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 di ff raction show that Ba 8 Cu 14 Ge 6 P 26 crystallizes in the cubic space group Pm (cid:1) 3 n (no. 223). Ba 8 Cu 14 Ge 6 P 26 is the fi rst representative of anionic clathrates whose framework is composed of three atom types of very di ff erent 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 con fi rmed by synchrotron X-ray di ff raction, electron di ff raction and high-angle annular dark fi eld 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 (cid:1) 1 K (cid:1) 1 at 812 K. The thermoelectric fi gure 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 m W cm (cid:1) 1 K (cid:1) 2 . The thermoelectric e ffi ciency 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 e ffi cient thermoelectric material.