THE MONOCLINIC-TETRAGONAL PHASE TRANSITION AND TUNNEL ION ORDERING IN CESIUM SUBSTITUTED BARIUM MAGNESIUM HOLLANDITES. Robert W. Cheary and Nirmala Maharaj. Department of Applied Physics, University of Technology Sydney, Broadway, New South Wales, Australia 2007.

We have investigated the substitution of Cs for Ba in Ba_xMg_xTi_8-xO₁₆ as part of a research program to understand the stability of the hollandite structure when loaded with simulated radioactive waste. The general composition of the substituted material is (Ba_1-ßCs_ß)_x Mg_yTi_8-y O₁₆ where y = x[1 - ß/2] and x is the number of the Ba and Cs per unit cell located in the tunnels sites of the structure. In these hollandites the tunnel sites are not fully occupied, and the Ba and Cs along with the vacant sites can adopt different ordered configurations along the tunnels depending on the occupancy level x/2. When this occurs superlattice lines appear in the X-ray powder pattern. The transition from order to substitutional disorder occurs either when the temperature is raised, or when the concentration of Cs is increased. The superlattice lines also display line broadening, through the formation of anti-phase domains, which increases with Cs concentration. Ba hollandites also undergo a distortional transition from a high temperature tetragonal phase to a low temperature monoclinic phase. In pure Ba hollandites this transition can occur at temperatures as high as 500[[ring]]C depending on the tunnel occupancy. When Cs is introduced into the tunnels the transition temperature is reduced dramatically and beyond ß ~ 0.15 the transition temperature is below room temperature for all occupancies. In this paper we have investigated the ordering of the tunnel ions and the monoclinic-tetragonal phase transition using X-ray diffraction data collected with a Siemens D5000 powder diffractometer fitted with a high temperature stage. The particular aspects reported include;

* the compositions that form when Cs is substituted into barium magnesium hollandite over the known range of stable occupancies (x = 1.14 to 1.33) and at increasing levels of Cs from ß = 0 to 0.3,

* the changes in lattice parameters, long range order parameter and superlattice line breadth with Cs concentration,

* the effect of temperature on both the ordering and the lattice parameters at different Cs concentrations.