Andrew F. May

Giant anharmonic phonon scattering in PbTe

Understanding the microscopic processes affecting the bulk thermal conductivity is crucial to develop more efficient thermoelectric materials. PbTe is currently one of the leading thermoelectric materials, largely thanks to its low thermal conductivity. However, the origin of this low thermal conductivity in a simple rocksalt structure has so far been elusive. Using a combination of inelastic neutron scattering measurements and first-principles computations of the phonons, we identify a strong anharmonic coupling between the ferroelectric transverse optic mode and the longitudinal acoustic modes in PbTe. This interaction extends over a large portion of reciprocal space, and directly affects the heat-carrying longitudinal acoustic phonons. The longitudinal acoustic-transverse optic anharmonic coupling is likely to play a central role in explaining the low thermal conductivity of PbTe. The present results provide a microscopic picture of why many good thermoelectric materials are found near a lattice instability of the ferroelectric type.

Glass-like phonon scattering from a spontaneous nanostructure in AgSbTe2.

Materials with very low thermal conductivity are of great interest for both thermoelectric and optical phase-change applications. Synthetic nanostructuring is most promising for suppressing thermal conductivity through phonon scattering, but challenges remain in producing bulk samples. In crystalline AgSbTe2 we show that a spontaneously forming nanostructure leads to a suppression of thermal conductivity to a glass-like level. Our mapping of the phonon mean free paths provides a novel bottom-up microscopic account of thermal conductivity and also reveals intrinsic anisotropies associated with the nanostructure. Ground-state degeneracy in AgSbTe2 leads to the natural formation of nanoscale domains with different orderings on the cation sublattice, and correlated atomic displacements, which efficiently scatter phonons. This mechanism is general and suggests a new avenue for the nanoscale engineering of materials to achieve low thermal conductivities for efficient thermoelectric converters and phase-change memory devices.

Low-temperature heat capacity and localized vibrational modes in natural and synthetic tetrahedrites

The heat capacity of natural (Cu12−x (Fe, Zn, Ag)x(Sb, As)4S13) and synthetic (Cu12−xZnxSb4S13 with x = 0, 1, 2) tetrahedrite compounds was measured between 2 K and 380 K. It was found that the temperature dependence of the heat capacity can be described using a Debye term and three Einstein oscillators with characteristic temperatures that correspond to energies of ∼1.0 meV, ∼2.8 meV, and ∼8.4 meV. The existence of localized vibrational modes, which are assigned to the displacements of the trigonally coordinated Cu atoms in the structure, is discussed in the context of anharmonicity and its effect on the low lattice thermal conductivity exhibited by these compounds.

Phonon Self-Energy and Origin of Anomalous Neutron Scattering Spectra in SnTe and PbTe Thermoelectrics

The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering (INS) and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe exhibit a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured INS spectra and their temperature dependence. We establish that the phase space for three-phonon scattering processes, combined with the proximity to the lattice instability, is the mechanism determining the complex spectrum of the transverse-optic ferroelectric mode.

Thermoelectric properties of p-type LiZnSb: Assessment of ab initio calculations

In response to theoretical calculations on the thermoelectric performance of LiZnSb, we report the pertinent transport properties between room temperature and 523 K. Nominal LiZnSb samples are found to be p-type, with a carrier concentration in the range (4–7)×10^(20) cm^(−3). The thermoelectric figure of merit (zT) is found to be 0.02–0.08 at 523 K. Analysis of material transport parameters and previously reported ab initio calculations demonstrates that even with optimal doping, p-type LiZnSb is unlikely to achieve zT>0.2 at 523 K. The accuracy of the high zT estimate (zT>2) for n-type compositions from ab initio calculations is discussed within the current synthetic limits.

Thermoelectric properties of Co-, Ir-, and Os-doped FeSi alloys: Evidence for strong electron-phonon coupling

The effects of various transition-metal dopants on the electrical and thermal transport properties of Fe{sub 1-x}M{sub x}Si alloys (M = Co, Ir, Os) are reported. The maximum thermoelectric figure of merit ZT{sub max} is improved from 0.007 at 60 K for pure FeSi to ZT = 0.08 at 100 K for 4% Ir doping. A comparison of the thermal conductivity data among Os-, Ir-, and Co-doped alloys indicates strong electron-phonon coupling in this compound. Because of this interaction, the common approximation of dividing the total thermal conductivity into independent electronic and lattice components (K{sub total} = K{sub electronic} + K{sub lattice}) fails for these alloys. The effects of grain size on thermoelectric properties of Fe{sub 0.96}Ir{sub 0.04}Si alloys are also reported. The thermal conductivity can be lowered by {approx}50% with little or no effect on the electrical resistivity or Seebeck coefficient. This results in ZT{sub max} = 0.125 at 100 K, still approximately a factor of 5 too low for solid-state refrigeration applications.

Thermoelectric transport properties of CaMg2Bi2, EuMg2Bi2, and YbMg2Bi2

The thermoelectric transport properties of CaMg

Transport properties of the layered Zintl compound SrZnSb2

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Phonon anharmonicity and negative thermal expansion in SnSe

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Properties of single crystalline AZn2Sb2 (A=Ca,Eu,Yb)

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