In-situ x-ray diffraction study of phase transitions in epitaxial KNbO₃ thin films

Epitaxial and stoichiometric thin films of ferroelectric KNbO₃ (110) were deposited on SrTiO₃ (100) substrate by Jaser ablation. From X-ray diffraction, in the orthorhombic phase of KNbO₃, KNbO₃[11̄0]//SrTiO₃[010], [01̄0], [001] or [001̄] which gives rise to polarization at 45° to the substrate plane. In the tetragonal KNbO₃ phase, KNbO₃ 〈100〉//SrTiO₃ 〈100〉, which gives rise to the polarization perpendicular to or in the substrate plane. The variation of lattice parameter with temperature was determined using in-situ X-ray diffraction measurements with temperature. The phase transitions show hysteresis of ∼10° to 25° with heating and cooling which is typical of first order transitions. The transition temperatures were determined to be in the range of 207 ± 12°C for orthorhombic-tetragonal transition and 410 ± 5°C for tetragonal-cubic transition Evidence for in-plane biaxial stresses in the high temperature ferroelectric tetragonal phase of KNbO₃ is observed.