Tetragonal to monoclinic phase transformation at room temperature in BaxFe2xTi8−2xO16 hollandite due to increased Ba occupancy

A series of barium-rich hollandite samples, of nominal stoichiometry BaxFe2xTi8−2xO16, with x=0.8–1.5 have been studied using scanning electron microscopy and synchrotron powder diffraction in order to establish the influence of Ba content on the phases formed. Samples with x between 1.1 and 1.3 were found to be single phase, samples with x<1.1 contained rutile as a secondary phase, and samples with x>1.3 contained (Ba, Fe) titanates as secondary phases. From synchrotron X-ray diffraction patterns, at room temperature, a transformation from the tetragonal (I4/m) to the monoclinic (I2/m) structure was observed between x=1.1 and 1.2. The monoclinic angle, β increases rapidly from 90.48(2) to 91.06(6)° as x increased from 1.2 to 1.4.