Strain-induced phase transitions in epitaxial NaNbO₃ thin films grown by metal-organic chemical vapour deposition

Epitaxially strained NaNbO₃ films were grown by liquid-delivery spin metal-organic chemical vapour deposition on several oxide substrates, inducing tensile and compressive lattice strain. High-resolution X-ray diffraction measurements reveal that coherently grown compressively strained NaNbO₃ films on NdGaO₃ exhibit the orthorhombic c phase. With increasing in-plane strain a first structural phase transition to the monoclinic r phase and, further on, for films grown under tensile strain on rare earth scandates, a second phase transition to the aa phase, are observed. Our results are in good agreement with the pathway of phase transitions predicted by Diéguez, Rabe & Vanderbilt [Phys. Rev. B, (2005), 72, 144101] for NaNbO₃ films.