LOCAL OCTAHEDRAL TILTS IN LA2-XBAXCUO4 - EVIDENCE FOR A NEW STRUCTURAL LENGTH SCALE

The short-range structure in La2-xBaxCuO4 is examined by obtaining the pair distribution function from neutron powder diffraction data. In contrast to the predictions of the crystal structure models, locally, the CuO6 octahedra do not change their tilt direction at the low-temperature tetragonal (LTT) to low-temperature orthorhombic (LTO) phase transition: they always tilt in the [110] (LTT) directions. The long-range structures of the high-temperature phases result from coherent superpositions of the local LTT variants. The local tilt order extends over approximately 1 nm. We discuss the implications of this local structure for the properties.