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+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 31P 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.
A rule‐breaking clathrate: A new clathrate type of the Ba/Cu/Zn/P system was synthesized and characterized by a combination of X‐ray and neutron diffraction. The structure was confirmed by solid‐state 31P NMR spectroscopy and shows that the framework atoms are not tetrahedrally coordinated.