Phase‐Transformation‐Induced Anti‐Phase Boundary Domains in Barium Cerate Perovskite

Abstract

Anti‐phase boundary (APB) domains resulting from the orthorhombic (o1, Ibmm (No. 74)) to orthorhombic (o2, Pbnm (No. 62)) phase transformation at ∼290°C in pressureless‐sintered barium cerate (BaCeO3) ceramics have been investigated by transmission electron microscopy and large‐angle convergent beam electron diffraction (LACBED). APB exhibiting symmetric π‐fringe patterns in both bright‐field and dark‐field images, lying in {011) with the fault vector R=1/2〈111〉, were determined adopting the invisibility criteria of 2πg·R=0 or 2nπ. The curved boundaries due to symmetry change can be related to the low symmetry phase by a pure translation upon phase transition. The displacement vector, R, relating the domains is the translational symmetry element from the origin to the lattice point that was lost upon transition. The formation of such domains induced by the o1→o2 phase transition is discussed in terms of change in the Bravais lattice and loss of lattice point upon the transition. The identification has also confirmed the existence of a higher symmetry orthorhombic o1 phase along the transition sequence of R3c→Ibmm→Pbnm. The increased spatial frequency of APB domains is attributed to A‐site ordering due to vacancies created by BaO loss and/or disordering of the B‐site substitutional defects generated extrinsically when doped with either a Y3+‐acceptor or a Nb5+‐donor.