Electron microscopy study of domain structure due to phase transitions in natural perovskite

Abstract

Abstract. Transmission electron microscopy on natural calcium metatitanate perovskite (dysanalyte) reveals the following twin laws in the orthorhombic (space group Pbnm) phase: reflection twins on the {110} and {112} planes, and 90 ~ rotation twins about the [001] axis (referred to as [001190o twin). Single crystals that were heattreated and quenched from above 1585 K exhibit a dramatic change in domain structure compared with the starting material and specimens quenched from T <1470 K. Mutually perpendicular {110} and [001190o twins are observed throughout the crystal, forming a cross-hatched domain texture. 1/21001] antiphase domains, which are very rarely observed in the starting material, also become dominant in the crystal. This change in domain structure is interpreted as due to a structural phase transition in perovskite at a temperature below 1585 K. From the point symmetry elements that describe the twin laws and the translational elements that relate the antiphase domains, the most likely phase near 1585 K is tetragonal with space group P4/mbm. These results are consistent with high-temperature powder X-ray diffraction study. On the other hand, density of the {112} twins is increased significantly in the crystal quenched from 1673 K. Twin domains are either bound by mutually perpendicular {110} and (001) walls, or by {112} walls with {110} twin domains within the polygonal {112} domains. Both twin density variation and domain morphology suggest that the crystal may be cubic at this temperature. Microstructure of a single crystal deformed at 1273 K and 3.5 GPa (within the orthorhombic stability field) is morphologically quite distinct from that of the heat-treated specimens. Dislocations dominate the microstructure and often interact with twin domain boundaries.

Phase changes and thermodynamic properties of CaTiO3. Spectroscopic data, vibrational modelling and some insights on the properties of MgSiO3 perovskite

“…This anomalous behaviour has been interpreted as due to the overlapping of an orthorhombic(Cmcm)-tetragonal and tetragonal-cubic transition or to a direct orthorhombic(Cmcm)-cubic transition. The orthorhombic-tetragonal-cubic sequence of transitions is supported by (1) the X-ray data of Liu and Liebermann (1993) which indicate a transition to a tetragonal form at around 1600 K; (2) the observation of the density and morphology of twins by transmission electron microscopy on samples quenched from 1673 K suggesting that above 1600 K CaTiO 3 is cubic (Wang and Liebermann 1993). The present Raman data can also be interpreted in the framework of such a transition sequence above 1500 K. The disappearance of all the first order bands as well as changes in second-order Raman scattering can be related to changes from orthorhombic (or tetragonal) to cubic symmetry with no allowed Raman modes as observed in other perovskite compounds like SrTiO3 (Perry et al 1967;Nilsen and Skinner 1968).…”

Effect of Al2O3 on the structure and microwave dielectric properties of Ca0.7Ti0.7La0.3Al0.3O3

“…Additions of combinations of Bi 2 O 3 with Al 2 O 3 or NiO lowered the sintering temperature to 1450 • C. However, the additives reduced the Q × f value by at least 15%. The microwave dielectric properties of ε r ≈ 45, Q × f ≈ 22,500 GHz and τ f ≈ +10 ppm/ • C were reported for the undoped 0.7CaTiO 3 -0.3LaAlO 3 In the present study, the effect of excess Al 2 O 3 and the change of cooling rate on the sintering behaviour, microstructure and microwave dielectric properties of 0.7CaTiO 3 -0.3LaAlO 3 ceramics have been investigated.…”

“…Such as twining domains and ordering anti-phase boundaries due to the structural phase transitions, which are encountered during cooling from the sintering temperature. [2][3][4] CaTiO 3 is an orthorhombic distorted perovskite 5 at room temperature with high relative permittivity (ε r = 170), modest quality factor (Q × f = 3500 GHz) and a very high positive temperature coefficient of resonant frequency (τ f = +800 ppm/ • C). 4,6 In contrast LaAlO 3 is a rhombohedral perovskite at room temperature 7 with low relative permittivity (ε r = 23.4), high quality factor (Q × f = 68,000 GHz) * Corresponding author.…”

MgSiO3 perovskite: a HRTEM study

“…However, in the case of MgSiO3, the structure refinement (Horiuchi et al, 1987) does not indicate positional ordering of the Mg atoms. It should be noted that 89 co APBs were observed in CaTiO3 perovskite (Wang and Liebermann, 1993).…”