TETRAGONAL-ORTHORHOMBIC STRUCTURAL MODULATION AT LOW-TEMPERATURE IN LA2-XBAXCUO4

Below similar to 60 K La2-xBaxCuO4 (0.10 less than or equal to x less than or equal to 0.15) reveals superstructure reflections consistent w ith the tetragonal space group P4(2)/ncm [low temperature tetragonal ( LTT)]. However, twins, characteristic of the related orthorhombic phas e [low temperature orthorhombic (LTO)], persist in this material. Elec tron microscopy shows that the LTT superstructure reflections originat e from the twin-boundary region. This suggests a phase mixture of orth orhombic and tetragonal with a gradual change from LTO (space group Bm ab) in the interior of the twin domains, via the Pccn, to LTT at the t win boundary. This is modeled as a spatial modulation, the tipping ang le of CuO6 octahedra being an order parameter.