The charge, bonding, and optical properties of the calcium-doped boron cluster B$_7$Ca$_2$ have been systematically investigated using density functional theory calculations. Extensive global basin-hopping searches identify a single-ring B$_7$ geometry stabilized by two calcium atoms symmetrically located on opposite sides of the boron ring as the global minimum. Electronic structure analysis reveals pronounced charge redistribution and strong Ca--B interactions that promote electron delocalization over the boron framework. Hirshfeld charge analysis indicates substantial electron donation from the electropositive calcium atoms to the electron-deficient B$_7$ ring, leading to effective electronic stabilization without the involvement of transition-metal $d$ orbitals. Optical absorption spectra further reflect the delocalized nature of the frontier electronic states. Real-space bonding analyses based on the electron localization function (ELF), Interaction Region Indicator (IRI), and the Laplacian of the electron density reveal a multicenter bonding pattern dominated by electron delocalization within the boron ring, with calcium acting primarily as an electrostatic and charge-donating stabilizer rather than forming localized two-center Ca--B bonds. These results establish B$_7$Ca$_2$ as a prototypical example of an alkaline-earth-metal-stabilized boron ring and highlight the ability of non-transition metals to stabilize aromatic boron clusters through charge transfer and multicenter bonding.
We present a new class of boron sheets, composed of triangular and hexagonal motifs, that are more stable than structures considered to date and thus are likely to be the precursors of boron nanotubes. We describe a simple and clear picture of electronic bonding in boron sheets and highlight the importance of three-center bonding and its competition with two-center bonding, which can also explain the stability of recently discovered boron fullerenes. Our findings call for reconsideration of the literature on boron sheets, nanotubes, and clusters.
A scientific discussion was recently raised on the interpretation of chemical bonding in the high pressure structure of Boron. Some remarks are given in this short commentary
Bonding characteristics of liquid boron at 2500K are studied by using high resolution Compton scattering. An excellent agreement is found between the measurements and the corresponding Car-Parinello molecular dynamics simulations. Covalent bond pairs are clearly shown to dominate in liquid boron along with the coexistence of diffuse pairs. Our study reveals the complex bonding pattern of liquid boron, and gives insight into the unusual properties of this high temperature liquid.
We used the binding energy-bond-charge model to study the atomic bonding and electrical properties of the two-dimensional graphene/BN van der Waals heterostructure. We manipulated its atomic bonding and electrical properties by manufacturing defects. We discovered that this process yielded a band structure with a flat band, i.e., a horizontal band structure without dispersion at the Fermi level. Thus, our research is significant because it is the first report on this flat band of defect graphene/BN van der Waals heterostructures.
The complex crystal chemistry of elemental boron has led to numerous proposed structures with distinctive motifs as well as contradictory findings. Herein, evolutionary structure searches performed at 100 GPa have uncovered a series of potential new metastable phases of boron, and bonding analyses were carried out to elucidate their electronic structure. These polymorphs, dynamically stable at 100 GPa, were grouped into two families. The first was derived from the thermodynamic minimum at these conditions, $α$-Ga, whereas channels comprised the second. Two additional intergrowth structures wer…
Usually the ions in solid are in the positive oxidation states or in the negative oxidation state depending upon the chemical environment. It is highly unusual for an ion having both positive as well as negative oxidation state in a particular compound. Structural analysis suggest that the alkaline earth ammine borohydrides (AABH) with the chemical formula M (BH4)2(NH3)2 (M = Mg, Ca, or Sr) where hydrogen is present in +1 and -1 oxidation states. In order to understand the oxidation states of hydrogen and also the character of chemical bond present in AABH we have made charge density, electron…
A symmetry analysis of the valence shell in the boron buckyball B80, shows a perfect match with the symmetry of the valence shell in C60 .The cap atoms participate in the bonding by 30 orbitals which transform as the sigma bonds along the 6-6 edges. Far reaching consequences for chemical and physical properties of B80, which until now only exists in silico, are expected.
