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.