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Recent studies have reported the experimental discovery that nanoscale specimens of even a natural material, such as diamond, can be deformed elastically to as much as 10% tensile elastic strain at room temperature without the onset of permanent damage or fracture. Computational work combining ab initio calculations and machine learning (ML) algorithms has further demonstrated that the bandgap of diamond can be altered significantly purely by reversible elastic straining. These findings open up unprecedented possibilities for designing materials and devices with extreme physical properties and performance characteristics for a variety of technological applications. However, a general scientific framework to guide the design of engineering materials through such elastic strain engineering (ESE) has not yet been developed. By combining first-principles calculations with ML, we present here a general approach to map out the entire phonon stability boundary in six-dimensional strain space, which can guide the ESE of a material without phase transitions. We focus on ESE of vibrational properties, including harmonic phonon dispersions, nonlinear phonon scattering, and thermal conductivity. While the framework presented here can be applied to any material, we show as an example demonstration that the room-temperature lattice thermal conductivity of diamond can be increased by more than 100% or reduced by more than 95% purely by ESE, without triggering phonon instabilities. Such a framework opens the door for tailoring of thermal-barrier, thermoelectric, and electro-optical properties of materials and devices through the purposeful design of homogeneous or inhomogeneous strains. 

First principles simulations are utilized to calculate the electronic and vibrational properties of several metastable structural phases of the CuZn 2 InSe 4 quaternary chalcogenide, including stanite, kesterite, primitive mixed CuAu, wurtzite-stanite, and wurtzite-kesterite lattices. We find that although each phase is formed by nearest cation-chalcogen bonds, the structural diversity due to cation and polyhedral arrangements has direct consequences in the electronic structure. The simulations further indicate that hybrid functionals are needed to account for the s–p and p–d orbital hybridization that is found around the Fermi level, which leads to much enhanced energy band gaps when compared with standard exchange-correlation approaches. We also find that the thermal conductivities for all phases are relatively low, and the main scattering channel comes from a low frequency optical band hybridized with acoustic phonons. Given that CuZn 2 InSe 4 is a material from a larger class of quaternary chalcogenides, other materials may exhibit similar electronic and vibrational properties, which may be useful for electronic and thermal management applications. 

Thermoelectric (TE) transport in anisotropic materials is investigated based on most general thermodynamical concepts. Currents and power conversion efficiency are studied in SnSe and SnS in different directions. The design of composites whose TE performance along different principles directions is the same is proposed. Although such features do not occur naturally, such man-made anisotropic materials can be constructed using bilayers achieving much broadened working conditions of TE conversion devices. Intricate relationships between the anisotropy and the direction of the electric and heat currents are revealed, which further help us understand how transport occurs in such composites.

 

The synthesis, electronic structure and temperature dependent transport properties of polycrystalline Cu 1+x Mn 2−x InTe 4 ( x = 0, 0.2, 0.3) are reported for the first time. These quaternary chalcogenides were synthesized by direct reaction of the elements, followed by solid state annealing and hot press densification. The thermal conductivity is low for all specimens and intrinsic to the material system. Furthermore, the off-stoichiometry specimens illustrate the sensitivity of the transport properties to stoichiometry, with a greater than two-orders-of magnitude increase in carrier concentration with increased Cu content. First principles calculations of the electronic structure are also reported, and are in agreement with the experimental data. This fundamental investigation shows the potential towards further optimization of the electrical properties that, in addition to the intrinsically low thermal conductivity, provides a basis for further research into the viability of this material system for potential energy-related applications.