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

Abstract

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.

Concurrent La and A-Site Vacancy Doping Modulates the Thermoelectric Response of SrTiO3: Experimental and Computational Evidence

“…It should be noted that HAADF images for ceramic x90 are reminiscent of HAADF images of LaAlO 3 stabilized La 2/3 TiO 3 , that we reported for the first time, showing the vacancies in a A-site deficient perovskite using HAADF imaging. 26,31 It also resembles the Z-contrast images of recently published data for CaTiO 3 stabilized La 2/3 TiO 3 . 44 perovskite using HAADF imaging.…”

Analysis of Bi Distribution in Epitaxial GaAsBi by Aberration-Corrected HAADF-STEM

“…Moreover, by applying the quantitative HAADF (qHAADF) image analysis algorithm, developed by Molina et al [17], it is possible to correlate effectively HAADF intensity and atomic column composition in III–V ternary semiconductor materials [12, 16, 18] and consequently in GaAsBi compounds. Additionally, high-resolution HAADF-STEM images offer information about crystalline quality [19–22]. …”

Tuning piezoelectric properties through epitaxy of La2Ti2O7 and related thin films

“…The correlation between the schematic atomic positions and the lattice positions in the image confirms that the epitaxial phase is La 2/3 TiO 3 with the orthorhombic [102] direction perpendicular to the interface. In this crystalline orientation, the superstructure contrast arising from the ordered A -site vacancies is not visible, in contrast to STEM images of the [100] projection 23 . Inspection of the lattice schematics confirms that the orthorhombic La 2/3 TiO 3 [102] direction corresponds closely to the LTO[010] direction.…”

“…While La 2/3 TiO 3 may not be of interest as a piezoelectric material, it has been explored as a microwave dielectric with promising properties 23–25 . La 2/3 TiO 3 is also the parent compound of ionic 22 and lithium ion 26 conductors.…”

“…La 2/3 TiO 3 is also the parent compound of ionic 22 and lithium ion 26 conductors. The orthorhombic structure of La 2/3 TiO 3 , with ordered A-site cation vacancies, is difficult to stabilize in bulk form without the addition of dopant cations 23,25 . Epitaxial stabilization of undoped La 2/3 TiO 3 on TiO 2 (110) is a promising route to realize pure La 2/3 TiO 3 for both fundamental studies and device applications.…”