Understanding the lattice dynamics and low thermal conductivities of IV-VI, V2-VI3 and V materials is critical to the development of better thermoelectric and phase-change materials. Here we provide a link between chemical bonding and low thermal conductivity. Our first-principles calculations reveal that long-ranged interaction along the 〈100〉 direction of the rocksalt structure exist in lead chalcogenides, SnTe, Bi2Te3, Bi and Sb due to the resonant bonding that is common to all of them. This long-ranged interaction in lead chalcogenides and SnTe cause optical phonon softening, strong anharmonic scattering and large phase space for three-phonon scattering processes, which explain why rocksalt IV-VI compounds have much lower thermal conductivities than zincblende III-V compounds. The new insights on the relationship between resonant bonding and low thermal conductivity will help in the development of better thermoelectric and phase change materials.

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Resonant bonding leads to low lattice thermal conductivity.

The Si3N4-SiC composite system was investigated to better understand the effect of microstructure on the strength-controlling factors, i.e. fracture energy, elastic modulus, and crack size. Silicon carbide dispersions with average particle sizes of 5, 9, and 32 μm were used to form 3 composite series within this system, each containing 0.10, 0.20, 0.30, and 0.40 vol fraction of the dispersed phase. These composites were fabricated by hot-pressing. Fracture energy and strength values were measured for each composite. A linear relation between the elastic modulus of the two phases was assumed. The crack size was calculated for each composite using the appropriate property values. The strength behavior of the 9- and 32-μm series was controlled by the crack size, which, in turn, was controlled by the particle size and volume fraction of the SiC phase. Particle size and volume fraction did not affect the crack size of the 5-μm series, in which strength was controlled by both fracture energy and elastic modulus. Strengths measured at 1400°C and thermal conductivity measurements indicate that several of these composites are promising as high-temperature structural materials.

Effect of Microstructure on Strength of Si3N4‐SiC Composite System

The lattice dynamics in the IV–VI compounds GeTe, SnTe and PbTe were studied by 125Te and 119Sn nuclear inelastic scattering and the obtained partial density of phonon states were compared with published theoretical calculations. The phase purity and structure were characterized by high energy X-ray diffraction. The effect of the atomic arrangement, rhombohedral for GeTe and cubic for SnTe and PbTe, is visible in the density of phonon states. Vibrational properties are found to be in good agreement with available calculated data and the softer character of the NaCl-type structures in comparison with the rhombohedral GeTe is confirmed.

Lattice dynamics and structure of GeTe, SnTe and PbTe

We report on the underlying mechanism that enables the SnTe–AgSbTe2 system to exhibit superior thermoelectric figure of merit (ZT) compared to its parent compound SnTe. We show that AgSbTe2 alloying has a profound impact on the band structure of SnTe by converging the energies of its light and heavy valence bands, leading to significantly enhanced Seebeck coefficients. We have also unraveled a significant connection between alloying and defect stability in this system, wherein the Sn vacancy concentration increases significantly when Ag and Sb are alloyed on the Sn site. The increased Sn vacancy concentration dramatically reduces the lattice thermal conductivity through both lattice softening and phonon–vacancy scattering to ∼0.4 W m–1 K–1 at 800 K. Consequently, a ZT value of 1.2 at 800 K for AgSn5SbTe7 can be achieved by doping I on Te sites. This represents a 300% improvement over pristine SnTe, outperforming many reported SnTe-based thermoelectric materials.

High Thermoelectric Performance in SnTe–AgSbTe2 Alloys from Lattice Softening, Giant Phonon–Vacancy Scattering, and Valence Band Convergence

The scattering of phonons by vacancies is estimated by a perturbation technique in terms of the missing mass and the missing linkages. An argument is given why distortion effects can be disregarded. The resonance frequency of the defect is sufficiently high so that resonance effects can be disregarded for phonons in the important frequency range for thermal conduction. The theory is applied to the thermal resistance by vacancies in cases where the vacancy concentration is known: potassium chloride with divalent cations, nonstoichiometric zirconium carbide, and tin telluride.

Scattering of Phonons by Vacancies

The thermal conductivity, electrical conductivity, and Seebeck coefficient of several specimens of SnTe have been measured between 100° and 500°K. The thermal conductivity is weakly dependent on both temperature and hole concentration. The total thermal conductivity is separated into an electronic and a lattice thermal conductivity. Because of the large concentrations of Sn vacancies in the samples, the phonons are scattered both by three‐phonon umklapp processes and by the Sn vacancies; this results in a lattice thermal conductivity that varies with temperature more like T−½ rather than T−1. The Lorenz number relating the electrical conductivity and the electronic thermal conductivity is an unusual function of hole concentration. The Lorenz number is larger than the Sommerfeld value L0, varies with hole concentration p, and has a maximum value of about 1.3 L0 at p=2×1020 cm−3. This is consistent with the variation of the electrical conductivity and the Seebeck coefficient with hole concentration.

D. H. Damon

Papers

Thermal Conductivity of SnTe between 100° and 500°K