Metallic Ternary Telluride with Sphalerite Superstructure

A new ternary compound with composition Cu₅Sn₂Te₇ has been synthesized using the stoichiometric reaction of Cu, Sn, and Te. The compound crystallizes in C2 space group with unit cell parameters of a = 13.549(2) Å, b = 6.0521(11) Å, c = 9.568(2) Å, and β = 98.121(2)°. Cu₅Sn₂Te₇ is a superstructure of sphalerite and exhibits tetrahedral coordination of Cu, Sn, and Te atoms, containing a unique adamantane-like arrangement. The compound is formally mixed valent with a high electrical conductivity of 9.8 × 10⁵ S m^-1 at 300 K and exhibits metallic behavior having p-type charge carriers as indicated from the positive Seebeck coefficient. Hall effect measurements further confirm holes as charge carriers with a carrier density of 1.39 × 10²¹ cm^-3 and Hall mobility of 4.5 cm² V^-1 s^-1 at 300 K. The electronic band structure calculations indicate the presence of a finite density of states around the Fermi level and agree well with the p-type metallic conductivity. Band structure analysis suggests that the effective mass of the hole state is small and could be responsible for high electronic conductivity and Hall mobility. The high thermal conductivity of 15.1 W m^-1 K^-1 at 300 K coupled with the low Seebeck coefficient results in a poor thermoelectric figure of merit (ZT) for this compound. Theoretical calculations indicate that if Cu₅Sn₂Te₇ is turned into a valence precise compound by substituting one Cu by a Zn, a semiconducting material, Cu₄ZnSn₂Te₇, with a direct band gap of ∼0.5 eV can be obtained.