Modeling of AlPO₄-8, VPI-5, and related structures

The structure and infrared spectrum of AlPO₄-8, the 18-ring aluminophosphate VPI-5, and related structures are simulated using atomistic potentials and a generalized valence force field. The various topologies theoretically possible for the VPI-5 structure indicate that hexagonal prisms and cubes in the structure increase its lattice energy, thus decreasing the plausibility of the structure. This is not the only energy-determining feature; the relative orientation of neighboring tetrahedra also has an effect. The calculated differences in lattice energy between the 14-ring structure AlPO₄-8, the 18-ring structure VPI-5, and a hypothetical 24-ring structure are mainly due to density differences; no direct effect of the ring size on the lattice energy has been observed. Both VPI-5 and AlPO₄-8 show a shift of layers when their lattice energy is minimized by optimizing the crystallographic coordinates and unit cell parameters. The hypothetical 24-ring structure does not show such a shift. This is probably due to the flexibility of the 24-rings that can completely accommodate strains in the initial structure. This is also indicated by the large loss of symmetry of the structure. The simulated infrared spectra reproduce essential differences between the experimental spectra of AlPO₄-8 and VPI-5. The highest frequency modes, however, are calculated to be too high. Spectra calculated with the structures obtained by lattice energy minimization agree better with experiment than those calculated with experimental structures. This can be explained by the presence of more physically acceptable angles in the relaxed structures compared with the experimental structures. The calculated spectrum of the hypothetical 24-ring structure is clearly distinguishable from that of VPI-5.