The structures of Li5BiO5 and Li5SbO5 from powder neutron diffraction

Li5BiO5 and Li5SbO5 have been synthesised by conventional ceramic methods and their structures examined using powder X-ray diffraction and time-of-flight neutron diffraction. The structures of both phases can be related to a simple rocksalt (NaCl) subcell, but cation and oxygen vacancy ordering results in a monoclinic superstructure, which is very similar to that observed in the ordered phase of TiO. The authors discuss subtle structural differences which exist between Li5BiO5 and Li5SbO5 since Li5BiO5 is acentric whereas Li5SbO5 is centrosymmetric. The acentric structure of Li5SbO5 is associated with cooperative oxygen displacements along <001> towards oxygen vacancies, to give pyro- or ferroelectric characteristics