arXiv subjects

Samir F. Matar

Publications and source records attributed to Samir F. Matar.

At least 19 recordsLinked to original sources

arXiv2024

From layered 2D carbon to 3D tetrahedral original allotropes C12 and C18 with physical properties related to diamond: Crystal chemistry and DFT investigations

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.

cond-mat.mtrl-sci↗

arXiv2024

Original orthorhombic tetrahedral and trigonal hybrid allotropes Cn with n= 8, 10, 12, 14 possessing ethene -like and propadiene -like units: Crystal chemistry and first principles

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.

cond-mat.mtrl-sci↗

arXiv2023

Evolution from quartz (qtz) to diamond (dia) carbon allotropes: Crystal engineering and DFT investigations

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.

cond-mat.mtrl-sci↗

arXiv2023

Novel Carbon allotropes with mixed hybridizations: ene-C10, and ene-yne-C14. Crystal chemistry and first principles investigations

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.

cond-mat.mtrl-sci↗

arXiv2023

Ultra-hard hexagonal C$_{12}$ with C3 cyclopropane-like moiety from first principles

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.

cond-mat.mtrl-sci↗

arXiv2022

Novel Ultra-hard tetragonal Octacarbon C8 close to Diamond from First Principles

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.

cond-mat.mtrl-sci↗

arXiv2022

First principles investigations in the carbon silicon system of novel tetragonal C8 (diamond) and Si8 allotropes and binary Si4C4 phase

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.

cond-mat.mtrl-sci↗

arXiv2021

Tricarbon: two novel ultra-hard metallic carbon allotropes from first-principle calculations

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° (smaller than ideal 109.4°) 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.

cond-mat.mtrl-sci↗

arXiv2021

Crystal chemistry and ab initio investigations of ultra-hard dense rhombohedral carbon and boron nitride

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.

cond-mat.mtrl-sci↗

arXiv2021

B12C2N Interstitial Boron subcarbonitride with peculiar magnetic properties: First principles investigations

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.

cond-mat.mtrl-sci↗

arXiv2021

Ultra-hard rhombohedral carbon from crystal chemistry rationale and first principles

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.

cond-mat.mtrl-sci↗

arXiv2021

Crystal chemistry and ab initio prediction of ultra-hard rhombohedral B2N2 and BC2N

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.

cond-mat.mtrl-sci↗

arXiv2020

First-principles studies of the electronic and magnetic structures and bonding properties of boron subnitride B$_{13}$N$_2$

Rhombohedral B$_{12}$ unit is viewed as a host matrix embedding linear tri-atomic arrangements of elements (E) resulting in a relatively large family of boron-rich compounds with B$_{12}${E-E-E} generic formulation. The present work focuses on boron subnitride, B$_{13}$N$_2$ that we express in present context as B$_{12}${N-B-N}. Within well established quantum density functional theory (DFT) a full study of its electronic properties is provided. Also linear triatomic arrangements in view of the existence in simple compounds such as sodium azide NaN$_3$, i.e., Na$^I${N-N-N} and calcium cyanamide, Ca$^{II}${N-C-N}, we devised Sc$^{III}${N-B-N} to establish comparison with B$_{12}${N-B-N}. ScBN$_2$ is calculated to be cohesive and possessing N-B-N isolated from ScIII with dB-N = 1.33 Å. In B$_{12}${N-B-N} an elongated dB-N=1.43 Å is identified due to the bonding of N with one of the two B12 boron substructures, B1 with the formation of "3B...N-B-N...3B"-like complex accompanied by a magnetic instability. Spin polarized (SP) calculations led to the onset of magnetization on central boron with M=1 $μ_B$ in a stable half-ferromagnetic ground state observed from the electronic density of states (DOS). The results are backed with total energy and calculations in both non-spin-polarized (NSP) and spin-polarized stabilizing the latter configuration over a broad range of volumes from M(V) plots. Further illustrative results are given with the charge densities (total and magnetic) and electron localization function (ELF).

cond-mat.mtrl-sci↗

arXiv2020

Electronic and magnetic properties of diiron in extended carbon networks Fe2C6 and Fe2C12 from first principles

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.

cond-mat.mtrl-sci↗

arXiv2019

Electronic and magnetic properties of new binary uranium-boron compounds with 2D and 3D boron networks: A revisit of the U:B system

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.

cond-mat.mtrl-sci↗

arXiv2018

Electronic structure and magnetic ordering of NiN and Ni$_2$N from first principles

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 $ σ$-type and $ π$-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.

cond-mat.str-el↗

arXiv2018

Lattice phonon modes of the spin crossover crystal [Fe(phen)2(NCS)2] studied by THz, IR, Raman spectroscopies and DFT calculations

[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.

cond-mat.mtrl-sci↗

arXiv2016

Drastic changes of electronic structure and crystal chemistry upon oxidation of SnII2TiO4E2 into SnIV2TiO6: an ab initio study

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.

cond-mat.mtrl-sci↗