Search

Two mechanisms of changes from 2D to 3D (D = dimensionality) involving 2D C(sp2) trigonal paving to C(sp3) tetrahedral stacking are proposed through puckering of the 2D layers on one hand and interlayer insertion of extra C on the other hand. Such transformations, led to original 3D hexagonal C12 and C18 allotropes respectively characterized by lon and bac topologies. Using density functional theory DFT calculations, the two allotropes were found cohesive and stable both mechanically (elastic properties) and dynamically (phonons). Comparisons of the physical properties with known uni C6 were established letting identify ranges of large Vickers hardness: HV (uni C6) = 89 GPa, HV (lon C12) = 97 GPa, and HV (bac C18) = 70 GPa. Whilst C6 was identified with acoustic phonons instability, C12 and C18 were found stable dynamically throughout the acoustic and optic frequency ranges. Furthering on the thermal properties the allotropes were characterized with a temperature dependence curve of the specific heat CV close to experimental data of diamond with best fit for novel C18. The electronic band structures reveal a small band gap of 1 eV for uni C6 and larger direct band gap of 3 eV for the two other 3D allotropes. Such modulations of the electronic and physical properties should open scopes of carbon research., Comment: 20 page, 2 tables, 5 figures; original research and findings by the author, submitted to Elsevier

Novel cubic allotrope yne-C$_{16}$ with sp$^3$/sp$^1$ carbon hybridizations is devised based on crystal chemistry and computations of the ground structure and the energy-dependent quantities within the quantum density functional theory DFT. With respect to srs-C8 characterized by trigonal C(sp$^2$), yne-C$_{16}$ is identified with original C -- C triple bond-like units and distorted tetrahedra. The new allotrope is qualified with unknown topology. At the elementary building unit, C$_{16}$ looks like a pyramidal molecule such as :PCl3, with ":" symbolizing the phosphorous lone pair. With this conceptualization C$_{16}$ has three bond pairs (SIGMA-like) making the pyramid base and the C(triple-bond)C with large PI character resembling phosphorous electron lone pair (:) at the apex of the pyramid. The new allotrope was found cohesive and stable both mechanically (positive elastic constants) as well as dynamically (phonons band structure). C$_{16}$ is characterized with a weakly metallic character., Comment: 14 pages; 1 Table, 4 Figures. arXiv admin note: text overlap with arXiv:2404.01813

Super-hard C5 with lon topology (lon: Lonsdaleite hexagonal diamond) and characterized by the presence of sp3 and sp2 -like carbon sites is devised from crystal chemistry and used as template matrix structure for identifying original carbonitrides C4N and C2N2 with lon topology except for the equiatomic belonging to a new topology (3,4L147). The steric effect of N(2s2) lone pair is highlighted in C2N2 in inducing an original structure of largely separated two-layered stacking of tetrahedra. The investigations based on crystal chemistry were backed by computations within the quantum density functional theory DFT. All systems were found cohesive and both mechanically (elastic constants) and dynamically (phonons band structures) stable. Super hardness characterizes the carbon allotrope C5 and the nitrides C4N and C2N2. Metallic-like conductivities and insulating characters were identified., Comment: 21 pages, 7 figures, 3 tables

Original carbon allotropes, orthorhombic C8, C10, C12 and C14 presenting mixed sp2 and sp3 carbon hybridizations exhibiting ethene like and propadiene like embedded units are proposed from crystal chemistry and calculations within the quantum density functional theory DFT. The carbon allotropes with topologies related with jeb, mog, as well as new topologies, show alternating tetrahedral and trigonal carbon stacking along the a orthorhombic direction (vertical) close to but different from isoglitter. The carbon allotropes were shown to be cohesive and stable both mechanically (elastic properties) and dynamically (phonons), with calculated Vickers hardness (HV) magnitudes ranging between 25 GPa and 52 GPa, the latter magnitude assigned to C12 possessing the largest number of tetrahedral versus trigonal stacking. High phonon frequencies close to 50 THz were attributed to stretching mode of ethene (in C8 and C12) and propadiene (in C10 and C14) components with good agreement with experiment for Raman molecular C double bond C intense stretching mode. Frequencies close to 40 THz were proposed as signatures of C...C simple bonds in the tetrahedra. The electronic band structure and electronic density of states DOS shown exemplarily for C8 point to metallic-like behavior assigned to the itinerant role of trigonal carbon pi-electrons.

