Structural studies onA-cation-deficient perovskite-related phases. II. Microdomain formation in ThNb₄O₁₂

Abstract

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-cationdeficient perovskite-related phase ThNb4Olz. 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 (20-50A) of the two orientation variants are statistically distributed in a type of tweed pattern, separated by boundaries that are aligned predominantly parallel to (ll0)p and (ll0)p. In the slow-cooled samples, the ordering of columns of thorium atoms and vacancies parallel to [ll0]p or [1.10]p occurs in microdomains, with domain boundaries parallel to (100)p and (010)p and with average separations of 6ap, 6bp =-24 A. 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 NbO 6 octahedra are tilted about the [ll0]p and [ll0]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 * Permanent address: CSIRO Division of Mineral Chemistry, PO Box 124, Port Melbourne, Victoria 3207, Australia. 0567-7394/82/060753-09501.00gives rise to characteristic clusters of split superlattice spots in the diffraction patterns for ThNb40~2. Optical transform methods were used to check the validity of microdomain models for both the quenched and the slow-cooled samples. IntroductionIn part I (Alario-Franco, Grey, Joubert, Vincent & Labeau, 1982), we presented electron and X-ray diffraction results for samples of the A-cation-deficient perovskite-related phase Th0.25NbO 3 (i.e. ThNb40~2), which were slowly cooled from the melt. The diffraction patterns showed both strong, sharp reflections and groups of very weak, diffuse satellite reflections. The sharp reflections, which indexed with a tetragonal perovskite supercell (a t = ap, c t = 2Cp), resulted from long-range ordering of thorium atoms into alternate (001)p layers of cuboctahedral A-cation sites, as originally reported by Trunov & Kovba (1966). The diffuse satellite reflections were interpreted as resulting from short-range ordering of thorium atoms and vacancies within the (001)p layers. Splitting of certain groups of satellite reflections was observed, characteristic of a periodic two-dimensional array of domains, with domain boundaries oriented parallel to (100)p and (010)p. However, in order to simplify the interpretation of the diffraction patterns, we consid...

Cation ordering in A-site-deficient Li-ion conducting perovskites La(1−x)/3LixNbO3

“…S4 †). 16,17,[21][22][23][24][25][26] All these features were observed in our LNO and LLNO samples. In fact, the cross-like reections were also observed in similarly modulated A-cation decient perovskite-related phases, such as ThNbO 3 (ref.…”

“…Various interpretations have been suggested, including formation of nanodomains, 20 microdomains, 16,21-23 unidentied sub-superstructures, 17 and the combined effects of crystal structure modulation, octahedral tilting, structural vacancies, and/or the formation of microdomains. In the case of RE y Li x NbO 3 materials the SAED superlattice reections are in the form of diffusion rods, 16,21,24 indicative of the presence of modulated crystal structures. In the case of RE y Li x NbO 3 materials the SAED superlattice reections are in the form of diffusion rods, 16,21,24 indicative of the presence of modulated crystal structures.…”

Crystal structure of thorium metavanadate (Th(VO3)4)

“…ThNb 4 O 12 [27,28] but the crystal structure is different due to the VO 4 network. In the ThNb 4 O 12 or similar compositions the structure is built by NbO 6 octahedra, and ordered absence of thorium atoms leads perovskite type superstructure.…”

Influence of Non‐Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics: II. Compositional Variations in Pure BZN

“…While formation of B‐site vacancies in a cubic perovskite is always limited to a very small range of stability, extensive A‐site non‐stoichiometry is observed in several compounds (e.g., La 2/3 TiO 3 ( x =0.33), 23 Th 1/4 NbO 3 24–26 . However, these examples are confined to simple perovskites with a single highly charged B‐site cation, and there are no examples of extensive A‐site vacancy formation in complex perovskites where at least one B cation has a low charge.…”