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The metallization of neon remains a controversial problem as there is no consensus in theoretical simulations and no experimental verification. In this work, the insulator-to-metal transition in fcc solid neon at high pressure was revisited with a coupling of the all-electron full-potential linear augmented-plane wave (FP-LAPW) method and the GW correction to avoid the potential unreliability of pseudopotential under high pressure and correct the inaccurate energy gaps caused by local density or generalized gradient approximation of the exchange-correlation. This FP-LAPW + GW calculation predicts that the bandgap closes at a density of 88.3 g/cm3 and a pressure of 208.4 TPa. Moreover, the reported positive pressure dependency of energy gap (increases with increasing density) for solid neon in 1.5–10.0 g/cm3 was confirmed with our FP-LAPW calculations, and the underlying mechanism was first revealed based upon analysis of the charge density distribution and the electron localization function. The results of this research will provide a valuable reference for future high pressure experiments and shed new insight into the planetary interiors. [ABSTRACT FROM AUTHOR]

An extensive exploration of high-pressure phase diagrams of NpHx (x = 1–10) compounds was performed by using swarm-intelligence-based CALYPSO structure searches. We propose five stable hydrogen-rich clathrate phases (P4/nmm-NpH5, Cmcm-NpH7, Fm3¯m-NpH8, P63/mmc-NpH9, and Fm3¯m-NpH10) that are composed of unusual H cages with stoichiometries H20, H24, H29, and H32 in which the H atoms are weakly covalently bonded to one another, with neptunium atoms occupying centers of the cages. The electronic structure analyses show that these predicted hydrogen-rich structures are all metallic phases, and Np–H and H–H bonds are formed by ionic and covalent bond interactions, respectively. The charge transfer from the Np atom plays an important role in the stability of the proposed structures. All hydrogen-rich clathrate structures show superconductivity behavior in their respective stability pressure range. Our work is an important step in understanding the phase stability and bonding behavior of NpHx under extreme conditions and provides a valuable reference for experimental synthesis and identification of cage-like neptunium hydrides. [ABSTRACT FROM AUTHOR]

The effect of atomic hydrogen adsorption on AnO2 (An = Th, U, and Pu) surfaces is studied in the framework of density functional theory and Hubbard-corrected density functional theory. Several adsorption coverages (1/3, 1/2, 2/3, and 1 monolayer) are considered. For the band insulator ThO2, surface metallicity induced by hydrogen adsorption is observed due to the electron donation of the hydrogen to the surface. But this effect is found to be strongly suppressed by electronic correlation for the Mott insulators UO2 and PuO2 because the electrons from the adsorbed hydrogen atoms occupy the localized 5f orbitals of the surface U/Pu atoms. [ABSTRACT FROM AUTHOR]

The density functional theory (DFT) + U method is adopted to study the electronic structure, magnetism, chemical bonding, and thermodynamic properties of USiO4. A bandgap of 3.1 eV is obtained by analyzing the band diagram of USiO4. The calculated structural parameters are consistent with the recent experimental results. The charge density and charge density differences are studied in order to analyze the chemical bonds in USiO4. The results indicate that interactions within USiO4 are mostly ionic and exhibit weak covalent character. In addition, phonon behavior is studied in detail. We predict phonon frequencies and assign and analyze the Raman-active and infrared-active modes at the Γ point. Furthermore, thermodynamic properties such as the internal energy ΔE, Helmholtz free energy ΔF, constant-volume specific heat CV, and entropy S are investigated in the region between 0 K and 1000 K. The results are expected to provide useful information for subsequent experiments on USiO4. [ABSTRACT FROM AUTHOR]

Fluorite-type XO2 (X = Ce, Th, U) have versatile technological and industrial applications, and the behavior of impurities in the oxides is one of the engaging topics for their application. However, the fundamental behaviors of impurities are still lacking. Herein, we conduct a systematic first-principles DFT+U screening to find the trends of transition metal (TM) behaviors in the three dioxides in terms of energetics and electronic states, with a particular focus on the dependency of f electronic states of the hosts. In order to overcome the long-standing bottleneck of determining the true oxidation state of multivalent TMs, Ce and U, a more rigorous method based on counting orbital occupation numbers of f and d orbitals is performed for clarification. The calculated incorporation energies and formation energies of TMs show that the relative stability of TMs in the three XO2 exhibits similar trends, indicative of the dominant roles played by the host oxides with the same crystal structure and very close lattice parameters. On the other hand, the quantitative differences in the stability and electronic state of doped TMs could be mainly attributed to the differences in the electronic structure of host XO2. The 5f electrons in UO2 are more delocalized than 4f in CeO2, suppressing the formation of high oxidation states of TMs in the former. For ThO2 with the negligible f electrons associated with monovalent Th4+, the doped TMs tend to adopt the oxidation states close to TM4+, achieving the electronically matched states. The appearance of a few unusual oxidation states of TMs sheds light on the flexible delocalization–localization mutual transition of f or d valence electrons. [ABSTRACT FROM AUTHOR]