Based on crystal engineering and density functional theory DFT calculations a transformational pathway is proposed from qtz (quartz-based) topology characterized by distorted tetrahedra to dia (diamond-like) regular tetrahedra topology. The protocol consists of carbon insertions into orthorhombic (space group P222, No. 16) within C5, C6, and C7, leading to ultimate C8 identified as diamond-like. The induced structural and physical changes are assessed with elastic properties pointing to ultra hardness, larger for qtz-C6 than dia-C8, whilst intermediate C7 is compressible due to its diamond-defective structure. The dynamic stability was shown from the phonons, and thermodynamic quantities as the specific heat CV was addressed in comparison with diamond experimental data. The electronic band structures reveal semi-conducting C6, metallic C7 characterized by diamond-defect structure, and insulating C8., Comment: 15 pages, 4 figures, 2 tables, 27 references

Based on C8, carbon 4C, with cfc topology, two hybrid carbon allotropes generated by inserting C(sp2) and C(sp1) carbon atoms into C8 diamond-like lattice were identified and labeled ene-C10 containing C(sp2) and ene-yne-C14 containing C(sp2 and sp1). The introduced double and triple chemical descriptions were illustrated from the projected charge densities. The crystal density and the cohesive energy were found to decrease due to the enhanced openness of the structures from inserted sp2/sp1 carbons, with a resulting decrease of the hardness along the series C8, C10, C12, and C14. The novel hybrid allotropes were found stable mechanically (elastic constants and their combinations) and dynamically (phonons band structures). The thermal properties from the temperature dependence of the heat capacity CV were found to increasingly depart from diamond-like C8 to higher values. From the electronic band structures, the inserted carbons were found to add up bands rigidly to diamond-like C8 while being characterized by metallic-like behavior for ene-C10 and ene-yne-C14., Comment: 18 pages, 32 references, 2 tables, 6 figures

A novel carbon allotrope, hexagonal C12, is proposed from crystal chemistry and quantum density functional theory DFT calculations of ground state and physical properties. The structure exhibits corner sharing distorted tetrahedra with the presence of C3 triangular cyclopropane-like moiety connecting planar carbon. C12 allotrope is found cohesive and stable both mechanically (elastic constants and their combinations) and dynamically -- phonons band structures -- and presents ultra-hardness with Vickers number of 70 GPa. The temperature dependence of the heat capacity CV shows close magnitudes to experimental results of diamond. The electronic band structure reveals closely insulating behavior with 2.5 eV band gap, half smaller than in diamond., Comment: 12 pages, 4 figures, 2 tables, 27 refeences

Original carbon allotropes C12 and C16 called superglitter and squarodiamond from relationships with literature glitter and squaroglitter respectively are shown through DFT-based geometry to be cohesive with energy dependent properties as hardness from the elastic constants, the phonon band structures, and thermal behavior related to diamond. Like C6 glitter, C12 superglitter exhibiting mixed sp2-sp3 carbon hybridization shows metallic behavior and moderate hardness and metallic behavior arising from trigonal C(sp2) forming C=C pairs connecting tetrahedra. Oppositely, C16 showing square C4 motifs as in squaroglitter characterized by both sp2-sp3 carbons, is characterized by edge and corner sharing tetrahedra with only sp3 carbons resulting in insulating behavior and shear modulus and Vickers hardness H(V) larger than 100 GPa as well as heat capacity alike diamond, whence its labeling as squarodiamond. The novel carbon allotropes are proposed as an opportunity to enrich the carbon database and the materials science with potentials of applications as abrasives and in electronic devices., Comment: A colleague mentioned that the structures I claim as original C12 and C16 are already known and studied before me. So I want to withdraw the paper. Apologies

