The high-pressure structural phase transitions of sodium bismuth titanate

The crystal structure of sodium bismuth titanate, Na0.5Bi0.5TiO3, as a function of pressure is reported. From a combination of neutron powder diffraction experiments using the instrument POLARIS at the ISIS pulsed neutron facility and synchrotron X-ray powder diffraction experiments at ID30 at ESRF, it is found that the rhombo-hedral ambient pressure perovskite structure (space group R3c on average) transforms to a lower-symmetry structure which has a strong resemblance to the CaTiO3 structure and is described by space group Puma, at 40 kbar and above. A new refinement of earlier angle-dispersive neutron data collected at ambient pressure and temperature at D2B of the ILL, which gives Bi-O bond-lengths closer to the values suggested by subsequent EXAFS experiments, is also reported. This yields a model for disorder on the bismuth site within the ambient-pressure phase, average symmetry R3c, with locally triclinic displacements of the Bi atom off the (pseudocubic) [111] direction along the (pseudocubic) [101] direction.