journal menu
research papers
Electron diffraction/microscopy and X-ray diffraction techniques have been used to study the thorium/ vacancy ordering and microdomain structures of quenched and slow-cooled samples of the A-cation deficient perovskite-related phase ThNb4O12. In both types of samples, there is primary ordering of thorium atoms into the cuboctahedral sites in alternate (001)p layers. The quenched and slow-cooled samples have different secondary orderings of thorium atoms and vacancies in the occupied (001)p layers. In the quenched samples, the thorium atoms and vacancies are ordered in alternate rows parallel to [100]p and [010]p. Short segments (20-50 Å) of the two orientation variants are statistically distributed in a type of tweed pattern, separated by boundaries that are aligned predominantly parallel to (110)p and (
10)p. In the slow-cooled samples, the ordering of columns of thorium atoms and vacancies parallel to [110]p or [10]p occurs in microdomains, with domain boundaries parallel to (100)p and (010)p and with average separations of 6ap, 6bp ≡ 24 Å. The domains corresponding to the two orientations of thorium columns form a checkerboard pattern of two interpenetrating sets of corner-shared squares. In either set, the ordering of columns is propagated along diagonal rows of corner-shared domains, but there is no correlation between adjacent rows. The NbO6 octahedra are tilted about the [110]p and [10]p axes, parallel to the thorium-column orientations, and the domain boundaries act as mirror-twin planes for the octahedral tilt systems. This periodic change in the tilt-axis orientation gives rise to characteristic clusters of split superlattice spots in the diffraction patterns for ThNb4O12. Optical transform methods were used to check the validity of microdomain models for both the quenched and the slow-cooled samples.
10)p. In the slow-cooled samples, the ordering of columns of thorium atoms and vacancies parallel to [110]p or [10]p occurs in microdomains, with domain boundaries parallel to (100)p and (010)p and with average separations of 6ap, 6bp ≡ 24 Å. The domains corresponding to the two orientations of thorium columns form a checkerboard pattern of two interpenetrating sets of corner-shared squares. In either set, the ordering of columns is propagated along diagonal rows of corner-shared domains, but there is no correlation between adjacent rows. The NbO6 octahedra are tilted about the [110]p and [10]p axes, parallel to the thorium-column orientations, and the domain boundaries act as mirror-twin planes for the octahedral tilt systems. This periodic change in the tilt-axis orientation gives rise to characteristic clusters of split superlattice spots in the diffraction patterns for ThNb4O12. Optical transform methods were used to check the validity of microdomain models for both the quenched and the slow-cooled samples.
