In recent years, LAST-m (AgPbmSbTem+2) and related materials have emerged as potential high performance high temperature thermoelectrics. These compounds are obtained by starting from PbTe, and replacing pairs of Pb2+ ions by (Ag1+, Sb3+) pairs. One example is LAST-18. When optimally doped, this compound has thermoelectric figure of merit ZT=1.7 at 700K. This large ZT is most likely due to very low lattice thermal conductivity, caused by phonon scattering from nanostructures. These nanostructures involve clustering and ordering of Ag, Sb, and Pb ions. Possible origins of this atomic ordering and how the presence of nanostructures affects the electronic structure near the band gap region are discussed. The temperature (T) dependence of electrical conductivity σ (∼T2.2 in the range 300K <T< 900K) in n-type PbTe is analyzed in terms of the T-dependence of different physical quantities contributing to transport. We find that the dominant contribution comes from the explicit T-dependence of relaxation time rather than its energy dependence. The T-dependence of chemical potential is also significant in the concentration range of interest. Electronic thermal conductivity for constant field (κel,E) and for constant current (κel,J) are found to differ considerably at high temperatures and the Weidemann-Franz (WF) law κel,J = LoσT, where Lo =2x10−8WΩ/K is the Lorentz number, overstimates κel,J by nearly 60% at 800K for carrier concentration n=5x1019/cm3. As a result, one tends to underestimate the lattice contribution κlatt = κexp - κel,J. We give theoretical values of effective Lorentz number L = κel.J/σT for different n and T.

In recent years, LAST-m (AgPbmSbTem+2) and related materials have emerged as potential high performance high temperature thermoelectrics. One example is LAST-18. When optimally doped, this compound has thermoelectric figure of merit ZT=1.7 at 700K. This large ZT is most likely due to the low lattice thermal conductivity, caused by phonon scattering from nanostructures. These nanostructures involve clustering and ordering of Ag, Sb, and Pb ions. The origin of these nanostructures has been studied using Monte Carlo (MC) simulation of an ionic model and ab initio studies of pair interaction energies. Effects of these substitutions on the band structure near the gap and their implications on transport properties are briefly discussed.

Novel quaternary compounds AgPbmSbTem+2 (LAST-m) with different m values have been synthesized recently and some of these compounds show promising thermoelectric properties at high temperatures. The two end members of the series, PbTe (m=∞) and AgSbTe2 (m=0), are also known to be good thermoelectrics. In this paper, we discuss the results of ab initio electronic structure calculations for these two end members and for LAST-2 and LAST-14 to see how the electronic structure near the chemical potential μ evolves with m. Whereas PbTe and LAST-14 are narrow band gap semiconductors, the other two compounds show pseudo-gap structure. Even in the absence of a true gap, the rapidly varying density of states (DOS) near μ may be conducive to large Seebeck coefficient in LAST-2 and AgSbTe2.

Ab initio electronic structure calculations have been carried out to understand the nature of anionic defect states in PbTe. We find that Te vacancies strongly perturb the electronic density of states (DOS) near the band gap region. New states of predominantly Pb p character appear in the band gap. Iodine is an ideal substitutional defect and a donor. Sulpher and Selenium do not affect the states near the conduction band minimum but suppress the DOS near the valence band maximum. These results have important implications on the thermoelectric properties of PbTe and PbTexM1−x (M=S, Se) ternary systems.

We describe the Tl2SnAs2Q6, (Q= S, Se) compounds which consist of [SnAs2S6]2- layers with the Tl+ cations lying in between. Tl2SnAs2S6 and Tl2SnAs2Se6 crystallize in the space group P-3 with a = 6.706(4) Å, c = 7.187(6) Å and a = 6.996(3) Å, c = 7.232(4) Å respectively. These compounds are semiconductors with band gaps of 1.68 eV for the sulfide and 1.08 eV for selenide corresponding to their dark red and black colors respectively. Band structure calculations suggest indirect band gaps in these materials.

To understand thermoelectric properties of multiplayer Bi2Te3/Sb2Te3 superlattices, especially their charge transport properties, electronic structure calculations were carried out using ab-initio gradient corrected density functional theory. The superlattice structures of (1,1) and (1,2) Bi2Te3/Sb2Te3 multilayers were optimized and their band structures were compared with each other. Different lattice relaxation effects are observed for the two structures. The cross-plane and inplane effective masses for both these systems are found to be comparable, consistent with experimental mobility measurements.

The known mineral Pavonite AgBi3S5 shows a complex structure composed of NaCl type fragments but has not been studied from the thermoelectric point of view. We present initial results on the synthesis and themoelectric properties of synthetic AgBi3S5, which shows n-type metallic conductivity. In addition, the examination of the solid solutions AgSbxBi3-xS5 (x=0.3) is reported.

