Large differences in the electronic structure and spectroscopic properties of three phases ofAlPO4fromab initiocalculations

The electronic structure, interatomic bonding, and spectroscopic properties of the three known phases of $\mathrm{Al}\mathrm{P}{\mathrm{O}}_{4}$ in trigonal, orthogonal, and monoclinic structures formed at different pressures were studied using the density functional method. These three crystals have the same chemical formula and the same percentages of different atomic species but distinct local atomic coordination which offers a unique opportunity to systematically investigate their structure-properties relationship. The $\mathrm{Al}\text{\ensuremath{-}}K$, $\mathrm{Al}\text{\ensuremath{-}}{L}_{3}$, $\mathrm{P}\text{\ensuremath{-}}K$, $\mathrm{P}\text{\ensuremath{-}}{L}_{3}$, and $\mathrm{O}\text{\ensuremath{-}}K$ x-ray absorption near edge structure (XANES) edges were calculated using a supercell approach including the effects of core hole. The large differences in these properties among the three crystals are demonstrated and explained in terms of the unique structural units present in each phase. These results are in contradiction to the prevailing notion of using fingerprinting for XANES spectra interpretation.