IN-SITU TRANSMISSION ELECTRON-MICROSCOPY STUDY OF PHASE-TRANSFORMATIONS IN KNBO3 PEROVSKITE

KNbO3 undergoes the phase transformations cubic (Pm3m) --> tetragonal (P4mm2) --> orthorhombic (Bmm2) at 438 degrees C and 234 degrees C res pectively. The cubic --> tetragonal transformation is accompanied by a paraelectric --> ferroelectric phase transition in the crystal. It is preceded by the appearance of microdomains at 440 degrees C and occur s with characteristic features of a first-order displacive transformat ion (temperature hysteresis, temperature-dependent growth and orientat ion relationship) with the orientation relationship {110}(c) parallel to{110}(t) and (001)(c) parallel to(001)(t). On the other hand, the te tragonal --> orthorhombic phase transformation takes place at exactly 234 degrees C with a front that swept through the thin foil once the t ransformation was initiated. It occurs with the orientation relationsh ip [001[(c) parallel to[001](o) and (100)(t) parallel to(100)(o). The cubic-to-tetragonal transformation involves a purely dilatational mech anism along the principal axes of the cubic phase while the tetragonal -to-orthorhombic transformation involves a shear along the diagonal of the tetragonal phase.