Large Bandgap Topological Insulator in Oxide APoO₃ (A = Be, Mg, Ca, Sr, Ba, and Ra) Perovskite: An Ab Initio Study

Under the density functional theory framework, we have calculated the electronic and elastic properties of APoO<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>3</mn></msub></semantics></math></inline-formula> (A = Be, Mg, Ca, Sr, Ba, and Ra) cubic perovskites. We found that CaPoO<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>3</mn></msub></semantics></math></inline-formula>, SrPoO<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>3</mn></msub></semantics></math></inline-formula>, BaPoO<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>3</mn></msub></semantics></math></inline-formula>, and RaPoO<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>3</mn></msub></semantics></math></inline-formula> are topological insulators (TIs) with very large bandgaps of 0.861, 0.871, 0.820, and 0.810 eV, respectively. The nontrivial band topology together with the Z<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>2</mn></msub></semantics></math></inline-formula> topological number of APoO<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>3</mn></msub></semantics></math></inline-formula> perovskite are investigated. We also theoretically determine the three independent elastic constants C<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>11</mn></msub></semantics></math></inline-formula>, C<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>12</mn></msub></semantics></math></inline-formula>, and C<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>44</mn></msub></semantics></math></inline-formula> of the APoO<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>3</mn></msub></semantics></math></inline-formula> perovskite. The bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and anisotropy factor are also calculated from the obtained elastic constants. We found that the Debye temperature for the APoO<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>3</mn></msub></semantics></math></inline-formula> perovskite is around 330-370 K. In the bulk APoO<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>3</mn></msub></semantics></math></inline-formula> perovskite, if the center Po atom is shifted 0.09Å away from the center, the induced electric polarization is quite large, being around 0.02 C/m<inline-formula><math display="inline"><semantics><msup><mrow></mrow><mn>2</mn></msup></semantics></math></inline-formula>. In the surface band calculation, we found that both AO and PoO<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>2</mn></msub></semantics></math></inline-formula> surfaces give rise to contributions to the conduction channel. If the Po atom moves both in-plane and out-of-plane, we show that both electric polarization and topologically protect surface conduction states exist in APoO<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>3</mn></msub></semantics></math></inline-formula> perovskite, indicating that these oxide APoO<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>3</mn></msub></semantics></math></inline-formula> perovskites are ferroelectric TIs and might be useful for spintronic applications.