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

• Published in: Journal of non-crystalline solids 2001-07, Vol.286 (3), p.187-199
• Main Authors: TOSSELL, J. A, COHEN, R. E
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Magnetic resonances and relaxations in condensed matter, mössbauer effect; Nuclear magnetic resonance and relaxation; Physics
• ISSN: 0022-3093; 1873-4812
• DOI: 10.1016/s0022-3093(01)00506-3

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 larger than those observed by NMR, casting doubt on the assignment of such features to triclusters. Authors 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 Al2SiO5, most of which have local geometries related to triclusters. Authors 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 nonplanar geometries about O or O-O edge-sharing geometries. The edge-sharing tricluster geometries give O EFG values which are 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 correlate well with the sum of the Pauling or Brown bond strength at the O, weighted by the fractional covalency of the bond. For 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. 50 refs.