• The general trends of stability of stoichiometric and non-stoichiometric Pu oxides are firstly reported. • A rigorous quantum definition of oxidation state is used to determine the atomic ratio of Pu3+/Pu4+ in Pu oxides. • The proposed ladder-like Pu oxide layer structure might be more reasonable than the sandwich-like structure. The oxide layer structure over Pu is the central issue of Pu corrosion science; however, the widely discussed sandwich-like structure (Pu//Pu 2 O 3 //PuO 2) might be just a phenomenological description. Here we perform first-principles calculations with the method verification on Ce oxides, the ideal analogues of Pu oxides, to establish a universal picture of Pu oxide layer structure. The results show that homogeneous Pu n O 2 n -2 between PuO 2 and Pu 2 O 3 are energetically more favorable than PuO 2- x with ordered arrays of oxygen vacancies. Thus, a ladder-like oxide layer structure is proposed to improve a simple sandwich-like model of the Pu oxide layers. [ABSTRACT FROM AUTHOR]

The nature and extent of the highest oxidation state (HOS) in solid-state actinide compounds are still unexplored compared with those of small molecules, and there is burgeoning interest in studying the actinide–ligand bonding nature in the condensed state. A comprehensive understanding of the electronic structure and unraveling the possibility of a HOS are of paramount importance in solid-state actinide chemistry. Here, we report the physical OS of the early to middle actinides (Th → Cm) in solid-state compounds via a more rigorous quantum mechanical definition of OS under the DFT+U theoretical frameworks for the first time. This work implies that the highest physical OS of the Pu solid ion is PuV in PuO2F and PuOF4, which can be achieved via tuning the ligand, thus improving our knowledge of oxidation states and chemical bonding in high OS solid-state compounds. We highlight the importance of ligand design in terms of the actinide HOS, employing a highly electronegative ligand and showing the capacity to form multiple bonds. [ABSTRACT FROM AUTHOR]

A systematic study of the phase stability, chemical bonding, and lattice dynamics properties of PuGaO3 is performed by means of unbiased particle-swarm optimization techniques combined with first-principles calculations. Phase stability analyses, including total energy, formation enthalpy, mechanical stability, and lattice dynamics, indicate that the ground-state structure of PuGaO3 at ambient pressure belongs to the space group Pnma. The fat-band dispersions and chemical bonding analysis suggest that Pnma PuGaO3 is an indirect band gap semiconductor (2.46 eV at the PBEsol + U level) at the Γ point and the Pu–O and Ga–O bonds in Pnma PuGaO3 are dominated by the ionic interactions. By examining the phonon spectra and phonon DOS, we provide a detailed insight into the lattice dynamics and thermodynamic properties of PuGaO3. In addition, the infrared absorption and Raman scattering spectra of PuGaO3 are predicted and further assigned at the center (Γ point) of the first Brillouin zone. [ABSTRACT FROM AUTHOR]

As B12 clusters exhibit significant structural stability due to double aromaticity, metal doped-B12 clusters often prefer a half sandwich structure. Herein, we report a systematic theoretical study on the geometric and electronic structures, and chemical bonding of the half sandwich AnB12 (An = Th to Cm) clusters to explore the stability and extent of covalency of the An–B bonds of these actinide borides. We have shown that in the gas-phase clusters, the significant stability of AnB12 is determined by electrostatic and orbital interactions between the An 5f6d7s orbitals and π-type molecular orbitals from B 2p orbitals of the B12 unit. A change-over of An–B bond length from An = Th to Cm is found at An = Pa as a result of actinide contraction combined with weakening An–B bonding due to an energy decrease and orbital localization of the 5f orbitals. Consistently, the oxidation states of the An atoms at first increase from Th(f0)IV to Pa(f0)V, and then due to the 5f-AO contraction, they smoothly decline to U(f2)IV, Np(f4)III and Pu(f5)III, and then eventually to Am(f7)II but Cm(f7)III, both with a half-filled 5f shell. [ABSTRACT FROM AUTHOR]

