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

Abstract

Epitaxial and stoichiometric thin films of ferroelectric KNbO₃ (110) were deposited on SrTiO₃ (100) substrate by laser ablation. From X-ray diffraction, in the orthorhombic phase of KNbO₃, KNbO₃[10]//SrTiO₃[010], [00], [001] or [00] 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.

Key Words:

• Keywords KNbO₃, in-situ X-ray diffraction, phase transitions