The accurate charge density of MgB2 was observed at room temperature(R.T.) and 15K by the MEM(Maximum Entropy Method)/Rietveld analysis using synchrotron radiation powder data. The obtained charge density clearly revealed the covalent bonding feature of boron forming the 2D honeycomb network in the basal plane, on the other hand, Mg is found to be in divalent state. A subtle but clear charge concentration was found on boron 2D sheets at 15K, which should be relating to superconductivity.
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 cyanamid…
New equiatomic nickel germanides MgNiGe, SrNiGe, and BaNiGe have been synthesized from the elements in sealed tantalum tubes using a high-frequency furnace. The compounds were investigated by X-ray diffraction both on powders and single crystals. MgNiGe crystallizes with TiNiSi structure type, space group Pnma, Z = 4, a = 6.4742(2) A, b = 4.0716(1) A, c = 6.9426(2) A, wR2 = 0.033, 305 F2 values, 20 variable parameters. SrNiGe and BaNiGe are isotypic and crystallize with anti-SnFCl structure type (Z = 4, Pnma) with a = 5.727(1) A, b = 4.174(1) A, c = 11.400(3) A, wR2 = 0.078, 354 F2 values, 20 …
Oxides with good p-type conductivity have been long sought after to achieve high performance all-oxide optoelectronic devices. Divalent Sn(II) based oxides are promising candidates because of their rather dispersive upper valence bands caused by the Sn-5s/O-2p anti-bonding hybridization. There are so far few known Sn(II) oxides being p-type conductive suitable for device applications. Here, we present via first-principles global optimization structure searches a material design study for a hitherto unexplored Sn(II)-based system, ternary alkaline-earth metal Sn(II) oxides in the stoichiometry …
We study the effect of electron doping on the bonding character and stability of two-dimensional (2D) structures of elemental boron, called borophene, which is known to form many stable allotropes. Our {\em ab initio} calculations for the neutral system reveal previously unknown stable 2D $ε$-B and $ω$-B structures. We find that the chemical bonding characteristic in this and other boron structures is strongly affected by extra charge. Beyond a critical degree of electron doping, the most stable allotrope changes from $ε$-B to a buckled honeycomb structure. Additional electron doping, mimickin…
When single-layer h-BN is subjected to a high-energy electron beam, triangular pores with nitrogen edges are formed. Because of the broken sp2 bonds, these pores are known to possess magnetic states. We report on the magnetism and electronic structure of triangular pores as a function of their size. Moreover, in the Bernal-stacked h-BN (AB-h-BN), multilayer pores with parallel edges can be created, which is not possible in the commonly fabricated multilayer AA'-h-BN. Given that these pores can be manufactured in a well-controlled fashion using an electron beam, it is important to understand th…
Structural stability and electronic properties of alkaline-earth metals (Ca, Sr, Ba) induced Si(111)-3x2 surfaces have been comprehensively studied by means of ab initio calculations. Adsorption energy and charge density difference calculations show the high structural stability due to the strong chemical bonding. Analysis of electronic band structures and band-decomposed charge density distributions indicates that the third valence band is deriving from top Si and metal atoms, while the top most two valence bands are deriving from the bulk silicon. These results suggest a larger surface band …
We investigated the possibility of superconductivity in monolayer hexagonal boron nitride (h-BN) doped using each group-1 (Li, Na, K) and group-2 (Be, Mg, Ca, Sr, Ba) atom via ab-initio calculations. Consequently, we reveal that Sr- and Ba-doped monolayer h-BN and Ca-doped monolayer h-BN with 3.5% tensile strain are energetically stable and become superconductors with Tc values of 5.83 K, 1.53 K, and 12.8 K, respectively, which are considerably higher than those of Ca-, Sr-, and Ba-doped graphene. In addition, the momentum-resolved electron-phonon coupling (EPC) constant shows that the scatter…
A set of general constructing schemes is unveiled to predict a large family of stable boron monoelemental, hollow fullerenes with magic numbers 32+8k (k>=0). The remarkable stabilities of these new boron fullerenes are then studied by intense ab initio calculations. An electron counting rule as well as an isolated hollow rule are proposed to readily show the high stability and the electronic bonding property, which are also revealed applicable to a number of newly predicted boron sheets and nanotubes.
