Low thermal conductivity materials are crucial for applications such as thermoelectric conversion of waste heat to useful energy and thermal barrier coatings. On the other hand, high thermal conductivity materials are necessary for cooling electronic devices. However, search for such materials via explicit evaluation of thermal conductivity either experimentally or computationally is very challenging. Here, we carried out high-throughput ab initio calculations, on a dataset containing 195 binary, ternary, and quaternary compounds. The lattice thermal conductivity κ l values of 120 dynamically stable and nonmetallic compounds are calculated, which span over 3 orders of magnitude. Among these, 11 ultrahigh and 15 ultralow κ l materials are identified. An analysis of generated property map of this dataset reveals a strong dependence of κ l on simple descriptors, namely, maximum phonon frequency, integrated Gruneisen parameter up to 3 THz, average atomic mass, and volume of the unit cell. Using these descriptors, a Gaussian process regression-based machine learning (ML) model is developed. The model predicts log-scaled κ l with a very small root mean square error of ∼0.21. Comparatively, the Slack model, which uses more involved parameters, severely overestimates κ l . The superior performance of our ML model can ensure a reliable and accelerated search for multitude of low and high thermal conductivity materials.

We investigate the electronic and thermal transport properties of bulk MX2 compounds (M = Zr, Hf and X = S, Se) by first-principles calculations and semi-classical Boltzmann transport theory. The band structure shows the confinement of heavy and light bands along the out of plane and in-plane directions, respectively. This results in high electrical conductivity (σ) and large thermopower leading to a high power factor (S2σ) for moderate n-type doping. The phonon dispersion demonstrates low frequency flat acoustical modes, which results in low group velocities (vg). Consequently, lowering the lattice thermal conductivity (κlatt) below 2 W/m K. Low κlatt combined with high power factor results in ZT > 0.8 for all the bulk MX2 compounds at high temperature of 1200 K. In particular, the ZTmax of HfSe2 exceeds 1 at 1400 K. Our results show that Hf/Zr based dichalcogenides are very promising for high temperature thermoelectric application.

Development of efficient thermoelectric materials requires a designing approach that leads to excellent electronic and phononic transport properties. Using firstprinciples density functional theory and semiclassical Boltzmann transport theory, we report unprecedented enhancement in electronic transport properties of A II B IV C 2 V (group II = Be, Mg, Zn, and Cd; group IV = Si, Ge, and Sn; and group V = P and As) chalcopyrites via isoelectronic substitution. Multiple valleys in conduction bands, present in these compounds, are tuned to converge by substitution of group IV dopant. Additionally, this substitution improves the convergence of valence bands, which is found to have a direct correlation with the tetragonal distortion of these chalcopyrites. Furthermore, several chalcopyrite compounds with heavy elements such as Zn, Cd, and As possess low phonon group velocities and large Gruneisen parameters that lead to low lattice thermal conductivity. Combination of optimized electronic transport properties and low thermal conductivity results in maximum ZT of 1.67 in CdGeAs 2 at moderate n-type doping. The approach developed here to enhance the thermoelectric efficiency can be generalized to other class of materials.

Thermal transport in a material is governed by anharmonicity of crystal potential, which depends on the type of interatomic interaction. Using first-principles calculations, we report that lattice thermal conductivity (κ latt ) and its anisotropy (κ x,y − κ z ) of transition metal dichalcogenides (TMDs) increase by orders of magnitude with the change of constituent metal atom from Zr/Hf to Mo/W. This unprecedented difference in κ latt is substantiated by lower phonon group velocity, and 4 times larger anharmonicity of Zr/Hf based TMDs compared to Mo/W based TMDs. The sign and the absolute value of the Born effective charges, which emerges from the ionicity of the bonds, are found to be different for these two classes of materials. This leads to a significant difference in their interlayer van der Waals (vdW) interaction strengths, which are shown to be inversely related to the anisotropy in κ latt .

A quantum magnetic state due to magnetic charges is never observed, even though they are treated as quantum mechanical variables in theoretical calculations. Here, the occurrence of a novel quantum disordered state of magnetic charges in a nanoengineered magnetic honeycomb lattice of ultra‐small connecting elements is demonstrated. The experimental research, performed using spin resolved neutron scattering, reveals a massively degenerate ground state, comprised of low integer and energetically forbidden high integer magnetic charges, that manifests cooperative paramagnetism at low temperature. The system tends to preserve the degenerate configuration even under large magnetic field application. It exemplifies the robustness of disordered correlation of magnetic charges in a 2D honeycomb lattice. The realization of quantum disordered ground state elucidates the dominance of exchange energy, which is enabled due to the nanoscopic magnetic element size in nanoengineered honeycomb. Consequently, an archetypal platform is envisaged to study quantum mechanical phenomena due to emergent magnetic charges.