Article pubs.acs.org/IC
Metallic Ternary Telluride with Sphalerite Superstructure Amit Adhikary,† Sudip Mohapatra,† Seng Huat Lee,‡ Yew San Hor,‡ Puja Adhikari,§ Wai-Yim Ching,§ and Amitava Choudhury*,† †
Department of Chemistry and ‡Department of Physics, Missouri University of Science and Technology, Rolla, Missouri 65409, United States § Department of Physics and Astronomy, University of MissouriKansas City, Kansas City, Missouri 64110, United States S Supporting Information *
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 × 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.
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INTRODUCTION Over the past few decades multinary copper chalcogenide materials have been attracting much attention from the scientific community because of their fascinating structural diversity,1 mixed valency,2−4 copper ion conduction,5,6 as well as potential application as thermoelectric7−12 and photovoltaic materials.13−17 One of the important characteristics of copper− chalcogen chemistry that stands out from other coinage metal− chalcogen chemistry is its propensity to form mixed-valent compounds, where the presence of Cu+ and Cu2+ needs to be formally invoked to describe a charge-balanced composition.2−4,18−25 However, it has now been widely accepted that the mixed valency arises from the chalcogen sublattice, where chalcogen can have oxidation states between −2 and −1.2,4,26 This electron defiency causes creation of holes in the chalcogen valence band as reflected in their p-type metallic conductivity.2,4,20 On the other hand, valence-precise copper−chalcogenides display a band gap around the Fermi level and are semiconducting in most cases.7−17,27,28 For these reasons a large number of copper−chalcogenides have been investigated in the ternary system A−Cu−Q (A = alkali ions and Tl; Q = chalcogen) with different structural dimensionality and electrical conductivity.3,4,18,29−31 Recently investigations on ternary and quaternary systems involving Cu−Sn−Q are on © XXXX American Chemical Society
the rise, especially those with diamond-like crystal structure due to their good thermoelectric and photovoltaic properties.7−10,32,34,35 These copper-containing chalcogenides show low thermal conductivities (0.3−3 W m−1 K−1)7−10 and are amenable to tuning of electrical properties by appropriate doping. Some of the intriguing properties, for example, mixed valency and p-type metallicity that are seen in A−Cu−Q series of compounds, can also be found in Cu−Sn−Q series.35 Though a large number of sulfides/selenide phases consisting of Cu and Sn were reported in the literature,7−10,28,32,34−40 synthesis of telluride phases containing Cu and Sn is rare.41−46 Since Cu-based chalcogenides display high intrinsic electrical conductivity, high Seebeck coefficient, and low thermal conductivity it may provide further improvements on thermoelectric properties if new phases of tellurides can be formed in the Cu−Sn−Te system. Motivated by this hypothesis we synthesized a new ternary composition Cu5Sn2Te7, which is isostructural to a recently reported selenide analogue.35 Cu5Sn2Te7 is a formally mixed-valent compound and a superstructure of sphalerite phase exhibiting p-type metallic conductivity. In this article, we present the synthesis, crystal Received: November 2, 2015
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DOI: 10.1021/acs.inorgchem.5b02516 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry structure, electronic band structure, and transport properties of Cu5Sn2Te7 and discuss how doping can be used to transform this material into a semiconductor and a possible thermoelectric material.
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Table 1. Crystal Data and Structure Refinement for Cu5Sn2Te7 compound empirical formula fw T (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) β (deg) vol. (Å3) Z ρcalc (Mg m−3) μ (mm−1) goodness of fit (S) Rint R [I > 2σ(I)]a wR2 [I > 2σ(I)]a R (all data) wR (F2) (all data)b absolute structure parameter δF (e Å−3)
EXPERIMENTAL SECTION
Synthesis. A single crystal of Cu5Sn2Te7 was first obtained accidently from a reaction of metallic Cu, Sn, and Te while attempting to synthesize Cu2SnTe3 at 800 °C. Once the composition was established from the single-crystal structure solution, a directed synthesis of Cu5Sn2Te7 was attempted from the stoichiometric combination of elements. The following reagents were used as received: Cu powder (99.9%, Sigma-Aldrich), Sn powder (99.9%, Sigma-Aldrich), and Te powder (99.9%, Sigma-Aldrich). All chemical loading was carried out in a glovebox under a dry nitrogen atmosphere. The starting elements in stoichiometric ratio were sealed in an evacuated fused quartz ampule under vacuum (
