Despite being successfully synthesized [Zhang et al., Nat. Mater. 20, 1073 (2021)], the monolayer structure of stable hexagonal ${\mathrm{TiO}}_2$ is unknown, and it is not even clear whether it can exist in a freestanding form. Through first-principles calculations, we have identified two previously uncharted stable structures, namely, distorted $1\times \sqrt{3} 1T-{\mathrm{TiO}}_2$ and $\sqrt{3}\times \sqrt{3} 1T-{\mathrm{TiO}}_2$, both of which are energetically more favorable than commonly adopted $1H$ and $1T$ phases. Here, structural distortions are characterized by the out-of-plane shifts of Ti atoms due to the pseudo-Jahn-Teller interactions, which break one and all two inversion symmetries of $1T$ configuration. As a consequence, the $1\times \sqrt{3} 1T$ remains centrosymmetric, while the $\sqrt{3}\times \sqrt{3} 1T$ exhibits out-of-plane ferroelectricity. Electronic structure calculations show that both are wide-band-gap semiconductors with band gaps larger than their bulk counterparts. In this paper, we not only deepen the understanding of structural instability in wide-gap semiconductors but also add a member to the rare family of two-dimensional out-of-plane ferroelectrics.

• Received 5 February 2024
• Revised 26 March 2024
• Accepted 12 April 2024

DOI:https://doi.org/10.1103/PhysRevB.109.165435