RELATION BETWEEN CRYSTAL SYMMETRY AND IONICITY IN SILICA POLYMORPHS

THE structure and stability of an inorganic solid is determined in general both by short-range covalent and by long-range electrostatic forces. Here we describe the use of interatomic force fields developed recently from first-principles quantum-chemical cluster calculations 1, 2 in the study of the structures of SiO2 tetrahedral networks. We find that the symmetry of these structures depends sensitively on the balance between ionic and covalent forces: high-symmetry structures are stabilized for relatively large ion partial charges, and low-symmetry structures are stabilized when the ionicity is small. For some SiO2 polymorphs, the low-symmetry structures found in our simulations correspond to the low-temperature phases of these polymorphs found experimentally. A reinterpretation of structural data on quartz provides evidence for temperature dependence of the ionicity, which can explain the change of symmetry observed when temperature is increased. Our preliminary calculations on aluminophosphates suggest that this symmetry-breaking mechanism may also provide insight into the structural changes observed for complex molecular sieves.