Breaking the Tetra‐Coordinated Framework Rule: New Clathrate Ba 8 M 24 P 28+ δ ( M =Cu/Zn)

A new clathrate type has been discovered in the Ba/Cu/Zn/P system. The crystal structure of the Ba 8 M 24 P 28+ 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 Ba 8 M 24 P 28+ δ a 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‐state 31 P 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.