Metallic Ternary Telluride with Sphalerite Superstructure

Abstract

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 × 10(5) 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 × 10(21) cm(-3) and Hall mobility of 4.5 cm(2) 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.

Copper-Based Diamond-like Thermoelectric Compounds: Looking Back and Stepping Forward

“…In addition, a record-high zT of 1.61 was obtained at 848 K by Qin et al [66] by constructing the intrinsic point defects, including high-dense stacking faults and endo-grown nanoneedles, to obstruct mid-as well as low-frequency phonons in Cu 2 SnSe 3 compounds. Except for Cu 2 SnSe 3 , Cu 5 A 2 B 7 (A = Si, Ge, Sn; B = S, Se, Te), with a centrosymmetric space group C2/m, is also a kind of distorted CBDL compound which has been considered to possess a non-centrosymmetric cubic structure, with the phase crystallized as C-centered, as shown in Figure 2a [91][92][93]. An undesirable characteristic of Cu 5 A 2 B 7 compounds is that they represent metal-like behaviors, such as the carrier concentration and κ of Cu 5 Sn 2 Te 7 at 300 K are 1.39 × 10 21 cm −3 and 15.1 W•m −1 •K −1 , respectively [92].…”

“…Except for Cu 2 SnSe 3 , Cu 5 A 2 B 7 (A = Si, Ge, Sn; B = S, Se, Te), with a centrosymmetric space group C2/m, is also a kind of distorted CBDL compound which has been considered to possess a non-centrosymmetric cubic structure, with the phase crystallized as C-centered, as shown in Figure 2a [91][92][93]. An undesirable characteristic of Cu 5 A 2 B 7 compounds is that they represent metal-like behaviors, such as the carrier concentration and κ of Cu 5 Sn 2 Te 7 at 300 K are 1.39 × 10 21 cm −3 and 15.1 W•m −1 •K −1 , respectively [92]. Simultaneously, zinc atoms have been proven to be effective dopants for strengthening the semiconductor properties of Cu 5 Sn 2 Te 7 compounds; Sturm et al [93] introduced a zinc dopant into Cu 5 Sn 2 Se 7 and Cu 5 Sn 2 Te 7 compounds, which also supports this conclusion.…”

Electronic Structure- and Entropy-Driven Design of Thermoelectric Chalcogenide Cu5Sn2Se7 Leading to the Optimization of Carrier Concentration and Reduction in Thermal Conductivity

“…The compound Cu 5 Sn 2 Se 7 (CSS), which consists of affordable, non-toxic, and earth-abundant elements, has a potential for practical application in thermoelectrics if its thermoelectric (TE) performance can be improved remarkably. Although the number of cations is equal to that of anions, there is electron deficiency in CSS, leaving one hole ( p + ) in terms of the valence state of each atom: [Cu 1+ ] 5 [Sn 4+ ] 2 [Se 2– ] 7 × 1 p + , which accounts for its high carrier concentration ( n H ∼ 3.0 × 10 21 cm –3 at room temperature). − Owing to its high n H , CSS has high electrical conductivity and electronic thermal conductivity ( κ e ) with the peak ZT value of only ∼0.13 at 700 K . Therefore, optimization in n H is required to improve its TE performance.…”

“…In recent years, some pioneering works have been done with respect to the CSS. − For instance, by replacing the monovalent Cu with divalent Zn, the n H reduces to 4 × 10 20 cm –3 , and the peak ZT increases to 0.51 at 750 K . This is the biggest improvement to date in this system.…”