We develop a simple model for the formation energies (FEs) of alkali and lkaline earth alanates and boranates, based upon ionic bonding between metal cations and (AlH4)- or (BH4)- anions. The FEs agree well with values obtained from first principles calculations and with experimental FEs. The model shows that details of the crystal structure are relatively unimportant. The small size of the (BH4)- anion causes a strong bonding in the crystal, which makes boranates more stable than alanates. Smaller alkali or alkaline earth cations do not give an increased FE. They involve a larger ionization p…
Structure and properties of magnesium silicate and borate melts and glasses were investigated by using newly parameterized interaction potentials in molecular dynamics simulations and compared with those of calcium silicate and borate. The competition between the depolymerization of the silica network and the formation of new bonds with modifier ions leads to the enhancement of the elastic moduli with increasing modifier content in alkaline earth silicate glasses. Compared with calcium silicate, the higher elastic moduli of magnesium silicate result from a higher connectivity of the overall gl…
Structural, Energetic, Electronic, and Vibrational Properties of Boron-Substituted Tungsten Clusters
University of Mumbai（孟买大学）；Chikitsak Samuha’s Sir Satam and Lady Shantabai Patkar College of Arts and Science（Chikitsak Samuha's Sir Satam 和 Lady Shantabai Patkar 文理学院）；V.P. Varde College of Commerce and Economics（V.P. Varde 商业与经济学院）
By devising a novel framework, we present a comprehensive theoretical study of solubilities of alkali (Li, Na, K, Rb, Cs) and alkaline earth (Be, Ca, Sr, Ba) metals in the he boron-rich Mg-B system. The study is based on first-principle calculations of solutes formation energies in MgB$_2$, MgB$_4$, MgB$_7$ alloys and subsequent statistical-thermodynamical evaluation of solubilities. The advantage of the approach consists in considering all the known phase boundaries in the ternary phase diagram. Substitutional Na, Ca, and Li demonstrate the largest solubilities, and Na has the highest (0.5-1 …
By electron and X-ray diffraction we establish that the CrB$_4$ compound discovered over 40 years ago crystallizes in the $oP10$ (\emph{Pnnm}) structure, in disagreement with previous experiments but in agreement with a recent first-principles prediction. The 3D boron network in the new structure is a distorted version of the rigid carbon $sp^3$ network proposed recently for the high-pressure C$_4$ allotrope. According to our density functional theory calculations and the analysis of the bonding, CrB$_4$ is a potentially superhard material. In fact, the calculated weakest shear and tensile str…
Isolated hydrogen and hydrogen pairs in bulk diamond matrix have been studied using density functional theory calculations. The electronic structure and stability of isolated and paired hydrogen defects are investigated at different possible lattice sites in pure diamond and boron doped diamond. Calculations revealed that isolated hydrogen defect is stable at bond center sites for pure diamond and bond center puckered site for boron doped diamond. In case of hydrogen pairs, H2 defect (one hydrogen at bond center and second at anti-bonding site) is stable for pure diamond, while for boron doped…
Engineering atomic-scale structures allows great manipulation of physical properties and chemical processes for advanced technology. We show that the B atoms deployed at the centers of honeycombs in boron sheets, borophene, behave as nearly perfect electron donors for filling the graphitic $σ$ bonding states without forming additional in-plane bonds by first-principles calculations. The dilute electron density distribution owing to the weak bonding surrounding the center atoms provides easier atomic-scale engineering and is highly tunable via in-plane strain, promising for practical applicatio…
We report on the detection and charge distribution analysis for nitrogen substitutional dopants in single layer graphene membranes by aberration-corrected high-resolution transmission electron microscopy (HRTEM). Further, we show that the ionicity of single-layer hexagonal boron nitride can be confirmed from direct images. For the first time, we demonstrate by a combination of HRTEM experiments and first-principles electronic structure calculations that adjustments to the atomic potentials due to chemical bonding can be discerned in HRTEM images. Our experiments open a way to discern electroni…
A highly efficient semi-empirical Hamiltonian has been developed and applied to model the compact boron clusters with the intermediate size. The Hamiltonian, in addition to the inclusion of the environment-dependent interactions and electron-electron correlations with the on-site charge calculated self-consistently, has contained the environment-dependent excitation orbital energy to take into account the atomic aggregation effect on the atomic orbitals. The Hamiltonian for boron has successfully characterized the electron deficiency of boron and captured the complex chemical bonding in variou…
