Growth and structure of Mg2TiO4 on MgO (001)*

The unique properties of two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, have been used for applications such as high mobility channels and as hosts for quantum optical phenomena. In transition metal oxides (TMOs), a similarly broad range of properties and devices may be possible to realize by leveraging the diverse properties and structures found in these materials and synthesizing them as ultrathin 2D films. In order to use 2D TMOs, understanding the limits of 2D oxide heteroepitaxy is crucial. Here, we report that growth of TiO2 on MgO using molecular beam epitaxy (MBE) results in formation of epitaxial Mg2TiO4. Using a combination of in situ crystal truncation rod (CTR) measurements and first-principles density functional theory (DFT) calculations, the layer-resolved structure of Mg2TiO4 that is a few unit cells thick is determined, and the thermodynamic driving force behind Mg2TiO4 formation is calculated. These measurements resolve distinct structural features at the Mg2TiO4 interface with MgO, which is due to the polarity of Mg2TiO4, pointing to new ways to synthesize distinct 2D electronic structures in thin films.