A tetragonal phase is predicted for Hf2O3 and Zr2O3 using density functional theory. Starting from atomic and unit cell relaxations of substoichiometric monoclinic HfO2 and ZrO2, such tetragonal structures are only reached at zero temperature by intr
oducing the oxygen vacancy pair with the lowest formation energy. The tetragonal Hf2O3 and Zr2O3 structures belong to space group P-4m2 and are more stable than their corundum structure counterparts. These phases are semi-metallic, as confirmed through further G0W0 calculations. The carrier concentrations are estimated to be 1.77E21 cm^{-3} for both electrons and holes in tetragonal Hf2O3, and 1.75E21 cm^{-3} for both electrons and holes in tetragonal Zr2O3. The tetragonal Hf2O3 phase is probably related to the low resistivity state of hafnia-based resistive random access memory (RRAM).
