Comprehensive First-Principles Modeling of Experimentally Synthesized BiPO₄ Polymorphs

Here, we present a theoretical and experimental investigation of hexagonal P3(1)21 and monoclinic P2(1)/n bismuth phosphate BiPO4 (BPO). The Hubbard U-corrected density functional theory (DFT+U) failed to tune the electronic band gap of BPO due to the negligible contribution of Bi 5d orbitals near the band edges of BPO. The Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional with Hartree-Fock exact-exchange mixing alpha(HF) = 25% suffered from band gap overestimation. The inclusion of spin-orbit coupling (SOC) and alpha(HF) tuning in the screened hybrid functional modeled the optical absorption and band gap in diffuse reflectance measurements. The experimentally measured indirect nature and magnitude of the band gap were validated from the SOC-incorporated tuned hybrid functional-based electronic band structure (BS) and density of states (DOS) simulations. The experimental Raman peaks were identified in reliable Raman tensor simulations. The simulated phonon BS and DOS from the finite difference supercell approach confirmed the dynamical stability and explained the infrared absorption of BPO, respectively. The mechanical stability was probed by an elastic tensor simulation with a sufficiently high energy cutoff. Overall, this work may help to understand the experimentally synthesized BPO polymorphs from a theoretical standpoint.