Twin structures in lanthanum, praseodymium, and neodymium aluminate ceramics

Ferroelastic domain reorientation in polycrystalline rhombohedral LaCoO3 has been demonstrated by backscatter electron (BSE) imaging and electron backscatter diffraction (EBSD) combined with in situ compression testing. Four different domain orientations were confirmed in line with expectation from crystallography. During in situ compression reorientation of domains were observed favoring domains with the c‐axis close to perpendicular to the stress field. In situ compression combined with BSE/EBSD is shown to be an excellent tool for studying the influence of mechanical stress on grain/domain structure in polycrystalline ceramics.

Section: Discussionsupporting

confidence: 89%

“…Wedge‐shaped twins, on the other hand, may form as a combination of both twin types. It has also been confirmed that {100} twins are predominant compared with {110} twins, because the former twin type relieves more strain inherent in the grains 34,35 …”

Section: Resultsmentioning

confidence: 91%

Backscatter Electron Imaging and Electron Backscatter Diffraction Characterization of LaCoO₃ During In Situ Compression

Karlsen

Einarsrud

Lein

et al. 2009

“…The microtwinning observed in various ceramic and metallic systems is a mechanism to reduce the lattice strain upon order–disorder or structural transitions 18 . Based on our HAADF/STEM results, we propose that the sharing of the 4( h ) and 4( g ) atomic sites at the twin boundaries is a mechanism to promote structural compatibility and minimize strain along the twin boundary, thereby reducing lattice strain upon the transformation from the high‐temperature form to the low‐temperature form during cooling 19–21 …”

Section: Resultsmentioning

confidence: 99%

Direct Observation of A‐Site Vacancies and a Twin Boundary Structure in La_2/3TiO₃‐Based Ceramics Using HAADF/STEM

Azough

Freer

Schaffer

2010

Ceramics of 0.9La 2/3 TiO 3 -0.1LaAlO 3 prepared by the mixed oxide route were sintered at 14001C in air for 4 h and cooled at rates 1801-11C/h. The products were at least 97% dense with grain sizes of 7-10 lm and relative permittivities 66.970.7 at 4 GHz. Rapidly cooled samples (1801C/h) were single phase and exhibited the highest dielectric Q Â f values of 16 3007900 GHz. High-resolution transmission electron microscopy revealed microtwins and domains in all the ceramics, with the highest density of both features in the rapidly cooled samples. A-site vacancies in rapidly cooled 0.9La 2/3 TiO 3 -0.1LaAlO 3 ceramics were imaged by aberration-corrected scanning transmission electron microscopy. It is shown that vacancies and La 1 atoms occupy the 4(h) sites and the 4(g) site is fully occupied by La 2 atoms in the orthorhombic Cmmm structure of lanthum titanate. The microtwin boundaries in the microstructure of La 2/3 TiO 3 stabilized by LaAlO 3 lie on the La 2 atomic planes. On twinning, La 2 and La 1 atoms share the same site. The sharing of the 4(h) and 4(g) atomic sites at the twin boundaries is a mechanism to reduce lattice strain upon the continuous transformation from a high-temperature simple cubic Fm3m form to a low-temperature orthorhombic Cmmm form, which involves vacancy/cation ordering and anti-phase tilting of the oxygen octahedra.

“…Both the shape and density of ferroelastic domains are directly dictated by the magnitude of the distortion. Kim et al 39 observed that domain shape changes from wedge‐like in LaAlO 3 to lamella‐like in NdAlO 3 due to increased lattice distortion. The occurrence of ferroelastic domains may affect the MW dielectric properties of both SrHfO 3 and CaHfO 3 ceramics as discussed in the following section.…”

Section: Discussionmentioning

confidence: 99%

Crystal Structure and Microwave Dielectric Properties of Alkaline‐Earth Hafnates, AHfO₃ (A=Ba, Sr, Ca)

Feteira

Sinclair

Rajab

et al. 2008

The crystal structure and microwave (MW) dielectric properties of AHfO3 (A=Ba, Sr, and Ca) ceramics prepared by the solid‐state route have been investigated. X‐ray and electron diffraction show BaHfO3 to be an untilted cubic perovskite (a0a0a0, space group ), whereas SrHfO3 and CaHfO3 are both orthorhombically distorted perovskites (a−b+a−, space group Pnma). AHfO3 ceramics with a relative density ∼90% of the theoretical X‐ray density and microstructures consisting of randomly oriented equiaxed grains were prepared by firing at 1750°C for 6 h. Conventional transmission electron microscopy reveals the occurrence of long parallel ferroelastic domains in the subgrain microstructure of CaHfO3 and SrHfO3. At room temperature and MW frequencies, AHfO3 ceramics, where A=Ba, Sr, and Ca, have ɛr∼24.2, Q×fr∼14 250 GHz (at 8.9 GHz) and τf∼+111 ppm/K; ɛr∼23.5, Q×fr∼33 534 GHz (at 9.3 GHz), and τf∼−63 ppm/K; and ɛr∼21.4, Q×fr∼15 950 GHz (at 8.9 GHz), and τf∼−33 ppm/K, respectively. The ion polarizability for Hf4+ in a cubic perovskite environment is calculated to be 3.32 Å3, whereas in a noncubic environment it increases to 4.36–4.47 Å3. Finally, the dielectric properties of AHfO3 (A=Ba, Sr, and Ca) are compared with their titanate and zirconate counterparts.