Chemical Bonding and Transport Properties in Clathrates‑I with Cu–Zn–P Frameworks

Quaternary clathrate-I phases with an overall composition of Ba8 M 16+yP30‑y (M = Cu,Zn) exhibit complex structural chemistry. Characterization of the electronic structures and chemical bonding using quantum-chemical calculations and 31P solid state NMR spectroscopy demonstrated that the Cu–Zn–P framework is flexible and able to accommodate up to six Zn atoms per formula unit via bonding rearrangements, such as partial Zn/P substitution and the formation of Cu–Zn bonds. Such perturbations of the framework’s bonding affect the thermal and charge transport properties. The overall thermoelectric figure-of-merit, ZT, of Ba8Cu14Zn2P30 is 0.62 at 800 K, which is 9 times higher than the thermoelectric performance of the ternary parent phase Ba8Cu16P30. Through a combination of inelastic neutron scattering and single crystal X-ray diffraction experiments at 10 K, low-energy rattling of the Ba guest atoms inside the large tetrakaidecahedral cages are shown to be the reason for the low thermal conductivities observed for the studied clathrates.