The formation and migration energies for various point defects, including vacancies and self-interstitials, in aluminium are systematically reinvestigated using the supercell approximation in the framework of orbital-free density functional theory. In particular, the finite-size effects and the accuracy of various kinetic energy density functionals are examined. It is demonstrated that as the supercell sizeincreases, the finite-size errors asymptotically decrease as. Notably, the formation energies of self-interstitials converge much more slowly than that of vacancies. With carefully chosen kinetic energy density functionals, the calculated results agree quite well with the available experimental data and those obtained through Kohn–Sham density functional theory, which has an exact kinetic term. [ABSTRACT FROM AUTHOR]

Herein, an experimental study of the vibrational spectra of HCeN was carried out in solid argon, followed by theoretical investigations of molecular structures and the nature of Ce≡N bond. The absorption band at 937.7 cm−1 with the 1.0311 14N/15N isotopic shift ratio is characteristic of Ce≡N stretching band for HCeN, showing a 94 cm−1 higher shift relative to that of the diatomic CeN molecule. This large frequency shift indicates a much stronger Ce≡N bond in HCeN, which is confirmed by DFT calculations. Qualitative orbital interaction and orbital composition analyses suggest that the addition of the H ligand to the Ce center will activate the 4f valence shell and strengthen the covalent bond between Ce and N, which may contribute to enhance the Ce≡N triple bond in the HCeN molecule. [ABSTRACT FROM AUTHOR]

Highlights: [•] Enhancement of superconductivity in 2D MgB4 nanosheets is reproduced. [•] Localization of surface electronic-state can be viewed as the origin of the enhancement. [•] Surface electronic-states depend on the surface terminations. [ABSTRACT FROM AUTHOR]

Abstract: Magnesium nickel hydrides (Mg2NiH4) are the prospective candidates for hydrogen storage and switchable mirror. The hydrides exist in two typical crystallographic forms, the low temperature (LT) phase in monoclinic structure, and the high temperature (HT) phase in cubic structure. LT has two modifications–untwinned (LT1) and microtwinned (LT2) structures. The electronic structures of the three polymorphs of Mg2NiH4 are investigated using ab initio calculations based on density functional theory. The calculated band gaps of LT1 and HT are in reasonable agreement with experimental observations and other theoretical predications, while the calculated band gap of LT2 is slightly lower than those of LT1 and HT. Electronic-structure analysis shows that strong interactions exist between Ni and H, whereas the interactions between Mg and H are negligible. The strong ionic character between Mg and NiH4 complex can be viewed as the origin of the semiconducting ground-state. [Copyright &y& Elsevier]

Oxidation state (OS) is one of the most important concepts in chemistry and, the challenging task is how to rigorously determine the OS of multivalent solid ions of which uranium is the outstanding example. Herein, we revisit the OS of solid uranium ions in a rich variety of compounds by calculating density functional theory (DFT)-based OS (OS DFT). The results disclose the variations of uranium OS DFT with chemical surrounding and local structure; OS DFT is generally consistent with formal OS (OS f) in the ionic approximation for the majority of compounds. Interestingly, uranium OS DFT in some compounds containing uranyl or uranyl-like units, e.g., β-/γ-UO 3 , and UO 2 X 2 (X = F, Cl, Br, OH), adopt U4+, instead of the textbook and never contradicted U6+ for gaseous and liquid uranyl ions. The associative effects from ligand-ligand interaction, steric repulsion, crystal-field splitting and delocalization-localization dual nature of 5f electrons are proposed to be responsible for the unusual OS behavior in solid-state uranium compounds. [ABSTRACT FROM AUTHOR]

Correction for ‘Insights into the enhanced Ce≡N triple bond in the HCe≡N molecule’ by Zhen Pu et al., Phys. Chem. Chem. Phys., 2017, 19, 8216–8222. [ABSTRACT FROM AUTHOR]

Uranium nitrides are one kind of accident-tolerant fuels and have been paid more attention recently. With the first-principles Density-Functional Theory (DFT) calculations, the adsorptions properties of some typical atoms, molecules and radical (including O, H, H 2 , O 2 , H 2 O and OH) adsorbed on the UN 2 (001) surface have been studied in the present work. The preferred sites and stable configurations for those adsorbates on the UN 2 (001) surface have been obtained. It's found that O or H atom prefers to be adsorbed at the bridge site; O 2 adsorption will dissociate into two O atoms and occupy the nearest neighbor bridge sites; the interaction between H 2 molecule and the UN 2 (001) surface is very weak; OH prefers to occupy the bridge site with its O-H bond vertical to the surface; the surface adsorption of H 2 O is non-dissociated and adsorption energies are dependent on the initial structures and adsorption modes. [ABSTRACT FROM AUTHOR]