Thermal vibrations in Ba2YCu3O7 studied by X-ray diffraction

Unlike neutron scattering, X-ray studies can be done on very small crystals; in the case of Ba2YCu3O7, a crystal of excellent quality and dimensions of 80 x 80 x μm3 dimensions was used. To account for the wavelength dependence of absorption and extinction, two complete Ewald spheres at two wavelengths were measured (CuΚα and MoΚα radiation) and simultaneously refined. All measurements were carried out at ambient temperature. Only a small amount of twinning was detected and accounted for mathematically. The thermal parameters were expanded beyond second order tensors (anisotropic Debye-Waller factors) to higher orders, using the Gram-Charlier formalism. It was possible to refine up to fourth order tensors for the metal atoms in Ba2YCu3O7 and second order tensors for the oxygen atoms due to their considerably smaller scattering power. In the following, the results for the second order terms are discussed separately from the higher order expansion