A structural analysis of potassium, rubidium and caesium substitution in barium hollandite

The changes in crystal structure of the barium hollandite Bax(Al2xTi8-2x)O16 (x=1.11 and 1.24) with barium being partially substituted by K, Rb or Cs, have been determined using the Rietveld method on high-resolution neutron powder diffraction data. When K and Rb are substituted, solid solutions form with compositions conforming to a linear combination of the end members Ba1.24(Al2.48Ti5.52)O16 and Rb1.5 or K1.5(Al1.5Ti6.5)O16. Cs substitution however, is limited to approximately 0.25 Cs ions per unit cell for specimens prepared in air at atmospheric pressure. The compositions in this range can be described as a linear combination of the end members Ba1.11(Al2.22Ti5.78)O16 and Cs1.32(Al1.32Ti6.68)O16. This limit is imposed by the large size of Cs which prevents the ion occupying sites in the c-axis tunnels next to sites already occupied in ±c direction. In the known ordered structures of tunnel ions, the number of occupied sites containing unoccupied neighbours decreases as the total A-site occupancy increases. The limit of substitution given by calculation is 0.28 Cs ions per unit cell which agrees well with the observed limit. The preference of Cs for 'lone' sites is also manifested through the c parameter and the average volume of each tunnel site which only change by small amounts, compared with Rb- and K-substituted hollandites. In addition, the average off-centre shift of the Ba and Cs ions decreases, whereas in K- and Rb-substituted hollandites this remains substantially constant. The lateral expansion of the tunnels and the change in a is proportionately greater for the Cs-substituted hollandite. (orig.)