In-situtransmission electron microscopy study of phase transformations in KNbO₃perovskite

KNbO3 undergoes the phase transformations cubic (Pm3m) →tetragonal (P4mm2) →orthorhombic (Bmm2) at 438°C and 234°C respectively. The cubic →tetragonal transformation is accompanied by a paraelectric →ferroelectric phase transition in the crystal. It is preceded by the appearance of o microdomains at 440°C and occurs with characteristic features of a first-order displacive transformation temperature hysteresis, temperature-dependent growth and orientation relationship with the orientation relationship {110}c‖{110}t and (001)c|(001)t. On the other hand, the tetragonal →orthorhombic phase transformation takes place at exactly 234°C with a front that swept through the thin foil once the transformation was initiated. It occurs with the orientation relationship [001[c‖[001]o and (100)t‖(100)o. The cubic-to-tetragonal transformation involves a purely dilatational mechanism along the principal axes of the cubic phase while the tetragonal-to-orthorhombic transformation involves a shear along the diagonal of the tetragonal phase.