From crystal chemistry rationale and density functional DFT calculations, novel tetragonal carbon C5 is proposed as simplest ultrahard allotrope with mixed hybrid carbon hybridizations (sp2 - sp3). Novel pentacarbon is identified as cohesive and stable both dynamically and mechanically. Whereas charge density is localized about tetrahedral C-sp3, it is found delocalized around trigonal C-sp2 resulting in metallic behavior. The anisotropic structure characteristics caused by the aligned trigonal carbon along the tetragonal c-axis provide high Vickers hardness with a magnitude close to diamond, Comment: 15 pages, 26 refs, 2 Tables, 5 figs. arXiv admin note: text overlap with arXiv:2206.05918

Novel extended networks of C8, Si8 and silicon carbide Si4C4 are proposed based on crystal chemistry rationale and optimized structures to ground state energies and derived physical properties within the density functional theory (DFT). The two carbon and silicon allotropes and the silicon carbide belong to primitive tetragonal space group P-4m2 Number 115. C8 allotrope structure made of corner sharing C4 and Si4 tetrahedra is illustrated by charge density projections exhibiting sp3 like carbon hybridization. From careful symmetry analysis, Symmetry analysis of C8 indicated that it is another representation of cubic diamond, space group F-d3m Number 227. C8 is identified as ultra-hard with a similar magnitude of Vickers hardness. The interest in C8 is to serve as template to study Si8 and Si-C binary. Si8 allotrope is found soft with HV =13 GPa alike cubic Si, and Si4C4 is identified with HV =33 GPa close to experimental SiC. All three new phases are mechanically (elastic constants) and dynamically (phonons) stable, and their electronic band structures are characteristic of insulating C8 (diamond) with large band gaps of about 5 eV, and semi-conducting Si8 and Si4C4 with band gaps of about 1 eV., Comment: 18 page, 2 Tables, 4 Figs (with subfigures). arXiv admin note: text overlap with arXiv:2206.05918

Novel ultra-hard carbon allotrope is proposed with mechanical, dynamic, and thermal properties like diamond. Based on energy criteria from computations within density functional theory DFT, tetragonal C8 stoichiometry is identified as a cohesive network of corner sharing C4 tetrahedra illustrated by charge density projections exhibiting sp3-like carbon hybridization. The new allotrope is mechanically (elastic constants) and dynamically (phonons) stable, exhibiting thermal properties (heat capacity CV) in close agreement with experimental data of diamond from the literature. From the used models to evaluate Vickers hardness, a larger magnitude with respect diamond is hypothesized for the new allotrope. Electronic band structure calculations show insulating behavior with large band gap of 5 eV like diamond., Comment: A careful analysis with symmetry finder "PLATON" software -that was not available at the time of publication- and exchanges with colleagues who drew my attention to the issue, reveal that the C8 allotrope is nothing else than diamond expressed in another space group but whose primitive cell is that of diamond. So there's no point in presenting well known results of tetragonal octacarbon C8

Based on crystal chemistry considerations and quantum density functional theory ground state calculations, rhombohedral rh-C3 and hexagonal h-C6 carbon allotropes are proposed and energetically calculated as new stable ultra-hard phases likewise lonsdaleite. Along the two kinds of carbon in linear C2-C1-C2 lattice, distorted tetrahedra C2(sp3) with an angle of 106.17{\deg} (smaller than ideal 109.4{\deg}) and C1(sp)-like hybridizations are inferred from charge density projections. The calculated elastic constants point to a strong anisotropy of mechanical properties of rh-C3 and h-C6 with an exceptionally large C33 values (1636 GPa and 1610 GPa, respectively), exceeding that of lonsdaleite (1380 GPa), due to the presence of aligned tricarbon units along the hexagonal c-axis. Both phases are characterized by large bulk moduli and high hardness values that are slightly less than those of lonsdaleite and diamond. Weak metallic behavior of both new phases is identified from electronic band structure calculations., Comment: arXiv admin note: text overlap with arXiv:2104.11024

