Anharmonic thermal motion in the 93-K superconductor ${\mathrm{Ba}}_2$Y${\mathrm{Cu}}_3$${\mathrm{O}}_7$ using multiple-wavelength x-ray diffraction

We have obtained anisotropic thermal parameters for ${\mathrm{Ba}}_2$Y${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ from a dual-wavelength x-ray crystal structure determination on a single crystal with a small amount of twinning. Anharmonic thermal motion was found for both Ba and Cu atoms with the largest anharmonic feature attributed to a Ba-Ba breathing mode. The anisotropic and anharmonic thermal parameters are consistent with the coordination polyhedra as well as the bonding distances involved.