ELECTRON AND X-RAY-DIFFRACTION FROM ANTIPHASE DOMAINS IN THE BARIUM MAGNESIUM HOLLANDITE BA1.33MG1.33TI6.67O16

The barium hollandite Ba1.33Mg1.33Ti6.67O16 undergoes an order-disorder transition at 1100 (10) K. Below this temperature the Ba and vacant sites in the tunnels of the hollandite structure are ordered with the sequence Ba-Ba-vacancy and b = 3b(hollandite. At the same time, antiphase domains form which produce broadened X-ray diffraction lines and electron diffraction peaks shifted off their commensurate superstructure positions. This shift in the diffraction peaks is explained by the formation of a boundary layer with a spacing of 2.5b(hollandite) between adjacent domains. Equations are developed relating the peak shift to the domain size which are consistent with observed electron diffraction patterns. The theory is extended to interpret the broadening of X-ray powder diffraction lines. During the early stages of domain growth boundaries form on (101), (101BAR) and (010) planes. Domain growth occurs mainly in the [101] and [101BAR] directions and after extended annealing only (010) boundaries remain.