Rhombohedral dense forms of carbon, rh-C2 (or hexagonal h-C6), and boron nitride, rh-BN (or hexagonal h-B3N3), are derived from rhombohedral 3R graphite based on original crystal chemistry scheme backed with full cell geometry optimization to minimal energy ground state computations within the quantum density functional theory. Considering throughout hexagonal settings featuring extended lattices, the calculation of the hexagonal set of elastic constants, provide results of large bulk moduli i.e. B0(rh-C2) = 438 GPa close to that of diamond, and B0(rh-BN) = 369 GPa close to that of cubic BN. The hardness assessment in the framework of three contemporary models enables both phases to be considered as ultra-hard. From the electronic band structures calculated in the hexagonal Brillouin zones, 3R graphite is a small-gap semiconductor, oppositely to rh-C2 that is characterized by a large band gap close to 5 eV, as well as the two BN phases., Comment: arXiv admin note: text overlap with arXiv:2104.01076

The subcarbides B12C3 and B13C2 known for their abrasive properties, can be structurally considered as carbon inserted rhombohedral alpha-B12 and expressed as B12{C-C-C} and B12(C-B-C). Using density functional theory DFT computations, the substitution of the central atom in the linear triatomic interstitials by N leads to an original new boron subcarbonitride B12C2N or B12(C-N-C) identified as slightly more cohesive than the two subcarbides. While B12C3 is insulating with a E(Gap) close to 0.2 eV, and B13C2 a weak metal with small density of state DOS at the Fermi level, B12C2N ground state is a half-metallic ferromagnet with M= 1 BM. From the equation of state, the magnetization is found unchanged over a broad volume range around equilibrium. Total and magnetic charge density projections, in accordance with charge transfer calculations, show the charge envelopes to be concentrated on (C:N:C) and the magnetic charge having the shape of a torus centered on N. The triatomic interstitials interact through terminal carbons specifically with one of the two boron substructures of alpha-B12 forming 3B1-C-N-C-3B1-like complex. Further syntheses and experimental characterizations are expected to extend the field of investigation of such an original class of materials., Comment: 17 pages; 22 references; 2 Tables; 5 Figures

A new ultra-hard rhombohedral carbon rh-C4 (or hexagonal h-C12) is reported as derived from 3R graphite through crystal chemistry construction and ground state energy within the density functional theory. An extended hexagonal three-dimensional network of h-C12 is formed of C4 tetrahedra alike in h-C4 lonsdaleite (hexagonal diamond). The electronic band structure of rh-C4 is characteristic of insulator with Egap = 4 eV similarly to diamond. From the set of elastic constants a larger value of bulk modulus versus lonsdaleite, and the largest Vickers hardness (HV) versus both forms of diamond were derived., Comment: arXiv admin note: text overlap with arXiv:2103.06802

New ultra-hard rhombohedral B2N2 and BC2N - or hexagonal B6N6 and B3C6N3 - are derived from 3R graphite based on crystal chemistry rationale schematizing a mechanism for 2D => 3D transformation. Full unconstrained geometry optimizations leading to ground state energy structures and energy derived quantities as energy-volume equation of states (EOS) were based on computations within the density functional theory (DFT) with generalized gradient approximation (GGA) for exchange-correlation (XC) effects. The new binary and ternary phases are characterized by tetrahedral stacking alike diamond, visualized with charge density representations, and illustrating ion characters. Atom averaged total energies are similar between cubic BN and rh-B2N2 on one hand, and larger stabilization of rhombohedral BC2N versus cubic and orthorhombic forms (in literature assessed from favored C-C and B-N bonding), on the other hand. The electronic band structures are characteristic of insulators with Egap ~ 5 eV. Both phases are characterized by large bulk and shear moduli and very high hardness values i.e. HV(rh-B2N2) = 74 GPa and HV(rh-BC2N) = 87 GPa.

