Calculation of the electric field gradients at `tricluster'-like O atoms in the polymorphs of Al 2SiO 5 and in aluminosilicate molecules: models for tricluster O atoms in glasses

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17O NMR studies and molecular dynamics calculations have suggested the presence in some aluminosilicate glasses of `tricluster' O atoms, O[3], O atoms which are bonded to three four-coordinated Al or Si atoms. Recent ab initio molecular orbital calculations on simple models for such species obtained 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-potential 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 Al 2SiO 5, most of which have local geometries closely related to triclusters. We also present calculated O EFG values for several different tricluster model species using Hartree–Fock and DFT methods within a cluster approach. These new model species contain tricluster O atoms with either non-planar geometries about O or O–O edge-sharing geometries. The edge-sharing tricluster geometries give O EFG values which are systematically much lower than those of the species previously considered. Many well-characterized examples of such compounds have been reported in the inorganic chemistry literature. Some of the edge-sharing clusters give O EFG values and NMR shieldings in the range attributed to tricluster O atoms in Ca aluminosilicate 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 the bond. For the more interesting edge-sharing species this EFG vs. ( bond strength × covalency ) correlation line is offset from that for the corner-sharing geometries by about 0.5 a.u., corresponding roughly to a systematic reduction of 2.5 MHz in the coupling constant.