title: Breaking the Tetra-Coordinated Framework Rule: New Clathrate Ba8M24P28+δ(M=Cu/Zn).
abstract: A new clathrate type has been discovered in the Ba/Cu/Zn/P system. The crystal structure of the Ba8M24P28+δ(M=Cu/Zn) clathrate is composed of the pentagonal dodecahedra common to clathrates along with a unique 22-vertex polyhedron with two hexagonal faces capped by additional partially occupied phosphorus sites. This is the first example of a clathrate compound where the framework atoms are not in tetrahedral or trigonal-pyramidal coordination. In Ba8M24P28+δa majority of the framework atoms are five- and six-coordinated, a feature more common to electron-rich intermetallics. The crystal structure of this new clathrate was determined by a combination of X-ray and neutron diffraction and was confirmed with solid-state31P NMR spectroscopy. Based on chemical bonding analysis, the driving force for the formation of this new clathrate is the excess of electrons generated by a high concentration of Zn atoms in the framework. The rattling of guest atoms in the large cages results in a very low thermal conductivity, a unique feature of the clathrate family of compounds.
authors.affiliation: Department of Chemistry, The University of California, Davis, One Shields Avenue, Davis, CA, 95616, USA.
authors.forename: Juli-Anna
authors.initials: JA
authors.lastname: Dolyniuk
authors.affiliation: Department of Chemistry, The University of California, Davis, One Shields Avenue, Davis, CA, 95616, USA.
authors.forename: Julia V
authors.initials: JV
authors.lastname: Zaikina
authors.affiliation: Department of Materials Science and Engineering, The University of California, Davis, One Shields Avenue, Davis, CA, 95616, USA.
authors.forename: Derrick C
authors.initials: DC
authors.lastname: Kaseman
authors.affiliation: Department of Materials Science and Engineering, The University of California, Davis, One Shields Avenue, Davis, CA, 95616, USA.
authors.forename: Sabyasachi
authors.initials: S
authors.lastname: Sen
authors.affiliation: Department of Chemistry, The University of California, Davis, One Shields Avenue, Davis, CA, 95616, USA.
authors.forename: Kirill
authors.identifier: 0000-0003-1152-1912
authors.initials: K
authors.lastname: Kovnir
keywords: NMR spectroscopy; chemical bonding; crystal structures; inorganic clathrates; thermoelectric materials