Considering rhombohedral alpha-boron, rh-B12 as a matrix hosting interstitials particularly triatomic linear ones as in B12C3 better known as B4C, the subnitride B12N3 is proposed herein. The N3 triatomic linear alignment labeled N2-N1-N2 resembles that found in the ionic sodium azide NaN3 characterized by a very short d(N1-N2)= 1.16 A. Within DFT-based calculations B12N3 is found more cohesive than pristine B12 with larger inter-nitrogen separation of d(N1-N2)= 1.38 A}. The N1-N2 elongation is explained from the bonding of the two terminal N2 with one of the two B12 boron substructures forming 3B-N2-N1-N2-3B-like complex. A resulting non-bonding charge density localized on central N1 leads to the onset of a magnetic moment of 1 Bohr Magneton in a half-metal ferromagnetic ground state illustrated by projections of the magnetic charge density and the site and spin electronic density of states DOS., Comment: 16 pages, 4 figures, 3 tables

From density functional DFT investigations helped with crystal chemistry rationale, diiron (pairs of Fe), mostly known in molecular diiron nona-carbonyl Fe2(CO)9 and diiron-mono-carbide Fe2C carbide, are embedded in hexagonal C6 substructures. Generated Fe2C6 and Fe2C12 are shown to be more cohesive than the mono-carbide on one hand, and increasingly cohesive from hexa- C6 to dodeca- C12 on the other hand. From energy differences, the ground state is spin-polarized SP, versus a non-spin-polarized NSP configuration, and identified as ferromagnetic versus a higher energy anti-ferromagnetic hypothesis. The projection of the magnetic charge density on Fe and C, shows that only Fe carries the magnetic moment, while carbon receives charges from Fe as illustrated by the electron localization function ELF 3D and 2D mapping. SP configuration induces an enlarged c/a hexagonal ratio, versus NSP, while a(hex.) remains constant thanks to the rigid C6 carbon substructure network, resulting in an anisotropic magneto-volume response. This feature, essentially due to in-plane diiron, is discussed from the energy-volume (E, V) NSP and SP equations of state EOS and derived quantities like volume- and d(Fe-Fe)- changes of the magnetization., Comment: 18 pages, 1 Table, 5 Figures

From density functional theory investigations helped with crystal chemistry rationale, single-atom C, embedded in layered hexagonal CCn with n = 6, 12, 18 networks, is stable in a magnetic state with M(C)= 2 Bohr Magneton (BM). The examined compositions, all inscribed within the P6/mmm 191 space group are characterized as increasingly cohesive with n, figuring mono, bi and tri layered honeycomb-like C networks respectively. The spin projected total density of states shows a closely half-metallic behavior with a gap at minority spins and metallic majority spins. Such results together with the large C-C intersite separation and the integer 2 BM magnetization, let propose an intra-band mechanism of magnetic moment onset on carbon 2p states. Support is provided from complementary calculations assuming C2C12 structure with planar 2 C with d(C-C)= 2.46 Angstroms resulting into a lowering of the magnetization down to 0.985 BM per C atom involving a ferromagnetic order arising from interband spin polarization on C where one nonbonding spin polarizes whereas the other is involved with the bonding with the other carbon. Further illustrative proofs are provided with the magnetic charge density projected onto the different atoms, showing its prevalence around C, contrary to the C6 layers, as well as electron localization function ELF., Comment: 18 pages; 4 figures, 1 Tables (4 subtables)

