## 1 Introduction  
Topological insulators [1; 2], which are characterized by an insulating bulk state and a unique protected gapless surface state, have been of particular interest in the past decades. Topological insulators are a new kind of material that have attracted much attention due to their interesting properties and great potential for spintronic devices [3; 4]. For example, the metallic surface states of the TIs are linearly dispersed in the momentum space with the helical spin textures that are protected by time-reversal symmetry and are also characterized by the so-called \(Z_{2}\) topological invariant [5]. These interesting features have increased the interest in TIs for scientists in both experimental and theoretical [6] research fields. In 2006 [7], the first 3D topological insulator phase was theoretically predicted. The element with a large spin-orbit coupling (SOC) effect is crucial for the topology. Theoretically, not only TI phase itself is interesting, but the combination with other physical properties or phases, such as superconductivity and magnetism [8; 9], is also promising. Several kinds of crystal structures are found or predicted to have a topological insulator phase: for example, Bi\({}_{2}\)Te\({}_{3}\)[1] and Bi\({}_{2}\)Se\({}_{3}\)[1] in the hexagonal structure; LaPtBi [10; 11] half-Heusler compound [9; 12; 13; 14; 15; 16; 17], oxide perovskite [18; 19], halide perovskite [20; 21; 22], binary compound [23], Ba\({}_{2}\)BiO\({}_{6}\)[24], and CaTePoO\({}_{6}\)[25] double perovskites in the cubic structure. The bulk bandgap for these TIs are in general smaller than 0.5 eV; for example, bismuth-based compounds such as Bi\({}_{2}\)Te\({}_{3}\)[26] are 0.17 eV, Bi\({}_{2}\)Te\({}_{2}\)S [27] 0.3 eV, and PbBi\({}_{4}\)Te\({}_{4}\)S\({}_{3}\) is 0.2 eV in the experiment and 0.3 eV in theory. In 2012, oxide perovskite YBiO\({}_{3}\)[18; 19] was also theoretically predicted to be a topological insulator with a bandgapof 0.33 eV. Further investigating cubic perovskites, CsPbI3[20; 28], CsSnI3[29], CsSbBr3[29], and CsSnCl3[29] were also found to be topological insulators. The bandgap depends on the external pressure. Large bandgap topological insulators are also found in the double perovskites. For example, the bandgaps for Ba2 BiO6[24], Sr2 BiO6[24], Ca2 BiO6[24], Ba2 BiBrO6[24], Sr2 BiBrO6[24], and Ca2 BiBrO6[24] are 0.55, 0.53, 0.52, 0.33, 0.32, and 0.34 eV, respectively. Ca2 TePO6[25], Sr2 TePO6[25], and Ba2 TePO6[25] double perovskites are also predicted to have theoretically large bandgap topological insulators, with bandgaps of 0.325, 0.400, and 0.375 eV, respectively. Nonetheless, large bandgap TIs could merely be found in two-dimensional structures. For example, PbXF2[30], PbC2[31], and BiC2[31] (X = H, Cl, F, Br, and I) nanoribbons are theoretically predicted to have a dramatically large bandgap ranging between 0.79 eV and 0.99 eV; Bi (111) monolayer[32] on Bi2 Te3 also has a bandgap of 0.5 eV; PbAF2 (A = C, Si, Ge, and Sn) is also theoretically predicted to have an indirect bandgap ranging from 0.3 eV to 0.7 eV. Most recently, the PbCB3[33] (B = H, F, and Cl) monolayer is also theoretically reported as TI, and the bandgap ranges from 0.84 to 0.98 eV. Polonium is a material first isolated by [PERSON]. It is a metal and has the lowest electronegativity in the group of chalcogens[34]. Polonium usually shows +2 and +4 oxidation states in their compounds with oxygen[35]. Compared with selenium and tellurium, the magnitude of SOC from polonium is larger and expected to play an important role in the electronic properties of ABO3 with perovskite structures. This work aimed to provide a detailed characterization of the electronic structures of the large bandgap oxide APOO3 perovskites. This paper is organized as follows. The crystal structure and computational method are described in Section 2. The structural and electronic properties without/with spin-orbit coupling taken into account are discussed in Section 3. Further, mechanical and thermodynamic properties are then discussed.