BaC: a thermodynamically stable layered superconductor

Superconductive Sodium Carbides with Pentagon Carbon at High Pressures

The design of metal-bearing carbon-based materials with unique structures and intriguing properties is highly desirable in the fields of physics, chemistry, and materials science. Here, within swarm-intelligence structure search and first-principles computations, we uncovered several hitherto unknown sodium carbides (i.e., Na₄C, Na₃C₂, NaC, Na₂C₃, and NaC₂) under high pressure. Intriguingly, the C atom arrangement reveals multiple structure evolution behavior with increased carbon content, from isolated anions in Na₄C, tetramers in Na₃C₂, extended chains in NaC, pentagonal rings in Na₂C₃, to eventually hexagonal rings in NaC₂. Among predicted phases, the superconducting critical temperature T_c of NaC₂ could approach ∼42 K at 80 GPa, which is slightly higher than the T_c of 39 K in the highest phonon-mediated superconductivity of MgB₂ at ambient pressure. This work offers insights into the reaction of carbides containing alkali metals and paves the way for the future investigation of high superconductivity in metal carbide systems.

Nonmetallization and band inversion in beryllium dicarbide at high pressure

Carbides have attracted much attention owing to their interesting physical and chemical properties. Here, we systematically investigated global energetically stable structures of BeC₂ in the pressure range of 0–100 GPa using a first-principles structural search. A transition from the ambient-pressure α-phase to the high-pressure β-phase was theoretically predicted. Chemical bonding analysis revealed that the predicted phase transition is associated with the transformation from sp² to sp³ C-C hybridization. The electrical conductivity of the high-pressure phase changed from a metal (α-phase) to a narrow bandgap semiconductor (β-phase), and the β-phase had an inverted band structure with positive pressure dependence. Interestingly, the β-phase was a topological insulator with the metallic surface states protected by the time-reversal symmetry of the crystal. The results indicate that pressure modulates the electronic band structure of BeC₂, which is an important finding for fundamental physics and for a wide range of potential applications in electronic devices.

Investigation of exotic stable calcium carbides using theory and experiment

It is well known that pressure causes profound changes in the properties of atoms and chemical bonding, leading to the formation of many unusual materials. Here we systematically explore all stable calcium carbides at pressures from ambient to 100 GPa using variable-composition evolutionary structure predictions using the USPEX code. We find that Ca₅C₂, Ca₂C, Ca₃C₂, CaC, Ca₂C₃ and CaC₂ have stability fields on the phase diagram. Among these, Ca₂C and Ca₂C₃ are successfully synthesized for the first time via high-pressure experiments with excellent structural correspondence to theoretical predictions. Of particular significance is the base-centred monoclinic phase (space group C2/m) of Ca₂C, a quasi-two-dimensional metal with layers of negatively charged calcium atoms, and the primitive monoclinic phase (space group P2₁/c) of CaC with zigzag C₄ groups. Interestingly, strong interstitial charge localization is found in the structure of R-3m-Ca₅C₂ with semi-metallic behaviour.

Pressure causes profound changes in the properties of atoms and chemical bonding leading to unusual materials. Here, the authors investigate the Ca-C system and find that it becomes increasingly complex and develops a multitude of phases with various compositions and new structures at higher pressures.