Xuezhi Ke

Transition metal carbides have attracted considerable attention and are widely used in machining tools, hard coatings and aerospace components, owing to their excellent mechanical and thermal properties. The Zr–C system is a typical refractory and hard transition-metal carbide, and its structural integrity and stability under extreme conditions are critical for practical applications. Here, a computational study focusing on the structural stability and crystal evolution pattern of Zr₂C under ambient and high-pressure conditions was performed using a particle-swarm optimization algorithm, in combination with first-principles calculations. The calculations identified seven viable stable or metastable crystalline phases of Zr₂C, exhibiting Fd 3 m, R 3 m, Cmcm, Cmca, Pbcn, Pnma and I4/mcm symmetries; further, a series of structural phase transitions were determined as the pressure increased: Fd 3 m → R 3 m → Cmcm → Cmca. In addition, the mechanical and dynamical stabilities of these phases were verified, and their structural properties were investigated. Overall, this work reveals valuable information concerning the structural, mechanical and electronic properties of Zr₂C, providing key insights into the mechanisms underlying its crystal evolution behavior.

First-principles calculations and particle swarm optimization algorithm are combined to predict the crystal structures in the pressure range from 0 to 100 GPa. Four phases of ThBC are determined, including the P4₁22, Cmcm, Cmce and Immm phases, in which the Cmcm, Cmce and Immm phases are newly predicted structures. The mechanical, electronic and thermodynamic properties of the four phases are investigated. According to the enthalpy–pressure and volume–pressure curves, the phase transition pressure from the P4₁22 phase to the Cmcm phase is 15 GPa, the Cmcm to the Cmce is 36 GPa and the Cmce to the Immm is 69 GPa. All the transitions belong to the first-order phase transition. Based on the calculated elastic constants, the P4₁22, Cmcm, Cmce and Immm phases exhibit brittle nature. The Young’s moduli show that the P4₁22 phase has the largest degree of anisotropy, and the Immm phase has the smallest. The calculated density of states reveal that the P4₁22, Cmcm, Cmce and Immm phases are all metallic.

Within the past few years, intriguing graphene Dirac cones have attracted intense interest in novel two-dimensional (2D) Dirac materials as ultrahigh-mobility functional materials. In this work, the phonon-limited charge transport properties of α-graphyne (α-GY), α-graphdiyne (α-GDY), and β-graphyne (β-GY) were investigated using the Boltzmann transport equation within the first-principles framework while considering the electron–phonon coupling (EPC). Despite all three investigated compounds being 2D Dirac carbon materials, each demonstrated distinctly different carrier mobilities by one order of magnitude (2.2 × 10⁴ cm² V⁻¹ s⁻¹ for α-GY, 2.1 × 10³ cm² V⁻¹ s⁻¹ for α-GDY and 1.9 × 10³ cm² V⁻¹ s⁻¹ for β-GY at room-temperature and a carrier connection of n ∼ 3 × 10¹² cm⁻²). The essential differences in the mobilities of these materials originated from the acetylenic linkage limiting the group velocity and the E_2g phonon modes limiting the scattering time. For example, a few uniformly equivalent acetylenic linkages and E_2g phonon modes tend to generate high mobilities. A simple mobility relationship was determined using the number of E_2g photon modes, allowing for a quick estimation of the mobilities for Dirac materials. α-GY was identified as a promising alternative to graphene for next generation nanoelectronic devices.

Thorium-carbon systems have been thought as promising nuclear fuel for Generation IV reactors which require high-burnup and safe nuclear fuel. Existing knowledge on thorium carbides under extreme condition remains insufficient and some is controversial due to limited studies. Here we systematically predict all stable structures of thorium dicarbide (ThC₂) under the pressure ranging from ambient to 300 GPa by merging ab initio total energy calculations and unbiased structure searching method, which are in sequence of C2/c, C2/m, Cmmm, Immm and P6/mmm phases. Among these phases, the C2/m is successfully observed for the first time via in situ synchrotron XRD measurements, which exhibits an excellent structural correspondence to our theoretical predictions. The transition sequence and the critical pressures are predicted. The calculated results also reveal the polymerization behaviors of the carbon atoms and the corresponding characteristic C-C bonding under various pressures. Our work provides key information on the fundamental material behavior and insights into the underlying mechanisms that lay the foundation for further exploration and application of ThC₂.

Searching and designing materials with extremely low lattice thermal conductivity (LTC) has attracted considerable attention in material sciences. Here we systematically demonstrate the diverse lattice dynamics of the ternary Cu-Sb-Se compounds due to the different chemical-bond environments. For Cu₃SbSe₄ and CuSbSe₂, the chemical bond strength is nearly equally distributed in crystalline bulk and all the atoms are constrained to be around their equilibrium positions. Their thermal transport behaviors are well interpreted by the perturbative phonon-phonon interactions. While for Cu₃SbSe₃ with obvious chemical-bond hierarchy, one type of atoms is weakly bonded with surrounding atoms, which leads the structure to the part-crystalline state. The part-crystalline state makes a great contribution to the reduction of thermal conductivity that can only be effectively described by including a rattling-like scattering process in addition to the perturbative method. Current results may inspire new approaches to designing materials with low lattice thermal conductivities for high-performance thermoelectric conversion and thermal barrier coatings.