The refined occupancy is in good agreement with the value measured by TGA, corresponding to an oxygen excess as high as delta = 0.18.