Based on crystal chemistry rationale and calculations within the density functional theory DFT, the U:B system is complemented with additional binary compounds UB3, U2B6, and UB6 possessing two-dimensional 2D and 3D boron substructures. Observations are supported quantitatively with the trends of cohesive energies, charge transfers onto the boron sub-lattice and geometry optimized structures. The results point out to a structure crossover from hexagonal (layer B network) to 3D boron network at compositions above UB3 found to be connected with a threshold amount of charge onto boron which is ~0.46. From the energy-volume of states EOS considering spin degenerate and spin-polarized configurations, hexagonal UB3, and cubic UB6 were found in a stable ferromagnetic ground state with 1.47 and 2.40Bohr Magnetons spin-only moments. The volume variations of magnetization show respectively a smooth and abrupt evolution for UB3 and UB6., Comment: 18 pages, 6 figures, 1 Table, 21 references

Cohesive energies, energy-volume equations of states EOS, electron localization ELF maps, elastic constants, and band structures are reported for original octacarbon C8 three dimensional 3D and two-dimensional 2D chemical systems based on density functional theory calculations. Specifically, tetragonal C8 is identified cohesive with hardness close to experimentally identified cubic Ia-3 C8; both exhibiting comparable hardness to diamond. Also, isoelectronic and isostructural B4N4 is calculated with a slightly lower hardness due to the ionocovalent B-N bonding and a bandgap with the same magnitude as Diamond. 2D-C8 on the other side is proposed with interpenetrating two carbon hexagonal substructures, identified from energy calculations as stable in a ferromagnetic ground state. Critical pressure for the collapse of magnetization PC=12 GPa let assign a soft ferromagnetic behavior alike Invar alloys. Electronic band structures analyses identify specific bands differentiating magnetic carbon substructure (C1) from nonmagnetic semi-conducting honeycomb-like C26 layers. These observations let propose spin chemistry perspectives once such multilayered carbon 2D compounds are grown as stand-alone or on selected substrates as thin or thick films, Comment: 23 pages, 2 Tables, 4 Figs, 27 refs

[Fe(phen)2(NCS)2] is a prototype transition metal complex material, which undergoes a phase transition between low-spin (LS) and high-spin (HS) phases, induced by temperature, pressure or light. Vibrational modes play a key role for spin-state switching both in thermal and photo-induced cases, by contributing to vibrational entropy for thermal equilibrium transitions or driving the fast structural trapping of the photoinduced high spin state. Here we study the crystal phonon modes of [Fe(phen)2(NCS)2], by combining THz, IR, and Raman spectroscopies sensitive to modes in different frequency ranges and different symmetries. We compare the experimental results to DFT calculations performed in a periodic 3D crystal for understanding the phonon modes in the crystal, compared to molecular vibrations. Indeed, each vibrational mode of the isolated molecule combines into several modes of different symmetry and frequency in the crystal, as the unit cell contains four molecules. We focus our attention on the HS symmetric and anti-symmetric breathing modes in the crystal as well as on the N-CS stretching modes.

The results of first-principles electronic structure calculations for the nitrogen-rich nickel nitrides $ {\rm NiN} $ and $ {\rm Ni_2N} $ are presented. The calculations are based on density functional theory and used the generalized gradient approximation (GGA) as well as the GGA$ +U $ approach. The latter turned out to be crucial for a correct description of the crystal phase stability and magnetic instabilities of both compounds. While for $ {\rm NiN} $ GGA calculations predict a non-magnetic ground state with the zincblende structure, GGA$ +U $ calculations result in a half-metallic ferromagnet with the rocksalt structure in line with indications from the neighboring transition-metal nitrides making $ {\rm NiN} $ a possible candidate for spin-filter devices. For $ {\rm Ni_2N} $ GGA calculations likewise lead to a non-magnetic behavior, which is contrasted with a ferrimagnetic ordering obtained from the GGA$ +U $ approach. This ground state results from complex three-dimensional exchange interaction via $ \sigma $-type and $ \pi $-type overlap of the Ni $ 3d $ orbitals with the N $ 2p $ orbitals and may explain the reported sensitivity of the magnetic ordering to details of the crystal structure. For both nitrides, experimental data are called for to confirm our predictions., Comment: 8 pages, 8 figures, enthalpies of formation corrected inversion 2

