GeTe is highly sought-after due to its versatility as a high-performance thermoelectric material and phase change material, as well as a ferroelectric Rashba semiconductor. Compared to most thermoelectric materials, it has an additional degree of freedom of rhombohedral–cubic phase transition at 673 K. At this temperature, the lattice thermal conductivity approaches a theoretical minimum due to ferroelectric instability while the high-energy Σ and low-energy L bands converge to give outstanding electronic properties. Therefore, modulation of the phase transition temperature allows simultaneous and synergistic tuning of the electronic and thermal transport properties to achieve high zT. In this work, Sn alloying together with Bi,Sb doping is used to suppress the phase transition to achieve a pure cubic structure with a lattice thermal conductivity of around 0.4 W m⁻¹ K⁻¹ and peak zT of 1.7 at 723 K with an average zT of 1.23 between 400 and 800 K. Furthermore, the Vickers hardness of 270 and Young's modulus of 63.5 GPa in Ge_0.4Sn_0.4Bi_0.02Sb_0.12Te are by far the highest amongst binary chalcogenides. More importantly, the high quality factor achieved in this work gives ample room for further zT improvements. The fundamental insights drawn from this work provide a pathway towards engineering GeTe-based alloys to achieve high zT at any temperature of interest.

ThermoelectricZTof n-doped ZrSe₂/HfSe₂SLM at room temperature.

Heat energy in solids is carried by phonons and electrons. However, in most two-dimensional (2D) materials, the contribution from electrons to total thermal conduction is much lower than that for phonons. In this work, through first-principles calculations combined with non-equilibrium Green’s function theory, we studied electron and phonon thermal conductance in recently synthesized 2D hydrogen boride. The hexagonal boron network with bridging hydrogen atoms is suggested to exhibit comparable lattice thermal conductance (4.07 nWK⁻¹ nm⁻²) as graphene (4.1 nWK⁻¹ nm⁻²), and similar electron thermal conductance (3.6 nWK⁻¹ nm⁻²), which is almost ten times that of graphene. As a result, total thermal conductance of 2D hydrogen boride is about two-fold of graphene, being the highest value in all known 2D materials. Moreover, tensile strain along the armchair direction leads to an increase in carrier density, significantly increasing electron thermal conductance. The increase in electron thermal conductance offsets the reduction in phonon thermal conductance, contributing to an abnormal increase in thermal conductance. We demonstrate that the high electron density governs extraordinarily high thermal conductance in 2D hydrogen boride, distinctive among 2D materials.