Effect of (001) and (111) epitaxial strain on Pnma perovskite oxides

With recent advances in strain engineering and its widespread applications, it is becoming increasingly important to understand the effect of biaxial strain on the most common structural phase of perovskites - the orthorhombic Pnma structure. In this work, by using a combination of group theory and first-principles density functional theory, we study the effect of biaxial strain on (001)- and (111)-oriented CaTiO3, SrSnO3, and SrZrO3 films to find phenomena common in different compounds and strain planes. We observe manifestly different behaviors depending on the strain orientation, with common trends emerging between different materials. In addition to the many different structural phases previously observed in individual compounds, we identify a transition between two different phases with the same space group name (P21/c) but different symmetries in (111)-strained materials. We also find that allowing the relaxation of the out-of-plane monoclinic angles, often ignored in first-principles studies, leads to significant stabilization of certain phases and is essential to correctly determine the structural ground state.