In the context of characterizing nitrogen poor carbo-nitrides for different applications, identification of an unusual onset of spin polarization of N(p) states has been shown. A full saturation up to 3 Bohr magnetons is demonstrated in extended two-dimensional carbon networks of C6N and C12N hexagonal structures refined based on density functional theory calculations. From establishing the energy-volume equations of states in both compounds assuming spin degenerate (non spin polarized) and spin-polarized configurations, the ground state is identified as ferromagnetic. The variation of magnetization with volume points to strongly ferromagnetic behavior, Comment: 15 pages; 1 table 6 figures

Based on geometry optimization and magnetic structure investigations within density functional theory, unique uranium nitride fluoride UNF, isoelectronic with UO2, is shown to present peculiar differentiated physical properties. Such specificities versus the oxide are related with the mixed anionic sublattices and the layered-like tetragonal structure characterized by covalent like [U2N2]2+motifs interlayered by ionic like [F2]2- ones and illustrated herein with electron localization function graphs. Particularly the ionocovalent chemical picture shows, based on overlap population analyses, stronger U-N bonding versus N-F and d(U-N) < d(U-F) distances. Based on LDA+U calculations the ground state magnetic structure is insulating antiferromagnet with 2 Bohr Magnetons magnetization per magnetic subcell and ~2 eV band gap., Comment: 8 pages, 6 Figs 1 Table

From DFT based calculations establishing energy-volume equations of state and electron localization mapping, the electronic structure and crystal chemistry changes from Sn2TiO4 to Sn2TiO6 by oxidation are rationalized; the key effect being the destabilization of divalent tin SnII towards tetravalent state SnIV leading to rutile Sn2TiO6 as experimentally observed. The subsequent electronic structure change is highlighted in the relative change of the electronic band gap which increases from ~1eV up to 2.2 eV and the 1.5 times increase of the bulk modulus assigned to the change from covalently SnII based compound to the more ionic SnIV one. Such trends are also confronted with the relevant properties of black SnIIO characterized by very small band gap., Comment: 21 pages; 5 Figs, 1 table

The electronic and magnetic structures of A2CrN3 (A = Ce, U) ternary compounds calculated based on band magnetism within DFT exhibit different behaviors of the nf elements (n = 4, 5 resp.). Charge analysis allows to formally express the two compounds as A2Cr 5+ N3 5- thus classifying them as covalent nitrides, i.e. far from formal exchange of +- 9 electrons. From establishing the energy-volume equations of state, the two compounds are found with hardness magnitudes: B0(A=Ce) =192 GPa and B0(A=U) = 243 GPa, within range of oxides due to covalent metal-nitrogen bonding shown as based on overlap matrix analysis. The uranium compound is harder due to a smaller volume and less compressible U versus Ce metals. Ce2CrN3 exhibits large magnetization on Cr (1.94BM) and a very small moment develops on cerium (0.14 BM) pointing out to an intermediate valence state while in U2CrN3, M(Cr) = 0.49 BM and M(U) = 0.97BM. These results are stressed by broad band-like density of states (DOS) behavior for A=U and localized DOS for A=Ce. Both compounds are found ferromagnetic in the ground state, Comment: 17 pages; 3 tables, 5 figs

