Pressure-Induced Structural Phase Transitions in Ln2-xNdxCuO4 for Ln=La (0.6x2) and Ln=Pr (x=0)

We have investigated the structural properties of LaNdCuO4 and Pr2CuO4 under hydrostatic pressure up to 37 GPa at ambient temperature by high-resolution angle-dispersive X-ray powder diffraction. Upon increasing pressure a structural transformation into the T-structure (I4/mmm, K2NiF4-type) was observed at PT=11.4 GPa and PT=15.1 GPa, respectively. Full-profile refinements of the diffraction data give evidence for a gradual distortion of the T-structure into the orthorhombic O-phase (Cmca, distorted K2NiF4-type) during pressure release in a certain pressure interval. At low pressures the initial Tʹ-structure (I4/mmm, Nd2CuO4-type) is eventually recovered again. The axis and volume compressibilities are well described by a simple model that considers the linear compressibilities of different polyhedra building the unit-cell. The occurrence of the Ophase can be understood within the tolerance factor t versus PT phase diagram.