Calculation of the electric field gradients at 'tricluster'-like O atoms in the polymorphs of Al2SiO5 and in aluminosilicate molecules: models for tricluster O atoms in glasses

O-17 NMR studies and molecular dynamics calculations have suggested the pre sence in some aluminosilicate glasses of 'tricluster' O atoms, O[3], O atom s which are bonded to three four-coordinated Al or Si atoms. Recent ab init io molecular orbital calculations on simple models for such species obtaine d electric field gradients (EFG) and nuclear quadrupole coupling constants (NQCC) at these O atoms which were considerably larger than those observed by NMR, casting doubt on the assignment of such features to triclusters. We present calculations using density functional theory (DFT) and full-potent ial linearized augmented plane wave (FLAPW) method on the O EFG values for the eight chemically inequivalent O sites in the andalusite and sillimanite polymorphs of Al2SiO5, most of which have local geometries closely related to triclusters. We also present calculated O EFG values for several differ ent tricluster model species using Hartree-Fock and DFT methods within a cl uster approach. These new model species contain tricluster O atoms with eit her non-planar geometries about O or O-O edge-sharing geometries. The edge- sharing tricluster geometries give O EFG values which are systematically mu ch lower than those of the species previously considered, Many well-charact erized examples of such compounds have been reported in the inorganic chemi stry literature. Some of the edge-sharing clusters give O EFG values and NM R shieldings in the range attributed to tricluster O atoms in Ca aluminosil icate glasses. O EFG values calculated by both band and cluster methods for tricluster O atoms are found to correlate well with the sum of the Pauling or Brown bond strength at the O, weighted by the fractional covalency of t he bond. For the more interesting edge-sharing species this EFG vs. (bond s trength x covalency) correlation line is offset from that for the corner-sh aring geometries by about 0.5 a.u., corresponding roughly to a systematic r eduction of 2.5 MHz in the coupling constant. (C) 2001 Elsevier Science B.V . All rights reserved.