The structures of Li5BiO5 and Li5SbO5 from powder neutron diffraction

Abstract

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. Subtle structural differences exist between Li5BiO5 and Li5SbO5 since Li5BiO5 is acentric [Cm, a=9.9640(2), b=4.1480(2), c=5.9409(2) α, β =109.49(2)°] whereas Li5SbO5 is centrosymmetric [C2/m a=9.724(2), b=3.984(1), c=5.792(1) A ̊, β =109.04(2)°]. The acentric structure of Li5SbO5 is associated with cooperative oxygen displacements along 〈001〉 towards oxygen vacancies, to give pyro- or ferroelectric characteristics. © 1989.