P. Larson

STRUCTURE OF NANOCRYSTALLINE MATERIALS USING ATOMIC PAIR DISTRIBUTION FUNCTION ANALYSIS: STUDY OF LIMOS2

The structure of

Observed properties and electronic structure of RNiSb compounds (R = Ho, Er, Tm, Yb and Y). Potential thermoelectric materials

{\mathrm{LiMoS}}_{2}

Electronic Structure of Complex Bismuth Chalcogenide Systems

has been experimentally determined. The approach of atomic pair distribution function analysis was used because of the lack of well-defined Bragg peaks due to the short structural coherence (\ensuremath{\sim}50 \AA{}) in this intercalation compound. The reduction of Mo by Li results in Mo-Mo bonding with the formation of chains of distorted

Electronic Structure of CsBi 4 Te 6

\mathrm{Mo}\ensuremath{-}{\mathrm{S}}_{6}

Physics of the gap formation in half-Heusler compounds and bismuth chalcogenide systems

octahedra. Using refined structural parameters the electronic band structure for this material has been calculated and is in good agreement with observed material properties.

Structural stability of Ni-containing half-Heusler compounds

The RNiSb compounds (R=Ho, Er, Tm, Yb and Y) and some selected solid solution members such as (Zr 1-x Er x )Ni(Sn 1-x Sb x ) and ErNiSb 1-x Pn x (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 ErNiSb 1-x Pn x (Pn=As, Sb, Bi) solid solutions, but for (Zr 1-x Er x )Ni(Sn 1-x Sb x ) members vary from negative to positive values when 0

Electronic structure of rare-earth nickel pnictides: Narrow-gap thermoelectric materials

Narrow band gap semiconductor Bi2Te3and its alloys with Sb and Se are bulk materials with one of the highest figure of merit Z (= α2σ/κ, where α is the thermopower, σ is the electrical conductivity, and κ is the thermal conductivity) in the range of room temperature.1Because of this, several attempts have been made to improve Z by making novel Bi-Te-Sb-Se based materials using new concepts such as quantum confinement2(QC) and phonon glass electron crystal3(PGEC). Within the QC picture, first proposed by Hicks et. al.2, model calculations predict an increase in Z for a two dimensional layer having thickness smaller than about 300 A (for example an idealized Bi2Te3-multiple quantum well (MQW) structure) due to enhanced power factor P = α2σ. Even larger power factor enhancement occurs in one-dimensional quantum wires. (See the article on bismuth nanowires by Dresselahaus et. al.4in this volume.) In contrast to the QC idea, within PGEC picture one uses superlattice (SL) structures consisting of two materials both having favorable α and σ (such that electronic properties are not affected by the SL structure), but reduces к by engineering the phonon band structure in suitably chosen transport direction.5A third approach where both the above ideas have been exploited to certain extent is to chemically synthesize new ternary and quaternary narrow band gap semiconductors containing Bi, Te, Se atoms with different arrangements of Bi-Te-Se blocks which we will refer to as different quantum architectures.6These compounds have low and promisingly high values of P.This has been the focus of our research program at Michigan State University. Some of these new compounds are BaBiTe3, CsBi4Te6, K2Bi8Se13, of which the last two show considerable promise.

Electronic structure and transport of Bi2Te3 and BaBiTe3

Recently, CsBi 4 Te 6 has been reported as a high-performance thermoelectric material for low temperature applications with a higher thermoelectric figure of merit (ZT ∼ 0.8 at 225 Kelvin) than conventional Bi 2- x Sb z Te 3- y Se y alloys at the same temperature. First-principle electronic structure calculations within density functional theory performed on this material give an indirect narrow-gap semiconductor. Dispersions of energy bands along different directions in k-space display large anisotropy and multiple conduction band minima close in energy, characteristics of a good thermoelectric material.

Li2PbGeS4 and Li2EuGeS4: Polar Chalcopyrites with a Severe Tetragonal Compression

Using first principles electronic structure calculations based on the density functional theory, we discuss the reasons behind the formation of energy gaps in different classes of narrow-gap semiconductors which are either good or promising thermoelectrics. We find that in half-Heusler compounds such as ZrNiSn and YNiSb, the Ni atoms take active role in the gap formation, both through local symmetry breaking and hybridization. In Bi/sub 2/Te/sub 3/, the best known room temperature thermoelectric, the subtle gap structure is determined by both spin-orbit interaction and hybridization of Bi p and Te p bands. In other Bi chalcogenides and complex ternary systems containing Bi and Te, it appears that spin-orbit interaction does not play as important a role. We discuss possible reasons for this difference.