Optical Properties and van der Waals–London Dispersion Interactions in Berlinite Aluminum Phosphate from Vacuum Ultraviolet Spectroscopy

The interband optical properties of single-crystal berlinite AlPO4 have been investigated in the vacuum ultraviolet (VUV) range using VUV spectroscopy and spectroscopic ellipsometry. The complex optical properties were directly determined from 0.8 to 45 eV. Band gap energies, index of refraction and complex dielectric functions, oscillator index sum rule, and energy loss functions were calculated through Kramers–Kronig transformation. Direct and indirect band gap energies of AlPO4 over the absorption coefficient range of 33–11 000 cm – 1 are 8.06 and 7.89 eV, respectively. The index of refraction at 2 eV, nvis, is 1.51. The interband transition features at 10.4, 11.4, 14.2, 16.2, 17.3, 21, 22.5, 24.5, and 31 eV were indexed and correlated with the electronic structure of AlPO4. Strong similarities were observed between AlPO4 and its structural isomorph SiO2 in the exciton and interband transitions, resulting from the similarity of their constituent tetrahedra and the strong electron localization therein. The London dispersion spectrum for AlPO4 was calculated, and the Hamaker coefficients for AlPO4 with various interlayers were calculated using the Lifshitz method. These results elucidate the role of phosphate complex anions on the electronic structure and van der Waals forces in important organic and inorganic systems.