Structural, electronic, and optical properties of polymorphic phases of the LiBaPO4 compound

In this work, we present a comprehensive first-principles study of the structural, electronic, and optical properties of LiBaPO₄ in its four experimentally proposed crystalline phases: Cc, P3₁c, P6₃, and Pmcn. An improved electronic structure description for the low-temperature Cc and P3₁c phases is achieved using the mBJ exchange potential, revealing similar wide bandgaps (∼7.6 eV) in both phases. This result contrasts with earlier reports and is consistent with the similar Ba–O coordination environments in these phases. For the high-temperature phases, we predict the atomic positions for the P6₃ and Pmcn structures. However, due to convergence issues and structural inconsistencies with Pmcn, we propose that the correct space group is Pnma, which yields results in better agreement with experimental lattice parameters. Analysis of bond lengths, coordination numbers, and bandgap values supports a sequence of displacive phase transitions among the Cc, P3₁c, and P6₃ phases, as all three share similar local atomic environments and electronic structures. In contrast, the transition from P6₃ to Pnma is characterized as reconstructive, evidenced by a change in Li coordination from tetrahedral to hepta-coordinated, significant alterations in Ba–O bond lengths, and a slight reduction in bandgap. Additionally, we calculated the optical absorption and reflectivity spectra for all four phases, providing theoretical guidance for future experimental investigations.