High-efficiency thermoelectric Ba(8)Cu(14)Ge(6)P(26): bridging the gap between tetrel-based and tetrel-free clathrates

A new type-I clathrate, Ba(8)Cu(14)Ge(6)P(26), was synthesized by solid-state methods as a polycrystalline powder and grown as a cm-sized single crystal via the vertical Bridgman method. Single-crystal and powder X-ray diffraction show that Ba(8)Cu(14)Ge(6)P(26) crystallizes in the cubic space group Pm3n (no. 223). Ba(8)Cu(14)Ge(6)P(26) is the first representative of anionic clathrates whose framework is composed of three atom types of very different chemical natures: a transition metal, tetrel element, and pnicogen. Uniform distribution of the Cu, Ge, and P atoms over the framework sites and the absence of any superstructural or local ordering in Ba(8)Cu(14)Ge(6)P(26) were confirmed by synchrotron X-ray diffraction, electron diffraction and high-angle annular dark field scanning transmission electron microscopy, and neutron and X-ray pair distribution function analyses. Characterization of the transport properties demonstrate that Ba(8)Cu(14)Ge(6)P(26) is a p-type semiconductor with an intrinsically low thermal conductivity of 0.72 W m(–1) K(–1) at 812 K. The thermoelectric figure of merit, ZT, for a slice of the Bridgman-grown crystal of Ba(8)Cu(14)Ge(6)P(26) approaches 0.63 at 812 K due to a high power factor of 5.62 μW cm(–1) K(–2). The thermoelectric efficiency of Ba(8)Cu(14)Ge(6)P(26) is on par with the best optimized p-type Ge-based clathrates and outperforms the majority of clathrates in the 700–850 K temperature region, including all tetrel-free clathrates. Ba(8)Cu(14)Ge(6)P(26) expands clathrate chemistry by bridging conventional tetrel-based and tetrel-free clathrates. Advanced transport properties, in combination with earth-abundant framework elements and congruent melting make Ba(8)Cu(14)Ge(6)P(26) a strong candidate as a novel and efficient thermoelectric material.