A variable temperature structural study of the Jahn–Teller distortion in Ba2CuUO6

The structure of the ordered double perovskite Ba2CuUO6 has been investigated between room temperature and 800 °C using synchrotron X-ray powder diffraction. At room temperature Ba2CuUO6 is tetragonal, space group I4/m, a=8.82331(13) c=8.82330(13) Å, the structure being characterized by a large Jahn–Teller distortion of the CuO6 octahedra and small out-of-phase tilts of the BO6 octahedra. This Jahn–Teller distortion is also evident in the UV–Vis spectra. Analysis of the spontaneous tetragonal strain reveals a continuous ferroelastic phase transition near 420 °C. This appears to be related to the loss of the tilts whilst maintaining the Jahn–Teller distortion, so that the high temperature structure is in space group I4/mmm.