Polimorfismo cristalino e estrutura eletrônica de ortofosfatos sob alta pressão e alta temperatura: um estudo de primeiros princípios

Abstract

This thesis addresses two research topics involving a class of materials known as orthophosphates, which are of significant scientific and technological interest: (1) the investigation of the structural and electronic properties of APO4 compounds (A = Sc, Lu, Y) under hydrostatic pressure up to 100 GPa, and (2) the study of the structural, electronic, and optical properties of LiBaPO4 across its four experimentally observed crystalline phases up to a temperature of 1373 K. All investigations were carried out using ab initio calculations based on density functional theory. In the first topic, the results confirm the sequence of pressure-induced phase transitions observed experimentally: zircon > scheelite for ScPO4 (with a scheelite > monazite transition predicted at 94 GPa), zircon > monazite > scheelite for YPO4, and zircon > scheelite for LuPO4. The calculated critical pressures, pressure-induced volume collapses, and bulk moduli for each crystalline phase are in good agreement with available experimental data. It was found that, in the zircon phase, two distinct sets of A–O bonds within the AO8 dodecahedron exhibit significantly different compressibilities. This behavior was explained and correlated with an unusual increase in the AO8 polyhedral volume at the critical pressures triggering the zircon > scheelite phase transition. In the scheelite phase, both sets of A–O bonds display approximately equal compressibilities. Despite undergoing pressure-induced structural phase transitions, all compounds exhibit remarkable electronic stability, with consistent band characteristics dominated by O 2p states in the valence band and A d states in the conduction band. The band gap increases with pressure within each crystalline phase but decreases abruptly at the critical pressures due to the expansion of the more compressible set of A–O bonds. These findings highlight the exceptional mechanical resistance to compression and electronic stability of orthophosphates, supporting their potential for applications under extreme conditions. In the second topic, four crystalline phases of LiBaPO4 proposed experimentally were investigated: Cc, P31c, P63, and Pmcn. An improved description of the electronic structure of the low-temperature Cc and P31c phases revealed wide and nearly identical band gaps (≈ 7.6 eV) for both phases, in contrast to previous reports. This result is consistent with the similar Ba–O bond lengths, which play a key role in determining the band-gap width. For the high-temperature phases, the atomic positions of the P63 and Pmcn structures were determined. However, due to convergence issues and structural inconsistencies associated with the Pmcn phase, it is proposed that the correct space group is Pnma, which yields better agreement with experimental lattice parameters. Analysis of bond lengths, coordination numbers, and band-gap values supports a sequence of displacive phase transitions among the Cc, P31c, and P63 phases, as these structures 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, significant modifications in Ba–O bond lengths, and a slight reduction in the band gap. In addition, optical absorption and reflectivity spectra were calculated for all four phases, providing theoretical guidance for future experimental investigations.