Absence of low temperature phase transitions and enhancement of ferroelectric transition temperature in highly strained BaTiO3 epitaxial films grown on MgO Substrates

Recently, a large enhancement in the ferroelectric transition temperature of several oxides is reported by growing the respective thin films on appropriate substrates. This phenomenon is correlated with high residual strain in thin films often leading to large increase in the tetragonality of their crystal structure. However, such an enhancement of transition temperature is usually limited to very thin films of similar to 10nm thickness. Here, we report growth of fully strained epitaxial thin films of BaTiO3 of 400 nm thickness, which are coherently grown on MgO substrates by pulsed laser deposition technique. Conventional high resolution x-ray diffraction and also the reciprocal space map measurements confirm that the film is fully strained with in-plane tensile strain of 5.5% that dramatically increases the tetragonality to 1.05. Raman measurements reveal that the tetragonal to cubic structural phase transition is observed at 583K, which results in an enhancement of similar to 200K. Furthermore, temperature dependent Raman studies on these films corroborate absence of all the low temperature phase transitions. Numerical calculations based on thermodynamical model predict a value of the transition temperature that is greater than 1500 degrees C. Our experimental results are therefore in clear deviation from the existing strain dependent phase diagrams. (C) 2015 AIP Publishing LLC.