Computational Studies of Aluminum Phosphate Polymorphs

Lattice energy minimization calculations have been carried out on a series of aluminum phosphate polymorphs using a formal charge shell model potential. The experimental structures are reproduced to a reasonable accuracy, especially in cases where high-quality crystallographic data are available on calcined structures. Good agreement is also found with previous calculations on these systems. In cases where the experimental methods give conflicting results regarding the space-group symmetry, calculated structures having lower symmetry than those observed in the crystallographic studies are suggested. An approximately linear dependence of lattice energy on density is observed; the computed lattice energies are found to span a range of 11.7 kJ mol -1 higher than berlinite, which compares to an experimentally determined range of 9.7 kJ mol -1 . Proton binding calculations have been performed on the structure of H-SAPO-37 to determine the most favorable proton sites. Both periodic and isolated defect cluster calculations correctly reproduce the sites which have the highest fractional occupancies in a crystallographic study.