Perovskites

The main theme of the session was oxide materials with unusual electronic or magnetic properties. P. Bordet (CNRS, Grenoble) described structural anomalies at the superconducting transition in HgBa₂CuO_4+d from single crystal powder diffraction and EXAFS spectra. A general treatment of tilting in perovskites and a new computer program to derive the tilt systems was given by P. Woodward (Ohio State, USA). M. Fernandez-Diaz (ILL, Grenoble) reported a remarkable charge disproportionation coupled to a structural transition in the undoped lanthanide nickelate perovskites LnNiO₃. The session ended with two talks on layered manganite perovskites. J. Mitchell (Argonne, USA) showed how new CMR Ruddleston-Popper phases could be prepared by controlling oxygen stoichiometry, and V. Caignaert (Caen) described the preparation of La/Ba ordered and disordered polymorphs of La_0.5Ba_0.5MnO₃ and discussed their different physical properties. The session illustrated the continuing challenge to understand the relationships between composition, structure and properties in perovskite oxides.