Calculation of bridging oxygen O17 quadrupolar coupling parameters in alkali silicates: A combined ab initio investigation

Ab initio band-structure calculations based on the density functional theory have been performed for several crystalline Li, Na, and K-silicates to obtain electric-field gradients (efg) for oxygen atoms. The efg for bridging oxygen environments in these compounds were also investigated by performing ab initio self-consistent field Hartree-Fock molecular orbital calculations on silicate clusters, and there is good agreement between these two approaches. By performing additional ab initio quantum chemistry calculations on model silicate clusters the factors influencing the O17 quadrupole coupling parameters for bridging oxygen environments in alkali silicates have been examined. The quadrupolar asymmetry parameter was found to be dependent on the Si-O-Si angle and the nature of the modifier cation, in agreement with previous studies. In contrast, the quadrupolar coupling constant was found to have a strong dependence on Si-O distance, as well as Si-O-Si angle and the nature of the modifier cation. Analytical expressions describing these dependencies are proposed, which should assist in describing the local environments of bridging oxygen in crystalline and amorphous materials.