Huge impact of compressive strain on phase transition temperatures in epitaxial ferroelectric KxNa1-xNbO3 thin films

We present a study in which ferroelectric phase transition temperatures in epitaxial KxNa1-xNbO3 films are altered systematically by choosing different (110)-oriented rare-earth scandate substrates and by variation of the potassium to sodium ratio. Our results prove the capability to continuously shift the ferroelectric-to-ferroelectric transition from the monoclinic MC to orthorhombic c-phase by about 400 °C via the application of anisotropic compressive strain. The phase transition was investigated in detail by monitoring the temperature dependence of ferroelectric domain patterns using piezoresponse force microscopy and upon analyzing structural changes by means of high resolution X-ray diffraction including X-ray reciprocal space mapping. Moreover, the temperature evolution of the effective piezoelectric coefficient d33,f was determined using double beam laser interferometry, which exhibits a significant dependence on the particular ferroelectric phase. © 2019 Author(s)

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

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 manifestingitselfintheformationofahighlyregular,herringbone likedomainarrangement.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 amp; 9702;C to about 260 amp; 9702;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 superdomain