Predicted the structural diversity and electronic properties of Pt-N compounds under high pressure

The nitrogen-rich transition metal nitrides have attracted considerable attention due to their potential application as high energy density materials. Here, a systematic theoretical study of PtN_xcompounds has been performed by combining first-principles calculations and particle swarm-optimized structure search method at high pressure. The results indicate that several unconventional stoichiometries of PtN₂, PtN₄, PtN₅, and Pt₃N₄compounds are stabilized at moderate pressure of 50 GPa. Moreover, some of these structures are dynamically stable even when the pressure release to ambient pressure. TheP1-phase of PtN₄and theP1-phase of PtN₅can release about 1.23 kJ g^-1and 1.71 kJ g^-1, respectively, upon the decomposition into elemental Pt and N₂. The electronic structure analysis shows that all crystal structures are indirect band gap semiconductors, except for the metallic Pt₃N₄withPcphase, and the metallic Pt₃N₄is a superconductor with estimated critical temperatureT_cvalues of 3.6 K at 50 GPa. These findings not only enrich the understanding of transition metal platinum nitrides, but also provide valuable insights for the experimental exploration of multifunctional polynitrogen compounds.

Superconductivity and strong anharmonicity in novel Nb-S phases

In this work we explore the phase diagram of the binary Nb-S system from ambient pressures up to 250 GPa usingab initioevolutionary crystal structure prediction. We find several new stable compositions and phases, especially in the high-pressure regime, and investigate their electronic, vibrational, and superconducting properties. Our calculations show that all materials, besides the low-pressure phases of pure sulfur, are metals with low electron-phonon (ep) coupling strengths and critical superconducting temperatures below 15 K. Furthermore, we investigate the effects of phonon anharmonicity on lattice dynamics, ep interactions, and superconductivity for the novel high-pressure phase of Nb₂S, demonstrating that the inclusion of anharmonicity stabilizes the lattice and enhances the ep interaction.

IrN4 and IrN7 as potential high-energy-density materials

Transition metal nitrides have attracted great interest due to their unique crystal structures and applications. Here, we predict two N-rich iridium nitrides (IrN₄ and IrN₇) under moderate pressure through first-principles swarm-intelligence structural searches. The two new compounds are composed of stable IrN₆ octahedrons and interlinked with high energy polynitrogens (planar N₄ or cyclo-N₅). Balanced structural robustness and energy content result in IrN₄ and IrN₇ being dynamically stable under ambient conditions and potentially as high energy density materials. The calculated energy densities for IrN₄ and IrN₇ are 1.3 kJ/g and 1.4 kJ/g, respectively, comparable to other transition metal nitrides. In addition, IrN₄ is predicted to have good tensile (40.2 GPa) and shear strengths (33.2 GPa), as well as adequate hardness (20 GPa). Moderate pressure for synthesis and ambient pressure recoverability encourage experimental realization of these two compounds in near future.

Thermal Expansion of Incompressible U2S3-Type Nb2N3 Synthesized in a Diamond Anvil Cell

A novel niobium nitride, U₂S₃-type Nb₂N₃, has been successfully synthesized by nitridation of δ-NbN above approximately 30 GPa in a laser-heated diamond anvil cell. Nb₂N₃ crystallizes in the same orthorhombic structure (space group: Pnma) as η-Ta₂N₃. Nb₂N₃ consists of regular-shaped polyhedra, and the bulk modulus has been determined to K₀ = 300(2) GPa. The low-temperature X-ray diffraction measurements have been successfully conducted for the tiny novel Nb₂N₃ between 297.7(5) and 106.3(3) K under ambient pressure. Nb₂N₃ shows no structural phase transition down to 106.3(3) K, and investigation of the linear thermal expansion coefficients yields α_a = 3.36(9) × 10^-6 K^-1, α_b = 5.39(10) × 10^-6 K^-1, α_c = 6.77(15) × 10^-6 K^-1, respectively. Our study reveals that the incompressible novel nitride shows low thermal expansion behavior, which offers new insights for the development of functional nitrides and their crystal chemistry.

Emergent property of high hardness for C-rich ruthenium carbides: partial covalent Ru-Ru bonds

Hard materials are being investigated all the time by combining transition metals with light elements. Combining a structure search with first-principles functional calculations, we first discovered three stable stoichiometric C-rich ruthenium carbides in view of three synthesis routes, namely, the ambient phases of Ru₂C₃ and RuC, and two high pressure phases of RuC₄. There is a phase transition of RuC₄ from the P3[combining macron]m1 structure to the R3[combining macron]m structure above 98 GPa. The calculations of elastic constants and phonon dispersions show their mechanical and dynamical stability. The large elastic modulus, high Debye temperature and the estimated hardness values suggest that these hard ruthenium carbides have good mechanical properties. The analyses of electronic structure and chemical bonding indicate that chemical bonding, not carbon content, is the key factor for the hardness in these metallic C-rich ruthenium carbides. The partial covalent Ru-C bonds and strong covalent C-C bonds are responsible for the high hardness. Moreover, the emergence of partial covalent Ru-Ru bonds can enhance the hardness of RuC, while the ionic Ru-Ru bonds can weaken the hardness of Ru₂C₃.

Crystal structures of transition metal pernitrides predicted from first principles

We have extensively explored the stable crystal structures of early-transition metal pernitrides (TMN₂, TM = Ti, V, Cr, Mn, Zr, Nb, Mo, Hf, and Ta) at ambient and high pressures using effective CALYPSO global structure search algorithm in combination with first-principles calculations. We identified for the first time the ground-state structures of MnN₂, TaN₂, NbN₂, VN₂, ZrN₂, and HfN₂ pernitrides, and proposed their synthesis pressures. All predicted crystal structures contain encapsulated N₂ dumbbells in which the two N atoms are singly bonded to a [N₂]⁴⁻ pernitride unit utilizing the electrons transferred from the transition metals. The strong nature of the single dinitrogen bond and transition metal–nitrogen charge transfer induce extraordinary mechanic properties in the predicted transition metal pernitrides including large bulk modulus and high Vickers hardness. Among the predictions the hardness of MnN₂ is 36.6 GPa, suggesting that it is potentially a hard material. The results obtained in the present study are important to the understanding of structure–property relationships in transition metal pernitrides and will hopefully encourage future synthesis of these technologically important materials.

We identified for the first time the ground-state structures of MnN₂, TaN₂, NbN₂, VN₂, ZrN₂ and HfN₂ pernitrides and proposed their synthesis pressures.

The pressure-temperature phase diagram of pure Co based on first-principles calculations

We optimized the high pressure-temperature phase diagram of pure Co up to the liquidus temperature and 120 GPa, based on thermodynamic properties calculated using first-principles. The Gibbs energy for each phase was evaluated in the framework of a quasiharmonic approximation, with a consideration of the thermal electronic contribution at finite temperatures. Particularly, the liquidus temperature, as a function of pressure, was determined using classical Molecular Dynamics simulations. Our results in this work successfully integrated experimental observations and the previous theoretical predictions. The critical solid phase transitions of ε → γ_f and ε → β were clarified using the Gibbs energy as a function of pressure and temperature. In addition, the magnetism of β above 70 GPa was verified to be nonmagnetic. The difference between γ_p and β, which was unclear before, has been illustrated to be associated with the magnetic transformation from the paramagnetic state of the γ_p phase to the nonmagnetic state of the β phase rather than the structural transformation.

Stability and properties of the Ru–H system at high pressure

The calculated formation enthalpies of RuH_n (n = 1–8) with respect to Ru and H at different pressures.