CsBi4Te6 (ZT ∼ 0.8 at 225 K) shows highly anisotropic features in its crystal morphology and structure as expressed by the parallel infinite [Bi4Te6] rods which are linked via Bi-Bi bonds. Band calculations also point to a significant anisotropy in the carrier effective masses, and for this reason we examined the anisotropic thermoelectric properties of CsBi4Te6. The electrical conductivity, thermopower and thermal conductivity were measured along the three different crystallographic directions of the monoclinic structure of CsBi4Te6. These measurements were performed on samples with different degrees of doping. The strong charge transport anisotropy of these samples was confirmed and also observed that the thermopower values along the c-axis direction (which is perpendicular to the layer of Cs atoms) was negative (-80 μV/K) while those along the needle direction (b-axis) and parallel to the [Bi4Te6] layers (a-axis) were p-type (50–100 μV/K at room temperature. Other anisotropic features in the crystal growth habit, electronic band structure, and electrical and thermal conductivities are also presented.

An ab-initio study of the electronic structures and optical absorption spectrum of the guest Cs and Na atoms absorbed in the pure silica ITQ-4 zeolite channels was carried out using density functional theory (DFT) and plane wave basis. Completely different geometries were found for doped Cs and Na in the channels. For Cs doping, a zig-zag chain was formed along the channel and a metallic band structure was obtained. However, a dimer structure around the channel center was found for Na doping and a narrow band gap semiconductor was observed for this system. Correspondingly, the calculated optical absorption spectrum also showed distinctive characters for Cs and Na dopings. Our theoretical results for Cs doping are compared with experimental measurements and previous DFT calculation with local orbital basis; better agreement with experiments has been reached than the previous calculation. For the Na doping, our theoretical predictions need to be confirmed by future experiments.

There is considerable current effort to discover new thermoelectric materials with a high figure of merit Z. Some of these new materials are narrow-gap semiconductors with rather complex crystal structures. In this paper we discuss the results of electronic structure calculations in two classes of such systems. The first class consists of BaBiTe3, a structural and chemical derivative of the well-studied Bi2Te3. Similarities and differences in the band structures of these two systems are discussed. The second class consists of half-Heusler or “stuffed”-NaCl compounds MNiX, where M is Y, La, Lu, Yb, and X is a pnictogen; As, Sb, Bi. To understand the physical reason behind the energy gap formation, we compare the electronic structure of YNiSb with that of an isoelectronic system ZrNiSn, another isostructural compound of thermoelectric interest. These calculations were carried out within density functional theory (in generalized gradient approximation) using self-consistent full-potential LAPW method. Energy gaps and effective masses associated with the conduction band minimum and valence band maximum have been calculated and these quantities have been used to estimate transport properties. Large room temperature thermopower values in Bi2Te3 and BaBiTe3 can be understood in terms of multiple conduction and valence band extrema whereas similar large values in ZrNiSn and other half-Heusler compounds can be ascribed to large electron and hole effective mass.

The RNiSb compounds (R=Ho, Er, Tm, Yb and Y) and some selected solid solution members such as (Zr1-xErx)Ni(Sn1-xSbx) and ErNiSb1-xPnx (Pn=As, Sb, Bi) have been studied. They all crystallize in the MgAgAs structure type, which can be considered as a NaCI structure type in which half of the interstitial tetrahedral sites are occupied by Ni atoms. The measured values of the Seebeck coefficients, at room temperature, are positive for RNiSb (R=Ho, Er, Yb and Y) compounds and ErNiSb1-xPnx (Pn=As, Sb, Bi) solid solutions, but for (Zr1-xErx)Ni(Sn1-xSbx) members vary from negative to positive values when 0 < x < 1. Some of these compounds show metallic conductivity while others exhibit thermally activated charge transport. Solid solutions of these materials have lower thermal conductivities than the pure members, RNiSb (R=Ho, Er, Yb and Y) and ZrNiSn. The electronic structures of RNiSb compounds, where R is Y, La, Lu, and Yb, have been studied with density functional theory. The results of the calculations for these systems, except for the Yb compound, indicate narrow gap semiconductors with large effective masses near the conduction band extrema. The Yb system is expected to show heavy fermion characteristics.

Randomly intercalated layered alloys are excellent models for two-dimensional microporous systems. We have studied the nonlinear gallery expansion and the gallery height fluctuations by constructing a double layer model that describes the layer rigidity and the size and stiffness of the intercalant species. Exact solutions, simulations and an effective-medium theory (EMT) results are compared. Applications of the results to ternary intercalation compounds are discussed.

Structural and dynamical properties of intercalated solids in general [1], and layered silicates of the type AxB1−x -Vermiculite in particular, are of both fundamental and practical interest. In these systems, two types of ions A and B with different ionic radii occupy the space between two silicate layers. On the fundamental side, one is interested in studying the average interlayer spacing as a function of (1) the concentration x of the large ion, (2) sizes and compressibilities of the intercalated ions and (3) the transverse rigidity of the silicate layers. In addition, one is interested in the dynamic properties of these solids. On the practical side, when the size difference between the two intercalants is large, one obtains pillared clays which are characterized by widely spaced silicate layers that are propped apart by sparsely distributed larger interlayer cations (sometimes referred to as pillars) [2]. The enormous free volume of accessible interior space that is derived from such an open structure has significent practical application in the field of catalysis and sieving.