INFLUENCE OF OXYGEN ON STRUCTURAL TRANSITIONS IN PR2NIO4+DELTA

We have investigated structural transitions in Pr2NiO4+delta (delta gr eater-than-or-equal-to0) using DSC, TGA and XRD measurements. This stu dy reveals firstly that the so called LTT (Low Temperature Tetragonal) phase i.e. P4(2)/ncm symmetry exists not only for delta=0 but also fo r slightly sur-stoichiometric compounds. The transition temperature fr om P4(2)/ncm to Bmab is independent on delta in the small range of oxy gen excess allowed. Moreover, an orthorhombic-tetragonal transition at about 720 K in the oxidized Pr2NiO4+delta was emphasized and extensiv ely discussed. This transition occurs simultaneously with an oxygen lo ss. It is shown that the parameter controlling this transition is neit her the temperature nor the oxygen partial pressure but the value of d elta. This structural change occurs for the threshold value delta(c) = 0.192. We suggest that this threshold reflects a frontier between an oxygen disordered HTT (High Temperature Tetragonal) phase and an ortho rhombic phase LTO (Low Temperature Orthorhombic) in which excess oxyge n and/or holes might be ordered. Finally, general considerations on th e stability of this K2NiF4 type oxide and evolution of transition temp eratures have been done as a function of the rare earth ion size.