PNNL

Abstract

Control of crystal orientations in thin films of functional materials allows for predictive tuning of their strain states, electronic properties, and surface chemical reactivity. Here, we investigate conditions for orientation control in epitaxial PdO films. Due to its tetragonal structure, PdO can form two orientational relationships with the MgO (001). We show that, under an oxygen-rich environment provided by oxygen-plasma-assisted molecular beam epitaxy, both (00l)- and (100)-oriented PdO domains form on MgO (001). Subsequent thermal annealing in a vacuum promotes film restructuring to a predominantly (100)-oriented PdO with improved crystallinity. Ab initio calculations reveal that the (001) orientation has lower strain energy but weaker interfacial interactions and serves as an oxygen vacancy sink, whereas the (100) orientation benefits from significantly stronger MgO-PdO bonding. Consequently (100)- oriented domains become favored under oxygen-poor conditions. We propose a mechanism whereby vacuum annealing drives orientation transformation by generating oxygen vacancies that destabilize the (001) domains and promote (100) ordering. Our findings deepen the understanding of how oxygen content impacts interfacial stability and reorganization, thereby offering a route to tune domain orientations in oxide thin films.