Carbon network evolution from dimers to sheets in superconducting ytrrium dicarbide under pressure

Emerging Yttrium Phosphides with Tetrahedron Phosphorus and Superconductivity under High Pressures

Metal phosphides have triggered growing interest for their exotic structures and striking properties. Hence, within advanced structure search and first-principle calculations, several unprecedented Y-P compounds (e. g., Y₃ P, Y₂ P, Y₃ P₂ , Y₂ P₃ , YP₂ , and YP₃ ) were identified under compression. Interestingly, as phosphorus content increases, P atoms exhibit diverse behaviors corresponding to standalone anion, dumbbell, zigzag chain, planar sheet, crossing chain-like network, buckled layer, three-dimensional framework, and wrinkled layer. Particularly, Fd-3m YP₂ can be viewed as assemblage of diamond-like Y structure and rare vertex-sharing tetrahedral P₄  units. Impressively, electron-phonon coupling (EPC) calculations elucidate that Pm-3m Y₃ P possesses the highest superconducting critical temperature T_c of 10.2 K among binary transition metal phosphides. Remarkably, the EPC of Pm-3m Y₃ P mainly arises from the contribution of low-frequency soft phonon modes, whereas mid-frequency phonon modes of Fd-3m YP₂ dominate. These results strengthen knowledge of metal phosphides and pave a way for seeking superconductive transition metal phosphides.

Novel High-Pressure Yttrium Carbide γ-Y_{4}C_{5} Containing [C_{2}] and Nonlinear [C_{3}] Units with Unusually Large Formal Charges

Changes in the bonding of carbon under high pressure leads to unusual crystal chemistry and can dramatically alter the properties of transition metal carbides. In this work, the new orthorhombic polymorph of yttrium carbide, γ-Y_{4}C_{5}, was synthesized from yttrium and paraffin oil in a laser-heated diamond anvil cell at ∼50  GPa. The structure of γ-Y_{4}C_{5} was solved and refined using in situ synchrotron single-crystal x-ray diffraction. It includes two carbon groups: [C_{2}] dimers and nonlinear [C_{3}] trimers. Crystal chemical analysis and density functional theory calculations revealed unusually high noninteger charges ([C_{2}]^{5.2-} and [C_{3}]^{6.8-}) and unique bond orders (<1.5). Our results extend the list of possible carbon states at extreme conditions.

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.

Computational prediction of a +4 oxidation state in Au via compressed AuO2 compound

Much effort has been devoted to the investigation of the physical and chemical properties of the Au-O system over a range of pressures, owing to the considerable importance of these materials in fundamental and practical applications. To date, however, only Au¹⁺, Au²⁺, Au³⁺, and Au⁵⁺ oxidation states have been identified in the Au-O system, but tetravalent Au⁴⁺ has not been found. Here, we report the results of structure prediction for the Au-O system at high pressure via the effective structure prediction methodology within a first-principles electronic structure framework. We have uncovered an intriguing structure with AuO₂ composition and tetravalent Au, stable at high pressures. This phase shows an electronic transition from a metal to a semiconducting phase as a function of pressure. The present results provide fundamental understanding of the structural and physicochemical properties of compressed Au-O compounds.

The magnetism of 1T-MX2 (M = Zr, Hf; X = S, Se) monolayers by hole doping

The magnetism of hole doped 1T-MX₂ (M = Zr, Hf; X = S, Se) monolayers is systematically studied by using first principles density functional calculations. The pristine 1T-MX₂ monolayers are semiconductors with nonmagnetic ground states, which can be transformed to ferromagnetic states by the approach of hole doping. For the unstrained monolayers, the spontaneous magnetization appears once above the critical hole density (10¹⁴ cm⁻²), where the p orbital of S or Se atoms contributes the most of the magnetic moment. As the tensile strains exceed 4%, the magnetic moments per hole of ZrS₂ and HfS₂ monolayers increase sharply to a saturated value with increasing hole density, implying obvious advantages over the unstrained monolayers. The phonon dispersion calculations for the strained ZrS₂ and HfS₂ monolayers indicate that they can keep the dynamical stability by hole doping. Furthermore, we propose that the fluorine atom modified ZrS₂ monolayer could obtain stable ferromagnetism. The magnetism in hole doped 1T-MX₂ (M = Zr, Hf; X = S, Se) monolayers has great potential for developing spintronic devices with desirable applications.

The magnetism of zirconium and hafnium dichalcogenides by hole doping is studied by using first principles calculations.