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 () 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.
Highlights
The discovery of the ferroelectric properties of BaTiO3 in 1946 [1] led to an extended search for materials with similar properties and in 1951, Matthias & Remeika [2] showed the ferroelectric nature of potassium niobate, KNbO3
The difference between the calculated diffraction pattern from the models obtained by Rietveld refinement and the three experimental diffraction patterns are shown in figure 1
The converging unit cell parameters towards cubic values upon heating of the tetragonal phase point to a displacive nature of the transition from tetragonal to cubic, but based on the divergence of atomic positions we suggested a 5 significant order–disorder component to the phase transition
Summary
The discovery of the ferroelectric properties of BaTiO3 in 1946 [1] led to an extended search for materials with similar properties and in 1951, Matthias & Remeika [2] showed the ferroelectric nature of potassium niobate, KNbO3. They found evidence for two temperature-induced phase transitions, from orthorhombic to tetragonal symmetry at 224°C, and from tetragonal to cubic at 434°C by dielectric spectroscopy.
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