Phase-transformation-induced twinning in orthorhombic BaCeO3

Phase-transformation-induced twinning in orthorhombic barium cerate (BaCeO3) has been analyzed using transmission electron microscopy. Reflection twins with boundaries lying in {110) and {112) are generated by loss of point group symmetry elements during solid-state phase transition along the sequence Pm3m -> R3c -> o(1)-Ibmm -> o(2)-Pbnm when pressureless-sintered samples were cooled from 1400 degrees C to room temperature. Fault vectors R=epsilon < 110] and epsilon < 021], in which displacements are not related in a simple way to the lattice translation, are determined for the twin boundaries exhibiting delta-fringe patterns. The high- and low-symmetry phases, rhombohedral and orthorhombic, are not related by group-subgroup symmetry, and the transformation is discontinuous and first order in nature, where twin relationships in the low-symmetry phase are forbidden by the Landau theory. However, the transformation twins observed experimentally are consistent with those predicted directly from assuming a non-disruption condition between transition phases, where the new structure is described in the frame of the old one geometrically. Such a phase transition can be continuous and diffusionless if it occurs via second order to and from a metastable intermediate phase, which is a shared common space group of the high- and low-symmetry phases. Accordingly, two possible intermediate phases of the minimal common supergroup cubic Pm3m (No. 221) and of the maximal common subgroup monoclinic C2/c (No. 15) for the two end phases are identified.