Thermoelectric Properties of Selenides Spinels

Many compounds with the spinel structure type have been analyzed for their thermoelectric properties. Published data was used to augment experimental results presented here to select promising thermoelectric spinels. Compounds studied here include C~~,~Al~.5Cr2Se4, Cu05Coo.5Cr2Se4, C~o,~Ino,~Cr2Se~ and CuIr2Se4. Many exhibit low lattice thermal conductivity of about 20 mW/cmK, independent of temperature. Two series of compounds were selected for further study: GaxCul-,Cr2Se4 and ZnxCul.,Cr2Se4 and preliminary results are given. INTRODUCTION The growth of commercial applications of thermoelectric devices depends primarily on increasing the figure of merit, ZT, for thermoelectric materials. The figure of merit is defined as ZT = a20T/h, where a is the Seebeck coefficient, o the electrical conductivity, h the thermal conductivity, and T is the absolute temperature. Materials with a large a20 value, or power factor, are usually heavily doped semiconductors, such as Bi2Te3. The thermal conductivity of semiconductors is usually dominated by phonon or lattice thermal conductivity. Thus, one method for finding new, advanced thermoelectric materials is to search for semiconductors with low lattice thermal conductivity. Figure 1. Illustration of the Spinel unit cell (e.g. ZnCrzSe4) showing Se atoms as spheres Cr atoms (not shown) at the center of the shaded octahedra and Zn atoms (not shown) at the center of the shaded tetrahedra. The cubic unit cell is indicated. In this paper we evaluate compounds based on the Spinel structure with general composition AlB2& where A and B are transition metals and X is a chalcogen,primarily Se. Previous work on such compounds [ 11 have shown that a range of metals and insulators exist with this structure type. The structure of Spinel (Figure 1) consists of cubic close packed chalcogen atoms with metal B atoms in half the octahedral holes and metal A atoms in 1/8 of the tetrahedral holes. There can be significant mixing of the different metal atoms on the two metal sites. As suggested by Spitzer [2] the relatively high coordination number of the B atoms in this structure may favor low lattice thermal conductivity. The large cubic unit cells (about lOA) full of vacant octahedral holes should reduce the lattice thermal conductivity by increasing the scattering of phonons. The G. J. Snyder Page 1 of 6 multi-valley electronic structure produced by such cubic compounds can be expected to enhance the thermopower. There are approximately 300 known Spinels with X = Se or S. Many of these compounds have X = Se compounds with 3-d transition metals for A and B atoms (Figure 2), and constitute most of the samples in this investigation. Many oxide spinels have been studied for their magnetic properties but are not suitable for thermoelectric applications because they are too insulating. Only a few of the known spinel sulfides are metals or heavily doped semiconductors; such compounds based on the sulfides of V, Co, Fe or Ni, are somewhat unstable in the spinel structure, preferring the Cr3S4-type at high temperature and pressure[3]. Several of the known AB2S4 sulfides with the Cr3S4-type structure are high temperature/pressure polymorphs of compounds with the Spinel structure at room temperature and pressure. Known compounds with X = Te are metals with low thermopower (a) (Table 1). The Cr3S4-type selenides are attractive for thermoelectric applications not only because they may have low thermal conductivity as suggested above, but they also exhibit a range of electronic properties from metals to semiconductors. Precise, heavy doping of the semiconductor is critical to obtain optimal power factor for both nand p-type materials. Proven thermoelectric materials such as filled Skutterudites and Zn&3b3 are often difficult to dope to the optimal nor p-type carrier concentration. An advantage of the AxB3-xX4 compounds is the chemical versatility of the structure, allowing continuous doping from metal to nand ptype semiconductor. Experimental The thermoelectric properties of many sulfur and selenide spinel compounds reported in the literature [l] were used to narrow the search. For insulating compositions, which would need to be h avily doped for thermoelectric applications, the most useful of this information for therm electric considerations is the apparent band gap and Known Selenide Spinels carrier mobility. These data are summarized in Table 1. The metallic spinels, such as CuCr,S4, can be used to d pe or alloy with related semiconducting spinels. The solid solution FeXCul.,Cr2S4 changes from a p-type semimetal to n-type, and then back to p-type as x is increased[4]. The maximumRT power factor for this system is about 1 pW/cmK2. Most of the known selenide spinels contain Cr as the B atom, and a +2 element such as Zn or Cd as the A atom. The other elements than can go on the B site such are mostly rare earth group elements and A1 which will make less chemically stable spinels with a strongly ionic character (large band gap insulators). CuIr2Se4 is both a metal and a pressure induced semiconductor, and as such may have interesting thermoelectric properties [5]. Various doped chromium selenide Figure 2. Elements known to make A1B2Se4 compounds with the spinel structure type, where A elements are horizontally hatched and B elements vertically hatched.