Herein we cover the key concepts in the field of thermoelectric materials research, present the current understanding, and show the latest developments. Current research is aimed at increasing the thermoelectric figure of merit (ZT) by maximizing the power factor and/or minimizing the thermal conductivity. Attempts at maximizing the power factor include the development of new materials, optimization of existing materials by doping, and the exploration of nanoscale materials. The minimization of the thermal conductivity can come through solid-solution alloying, use of materials with intrinsically low thermal conductivity, and nanostructuring. Herein we describe the most promising bulk materials with emphasis on results from the last decade. Single-phase bulk materials are discussed in terms of chemistry, crystal structure, physical properties, and optimization of thermoelectric performance. The new opportunities for enhanced performance bulk nanostructured composite materials are examined and a look into the not so distant future is attempted.

New and Old Concepts in Thermoelectric Materials

A survey is given of the physical properties, composition and crystal structure of intermetallic compounds formed between rare-earth elements and 3d transition elements. Apart from binary compounds the results of pseudobinary series are also considered. The magnetic properties determined by the exchange interactions involving 4f as well as 3d electrons, are discussed together with experimental results available on magnetovolume effects and various resonance techniques such as NMR and the Mossbauer effect.

https://iopscience.iop.org/article/10.1088/0034-4885/40/10/002/pdf

Intermetallic compounds of rare-earth and 3d transition metals

We report measurements of the thermal conductivity of high-quality crystals of the cubic I-V-VI2 semiconductors AgSbTe2 and AgBiSe2. The thermal conductivity is temperature independent from 80 to 300 K at a value of approximately 0.70 W/mK. Heat conduction is dominated by the lattice term, which we show is limited by umklapp and normal phonon-phonon scattering processes to a value that corresponds to the minimum possible, where the phonon mean free path equals the interatomic distance. Minimum thermal conductivity in cubic I-V-VI2 semiconductors is due to an extreme anharmonicity of the lattice vibrational spectrum that gives rise to a high Gruneisen parameter and strong phonon-phonon interactions. Members of this family of compounds are therefore most promising for thermoelectric applications, particularly as p-type materials.

Intrinsically minimal thermal conductivity in cubic I-V-VI2 semiconductors.

As over 93% of the world's energy comes from thermal processes, new materials that maximize heat transfer or minimize heat waste are crucial to improving efficiency. Here we focus on fully dense electrical insulators at the low end of the spectrum of lattice thermal conductivity κL. We present an experimentally validated predictive tool that shows how the high deformability of lone-pair electron charge density can limit κL in crystalline materials. Using first-principles density-functional theory (DFT) calculations, we predict that several ABX2 (groups I–V–VI2) compounds based on the rocksalt structure develop soft phonon modes due to the strong hybridization and repulsion between the lone-pair electrons of the group V cations and the valence p orbitals of group VI anions. In many cases, this creates lattice instabilities and the compounds either do not exist or crystallize in a different structure. Marginally stable ABX2 compounds have anharmonic bonds that result in strong phonon–phonon interactions. We show experimentally how these can reduce κL to the amorphous limit.

Lone pair electrons minimize lattice thermal conductivity

The physical properties of many of the rare-earth intermetallic compounds have been collected together. They are discussed in terms of the role that the magnetic exchange and crystal field interactions play in determining these properties. It is pointed out that in this vast number of materials there is an ideal chance of establishing which of several second-order terms are effective in determining structural stability.

Intermetallic rare-earth compounds.

The crystal structure and magnetic properties of the rare-earth nickel (RNi) compounds

Pseudo-binary systems involving rare earth laves phases

Pits which are probably the sites of dislocations have been produced on the (111) cleavage planes of antimony by use of the CP4 etching reagent. Evidence that the pits probably are at dislocation sites was obtained from density counts on intersecting boundaries and from deformation and annealing studies. The Dash etching reagent chemically polishes antimony without producing pits. The Superoxol etch brings out spiral terraced pits and other fine structure.

J.H. Wernick

Papers

The crystal structure and magnetic properties of the rare-earth nickel (RNi) compounds

Pseudo-binary systems involving rare earth laves phases

Dislocation Etch Pits in Antimony

Constitution of the AgSbSe2-AgSbTe2−AgBiSe2−AgBiTe2 system

Superconducting, thermal, and magnetic susceptibility behavior of some intermetallic compounds with the fluorite structure.