Synthetic NaScSi2O6 clinopyroxene (cpx) is isomorphous with other Na clinopyroxenes such as widespread natural jadeite (NaAlSi2O6) and synthetic NaInSi2O6. Sc occupies a distorted octahedral (M1) site, while Na occupies an 8-fold coordinated M2 site which may be considered as a very distorted cubic antiprism. In the cpx structures, the SiO4 groups form chains of tetrahedra along c-axis which are linked to strips where octahedral sites M1 alternate with M2 sites along b-axis. The substitution of the cation in the M1 site is known to determine significant changes in the bonding topology of these structures, while overall structure topology is preserved. In a previous study, the dependence of the Si-O distances on the M1 population in a series of nine Na(In,Sc) Si2O6 pyroxenes, has been investigated. The different, i.e., bridging and nonbridging, Si-O bond lengths have been found to correlate in different way with the chemical composition of the cpx. In order to examine the role of the electron density of the M1 ion in determining the bonding topology of these structures, experimental multipole-refined charge density studies on both NaScSi2O6 and NaInSi2O6 synthesized at 6GPa have been undertaken. Here the results obtained for the former structure will be presented. Data have been interpreted with the multipole formalism of Stewart, up to the octopole level for Si, O, Na and the hexadecapole level for Sc. Topological analysis of the electron density and its Laplacian distributions indicates for the Si-O bonds an intermediate character between ionic and covalent, similar to previous charge density investigations of other silicates.3 An intermediate character has been as well obtained for the Sc-O bonds, though with smaller covalency degree with respect to the Si-O bonds, while the Na-O bonds are pure closed-shell ionic interactions. O...O interactions between the O atoms at the vertices of the SiO4 tetrahedron were also detected from the topological analysis of the electron density. They can be described within experimental uncertainty as pure closed-shell interactions and their topological properties are very similar to those found in other silicates.
Experimental multipole-refined charge density study of NaScSi2O6 clinopyroxene
Forni A;
2008
Abstract
Synthetic NaScSi2O6 clinopyroxene (cpx) is isomorphous with other Na clinopyroxenes such as widespread natural jadeite (NaAlSi2O6) and synthetic NaInSi2O6. Sc occupies a distorted octahedral (M1) site, while Na occupies an 8-fold coordinated M2 site which may be considered as a very distorted cubic antiprism. In the cpx structures, the SiO4 groups form chains of tetrahedra along c-axis which are linked to strips where octahedral sites M1 alternate with M2 sites along b-axis. The substitution of the cation in the M1 site is known to determine significant changes in the bonding topology of these structures, while overall structure topology is preserved. In a previous study, the dependence of the Si-O distances on the M1 population in a series of nine Na(In,Sc) Si2O6 pyroxenes, has been investigated. The different, i.e., bridging and nonbridging, Si-O bond lengths have been found to correlate in different way with the chemical composition of the cpx. In order to examine the role of the electron density of the M1 ion in determining the bonding topology of these structures, experimental multipole-refined charge density studies on both NaScSi2O6 and NaInSi2O6 synthesized at 6GPa have been undertaken. Here the results obtained for the former structure will be presented. Data have been interpreted with the multipole formalism of Stewart, up to the octopole level for Si, O, Na and the hexadecapole level for Sc. Topological analysis of the electron density and its Laplacian distributions indicates for the Si-O bonds an intermediate character between ionic and covalent, similar to previous charge density investigations of other silicates.3 An intermediate character has been as well obtained for the Sc-O bonds, though with smaller covalency degree with respect to the Si-O bonds, while the Na-O bonds are pure closed-shell ionic interactions. O...O interactions between the O atoms at the vertices of the SiO4 tetrahedron were also detected from the topological analysis of the electron density. They can be described within experimental uncertainty as pure closed-shell interactions and their topological properties are very similar to those found in other silicates.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
