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 M-C to orthorhombic c-phase by about 400 degrees 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 d(33,f) was determined using double beam laser interferometry, which exhibits a significant dependence on the particular ferroelectric phase.