Magnetic properties and bonding analyses of perovskite structure Co$_{4}$N nitride have been investigated within density functional theory using both pseudo potential and all electron methods. In the same time, the structural and magnetic stability of pure cobalt in hexagonal close packed (HCP), face centered cubic (FCC) and body centered cubic (BCC) structures are reviewed. At equilibrium, non-spin polarized (NSP) and spin polarized (SP) calculations of the energy versus volume show that the ground state is ferromagnetic in both materials. HCP-Co is found to be more stable than the cubic ones. Magnetic moments of Co atoms in Co$_4$N nitride respectively belonging to two different crystallographic sites are studied over a wide range of the cubic lattice constant, and a comparison with the FCC-cobalt one is given. The volume expansion in the nitride indicates that the corner Co$^{I}$ atoms show localized magnetism while face center Co$^{II}$ atoms exhibit an itinerant behavior. Like in FCC-Fe/Fe$_{4}$N, a "low volume-low moment" and "large volume-high moment" behavior is observed for FCC-Co/Co$_4$N. The density of states of the Co$_{4}$N ferromagnetic ground state is interpreted within the rigid band model. The different bonding characters of Co$^I$-N versus Co$^{II}$-N are shown with help of electron localization fucntion ELF plots and spin resolved chemical bonding criteria., Comment: 8 pages and 10 figures

The stereochemistry of 6s2 (E) lone pair of divalent Pb and trivalent Bi (PbII and BiIII designated by M*) in structurally related PbO, PbFX (X= Cl, Br, I), BiOX (X= F, Cl, Br, I) and Bi2NbO5F is rationalized. The lone pair LP presence determined by its sphere of influence E, equal to those of oxygen or fluorine anions, was settled by its center then giving M*-E directions and distances. Detailed description of structural features of both elements in the title compounds characterized by [PbEO]n and [BiEO]n layers allowed to show the evolution of M*-E distance versus the changes with the square pyramidal SP coordination polyhedra. All are different, in red PbO one finds {PbEO4E4} square antiprism, a {[Bi.E]O4X4Xapical} monocapped square antiprism in PbFX and BiOX and {BiEO4F4}square antiprism in Bi2NbO5F. To analyze the crystal chemistry results, the electronic structures of these compounds were calculated within density functional theory DFT. Real space analyses of electron localization illustrate a full volume development of the lone pair on PbII within {PbEO4E4} in PbOE, {PbEF4X4} in PbFXE and Bi(III) within {BiEO4X4} square antiprisms, contrary to Bi(III) within {[Bi.E]O4F4Fapical} monocapped square antiprism. Larger hardness (larger bulk modules B0) and band gap characterize BiOF versus PbO due to the presence of F which brings antibonding Bi-F interactions oppositely to mainly bonding Bi-O. In PbFX and BiOX series there is a systematic decrease of B0 with the increasing volume following the nature and size of X which is decreasingly electronegative and increasingly large. The electronic densities of states mirror these effects through the relative energy position and relative electronegativities of F/X and O/X leading to decrease the band gap., Comment: 20 text pages/ 10 multifigures/large review article, J. Progress Solid State Chemistry, under production MAY 2015

The results of electronic structure calculations for a variety of palladium hydrides are presented. The calculations are based on density functional theory and used different local and semilocal approximations. The thermodynamic stability of all structures as well as the electronic and chemical bonding properties are addressed. For the monohydride, taking into account the zero-point energy is important to identify the octahedral Pd-H arrangement with its larger voids and, hence, softer hydrogen vibrational modes as favorable over the tetrahedral arrangement as found in the zincblende and wurtzite structures. Stabilization of the rocksalt structure is due to strong bonding of the 4d and 1s orbitals, which form a characteristic split-off band separated from the main d-band group. Increased filling of the formerly pure d states of the metal causes strong reduction of the density of states at the Fermi energy, which undermines possible long-range ferromagnetic order otherwise favored by strong magnetovolume effects. For the dihydride, octahedral Pd-H arrangement as realized e.g. in the pyrite structure turns out to be unstable against tetrahedral arrangement as found in the fluorite structure. Yet, from both heat of formation and chemical bonding considerations the dihydride turns out to be less favorable than the monohydride. Finally, the vacancy ordered defect phase Pd3H4 follows the general trend of favouring the octahedral arrangement of the rocksalt structure for Pd:H ratios less or equal to one., Comment: 11 pages, 10 figures