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We introduce a lattice dynamics package which calculates elastic, thermodynamic and thermal transport properties of crystalline materials from data on their force and potential energy as a function of atomic positions. The data can come from density functional theory (DFT) calculations or classical molecular dynamics runs performed in a supercell. First, the model potential parameters, which are anharmonic force constants are extracted from the latter runs. Then, once the anharmonic model is defined, thermal conductivity and equilibrium properties at finite temperatures can be computed using lattice dynamics, Boltzmann transport theories, and a variational principle respectively. In addition, the software calculates the mechanical properties such as elastic tensor, Gruneisen parameters and the thermal expansion coefficient within the quasi-harmonic approximation (QHA). Phonons, elastic constants and thermodynamic properties results applied to the germanium crystal will be illustrated. Using the force constants as a force field, one may also perform molecular dynamics (MD) simulations in order to investigate the combined effects of anharmonicity and defect scattering beyond perturbation theory. 

GeTe-based alloys hold great promise for thermoelectric applications. Our comprehensive study investigates the intricate interplay between chemical bonding and transport properties in cubic GeTe. We demonstrate a balance between minimizing thermal conductivity and maximizing power factor, guided by the mediating influence of chemical bonding. Our primary findings reveal that Pb-doped GeTe exhibits low lattice thermal conductivity due to weak p–p orbital interactions, whereas In-doping boosts lattice thermal conductivity by reinforcing the chemical bonds, as elucidated by crystal orbital hamilton population (COHP) analysis. Further investigation reveals weak s–p interactions in Bi-, Sb-, and Pb-doped GeTe, and strong s–p interactions in In-doped GeTe compared to the pure GeTe, as probed by projected density of state (PDOS). These dual effects explain the experimentally observed high power factor and enhanced zT in Bi-, Sb-, and Pb- doping in contrast to In-doping. In our study, we find that weak s–p interactions improves electronic performance by modifying DOS whereas weak p–p interactions reduce thermal transport by diminishing the strength of chemical bonding. These findings underscore the correlation between doping-induced modifications in chemical bonding and resulting thermoelectric properties. Utilizing a first-principles framework, we systematically explore the temperature and carrier concentration-dependent transport properties of pure GeTe under relaxation time approximation. Optimization strategies yield a maximum peak power factor times temperature of 2.2 Wm−1 K−1 and a maximum zT value of ∼0.83 at 800 K, showcasing the potential for tailored thermoelectric performance. Finally, this research presents a systematic approach to improve thermoelectric performance by modifying chemical bonds through doping. 

Metavalent descriptors of LaP exhibit a stronger pressure dependence than those of LaBi. Strong anharmonicity in LaP bonds come from its antibonding π* valence bands. These features in LaP make its thermal conductivity lower than LaBi. 

Two-dimensional layered transition metal dichalcogenides are potential thermoelectric candidates with application in on-chip integrated nanoscale cooling and power generation. Here, we report a comprehensive experimental and theoretical study on the in-plane thermoelectric transport properties of thin 2H-MoTe2 flakes prepared in field-effect transistor geometry to enable electrostatic gating and modulation of the electronic properties. The thermoelectric power factor is enhanced by up to 45% using electrostatic modulation. The in-plane thermal conductivity of 9.8 ± 3.7 W m−1 K−1 is measured using the heat diffusion imaging method in a 25 nm thick flake. First-principles calculations are used to obtain the electronic band structure, phonon band dispersion, and electron–phonon scattering rates. The experimental electronic properties are in agreement with theoretical results obtained within energy-dependent relaxation time approximation. The thermal conductivity is evaluated using both the relaxation time approximation and the full iterative solution to the phonon Boltzmann transport equation. This study establishes a framework to quantitively compare first-principle-based calculations with experiments in 2D layered materials. 

In this review, motivated by the recent interest in high-temperature materials, we review our recent progress in theories of lattice dynamics in and out of equilibrium. To investigate thermodynamic properties of anharmonic crystals, the self-consistent phonon theory was developed, mainly in the 1960s, for rare gas atoms and quantum crystals. We have extended this theory to investigate the properties of the equilibrium state of a crystal, including its unit cell shape and size, atomic positions and lattice dynamical properties. Using the equation-of-motion method combined with the fluctuation–dissipation theorem and the Donsker–Furutsu–Novikov (DFN) theorem, this approach was also extended to investigate the non-equilibrium case where there is heat flow across a junction or an interface. The formalism is a classical one and therefore valid at high temperatures.