Structure, crystal chemistry and thermal evolution of the δ-Bi2O3-related phase Bi9ReO17

The thermal evolution and structural properties of fluorite-related δ-Bi2O3-type Bi9ReO17 were studied with variable temperature neutron powder diffraction, synchrotron X-ray powder diffraction and electron diffraction. The thermodynamically stable room-temperature crystal structure is monoclinic P21/c, a=9.89917(5), b=19.70356(10), c=11.61597(6) Å, β=125.302(2)° (Rp=3.51%, wRp=3.60%) and features clusters of ReO4 tetrahedra embedded in a distorted Bi–O fluorite-like network. This phase is stable up to 725 °C whereupon it transforms to a disordered δ-Bi2O3-like phase, which was modeled with δ-Bi2O3 in cubic Fm3¯m with a=5.7809(1) Å (Rp=2.49%, wRp=2.44%) at 750 °C. Quenching from above 725 °C leads to a different phase, the structure of which has not been solved but appears on the basis of spectroscopic evidence to contain both ReO4 tetrahedra and ReO6 octahedra.