The response of the worldwide scientific community to the discovery in 2008 of superconductivity at T c = 26 K in the Fe-based compound LaFeAsO1−x F x has been very enthusiastic. In short order, ot...

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

Mitigation of the global energy crisis requires tailoring the thermal conductivity of materials. Low thermal conductivity is critical in a broad range of energy conversion technologies, including thermoelectrics and thermal barrier coatings. Here, we review the chemical trends and explore the origins of low thermal conductivity in crystalline materials. A unifying feature in the latest materials is the incorporation of structural complexity to decrease the phonon velocity and increase scattering. With this understanding, strategies for combining these mechanisms can be formulated for designing new materials with exceptionally low thermal conductivity.

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Phonon engineering through crystal chemistry

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.

Giant anharmonic phonon scattering in PbTe

We present a first-principles theoretical approach to calculate the lattice thermal conductivity of diamond based on an exact solution of the Boltzmann transport equation. Density-functional perturbation theory is employed to generate the harmonic and third-order anharmonic interatomic force constants that are required as input. A central feature of this approach is that it provides accurate representations of the interatomic forces and at the same time introduced no adjustable parameters. The calculated lattice thermal conductivities for isotopically enriched and naturally occurring diamond are both in very good agreement with experimental data. The role of the scattering of heat-carrying acoustic phonons by optic branch phonons is also investigated. We show that inclusion of this scattering channel is indispensable in properly describing the thermal conductivity of semiconductors and insulators. The accurate adjustable-parameter-free results obtained herein highlight the promise of this approach in providing predictive descriptions of the lattice thermal conductivity of materials.

/pdf/ab-initio-theory-of-the-lattice-thermal-conductivity-in-5292erk3m8.pdf

Ab initio theory of the lattice thermal conductivity in diamond

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.

/pdf/resonant-bonding-leads-to-low-lattice-thermal-conductivity-2gglwa6n0j.pdf

Resonant bonding leads to low lattice thermal conductivity.

The physical properties of the layered iron superconductors and related phases are discussed starting from first principles calculations. The electronic structure is described as that of metallic Fe2+ square lattice sheets with substantial direct Fe-Fe hopping and interactions with the neighboring anionic pnictogens or chalcogens. The materials have a semi-metallic band structure, and in particular the Fermi surface consists of small cylindrical electron sections centered at the zone corner, and compensating hole sections at the zone boundary. The density of states N(E
F) is high placing the materials near itinerant magnetism in general, and furthermore the small Fermi surface sections are well nested leading to a tendency towards a spin density wave. Comparison of experimental and density functional results imply the presence of exceptionally strong spin fluctuations in these materials. Superconductivity is discussed within this context.

Electronic Structure, Magnetism and Spin-Fluctuations in Fe-As Based Superconductors

The layered iron superconductors are discussed using electronic structure calculations. The four families of compounds discovered so far, including Fe (Se, Te) have closely related electronic structures. The Fermi surface consists of disconnected hole and electron cylinders and additional hole sections that depend on the specific material. This places the materials in proximity to itinerant magnetism, both due to the high density of states and due to nesting. Comparison of density functional results and experiment provides strong evidence for itinerant spin fluctuations, which are discussed in relation to superconductivity. It is proposed that the intermediate phase between the structural transition and the SDW transition in the oxy-pnictides is a nematic phase.

Electronic structure, magnetism and superconductivity of layered iron compounds

Ab initio phonon dispersions for PbTe

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Electronic Structure, Magnetism and Spin-Fluctuations in Fe-As Based Superconductors

Electronic structure, magnetism and superconductivity of layered iron compounds

Ab initio phonon dispersions for PbTe