Ferroelectric phase transitions in multi-domain K0.9Na0.1NbO3 epitaxial thin films

Abstract

A high-temperature phase transition in strained ferroelectric K0.9Na0.1NbO3 thin films epitaxially grown on orthorhombic (110) NdScO3 substrates is identified and investigated by in situ x-ray diffraction and piezoresponse force microscopy. At room temperature, the thin films exhibit a highly anisotropic misfit strain, inducing the occurrence of monoclinic a1a2/MC phases and manifesting itself in the formation of a highly regular, herringbone-like domain arrangement. With increasing temperature, a ferroelectric-to-ferroelectric phase transition to an orthorhombic a1/a2 phase with exclusive lateral electrical polarization takes place. Within a wide temperature range from 180 °C to about 260 °C, a coexistence of the monoclinic a1a2/MC room temperature phases and the orthorhombic a1/a2 high temperature phase is observed. Finally, at higher temperatures only the orthorhombic a1/a2 phase, which is arranged in a regular stripe domain pattern, is present. Corresponding simulations of the scattered x-ray intensity patterns show that the orthorhombic unit cells undergo a small in-plane rotation. This leads to four different in-plane orientations of the orthorhombic unit cells and four corresponding variants of superdomains.