Thermal evolution of the crystal structure and phase transitions of KNbO3

Abstract

The thermal evolution of the crystal structure and phase transitions of KNbO₃ were investigated by high-temperature powder X-ray diffraction and Rietveld refinement of the diffraction data. Two phase transitions from orthorhombic (Amm2) to tetragonal (P4mm) and from tetragonal to cubic ( $Pm\overline{3}m$ ) were confirmed, both on heating and cooling. Both phase transitions are first order based on the observed hysteresis. The mixed displacive and order-disorder nature of the tetragonal to cubic transition is argued based on symmetry and apparent divergence of the atomic positions from pseudo-cubic values. The transition between the orthorhombic and tetragonal phase shows no temperature-dependence for atomic positions and only thermal expansion of the unit cell parameters and is thus discussed in relation to a lattice dynamical instability.

Keywords: KNbO3; high-temperature powder X-ray diffraction; phase transitions; thermal evolution.