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

Quaternary
clathrate-I phases with an overall composition of Ba8M16+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.

整体组成为Ba₈M₁₆₊ᵧP₃₀₋ᵧ（M=Cu、Zn）的四元I型笼型结构（clathrate-I）相展现出复杂的结构化学特性。借助量子化学计算与³¹P固态核磁共振（solid state NMR）光谱技术对电子结构及化学键合进行表征，结果表明Cu-Zn-P骨架具有柔性，可通过键重排（包括部分Zn/P取代以及Cu-Zn键的形成）为每个化学式单元容纳最多6个Zn原子。骨架键合的此类扰动会对热输运与电荷输运性能产生显著影响。Ba₈Cu₁₄Zn₂P₃₀的整体热电优值（thermoelectric figure-of-merit）ZT在800 K时可达0.62，较三元母相Ba₈Cu₁₆P₃₀的热电性能提升9倍。借助10 K下的非弹性中子散射（inelastic neutron scattering）与单晶X射线衍射（single crystal X-ray diffraction）联合实验，研究证实：大尺寸十四面体笼（tetrakaidecahedral cages）内的Ba客体原子所发生的低能rattling振动，正是所研究笼型结构呈现低热导率的核心原因。