Abstract
We have investigated structural transitions in Pr 2NiO 4+δ(δ≥0) using DSC, TGA and XRD measurements. This study reveals firstly that the so called LTT (Low Temperature Tetragonal) phase i.e. P4 2/ncm symmetry exists not only for δ=0 but also for slightly sur-stoichiometric compounds. The transition temperature from P4 2/ncm to Bmab is independent on δ in the small range of oxygen excess allowed. Moreover, an orthorhombic-tetragonal transition at about 720 K in the oxidized Pr 2NiO 4+δ was emphasized and extensively discussed. This transition occurs simultaneously with an oxygen loss. It is shown that the parameter controlling this transition is neither the temperature nor the oxygen partial pressure but the value of δ. This structural change occurs for the threshold value δ c=0.192. We suggest that this threshold reflects a frontier between an oxygen disordered HTT (High Temperature Tetragonal) phase and an orthorhombic phase LTO (Low Temperature Orthorhombic) in which excess oxygen and/or holes might be ordered. Finally, general considerations n the stability of this K 2NiF 4 type oxide and evolution of transition temperatures have been done as a function of the rare earth ion size.
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