We demonstrate that freeing a single-atom thick layer of hexagonal boron nitride (hbn) from tight chemical bonding to a Ni(111) thin film grown on a W(110) substrate can be achieved by intercalation of Au atoms into the interface. This process has been systematically investigated using angle-resolved photoemission spectroscopy, X-ray photoemission and absorption techniques. It has been demonstrated that the transition of the hbn layer from the "rigid" into the "quasi-freestanding" state is accompanied by a change of its lattice constant. Using chemical vapor deposition, graphene has been succe…
The discovery of superconductivity in Magnesium Diborate (MgB$_2$) has stimulated great interest in the search of new superconductors with similar lattice structures. Unlike cuprate or iron-based superconductors, MgB$_2$ is indisputably a phonon-mediated high temperature superconductor. The emergence of high temperature superconductivity in this material results from the strong coupling between the boron $σ$-bonding electrons around the Fermi level and the bond-stretching optical phonon modes. Here we show, based on the first-principles calculations, that Li$_2$B$_3$C is such a good candidate …
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 formin…
Boron phosphide has recently been identified as a potential high hole mobility transparent conducting material. This promise arises from its low hole effective masses. However, BP has a relatively small 2 eV indirect band gap which will affect its transparency. In this work, we computationally study both optical absorption across the indirect gap and phonon-limited electronic transport to quantify the potential of boron phosphide as a \emph{p}-type transparent conductor. We find that phonon-mediated indirect optical absorption is weak in the visible spectrum and that the phonon-limited hole mo…
We compute the electronic band structure and optical properties of boron arsenide using the relativistic quasiparticle self-consistent $GW$ approach, including electron-hole interactions through solution of the Bethe-Salpeter equation. We also calculate its electronic and optical properties using standard and hybrid density functional theory. We demonstrate that the inclusion of self-consistency and vertex corrections provides substantial improvement in the calculated band features, in particular when comparing our results to previous calculations using the single-shot $GW$ approach and variou…
The doping of graphene to tune its electronic structure is essential for its further use in carbon based electronics. Adapting strategies from classical silicon based semiconductor technology, we use the incorporation of heteroatoms in the 2D graphene network as a straightforward way to achieve this goal. Here, we report on the synthesis of boron-doped graphene on Ni(111) in a chemical vapor deposition process of triethylborane on the one hand and by segregation of boron from the bulk on the other hand. The chemical environment of boron was determined by x-ray photoelectron spectroscopy and an…
γ-B28 is a recently established high-pressure phase of boron. Its structure consists of icosahedral B12 clusters and B2 dumbbells in a NaCl-type arrangement (B2)δ+(B12)δ- and displays a significant charge transfer δ~0.5- 0.6. The discovery of this phase proved essential for the understanding and construction of the phase diagram of boron. γ-B28 was first experimentally obtained as a pure boron allotrope in early 2004 and its structure was discovered in 2006. This paper reviews recent results and in particular deals with the contentious issues related to the equation of state, hardness, putativ…
Boron nitride structures have excellent thermal and chemical stabilities. Based on state-of-art theoretical calculations, we propose a wide gap semiconducting BN crystal with a three-dimensional hyperhoneycomb structure (Hp-BN), which is both mechanically and thermodynamically stable. Our calculated results show that Hp-BN has a higher bulk modulus and a smaller energy gap as compared to c-BN. Moreover, due to the unique bonding structure, Hp-BN exhibits anisotropic electronic and optical properties. It has great adsorption in the ultraviolet region, but it is highly transparent in the visible…
Fabrication of ordered structures at the nanoscale limit poses a cornerstone challenge for modern technologies. In this work we show how naturally occurring moiré patterns in Ir(111)-supported graphene template the formation of 2D ordered arrays of substitutional boron species. A complementary experimental and theoretical approach provides a comprehensive description of the boron species distribution, bonding configurations, interfacial interaction with Ir(111) and the impact on graphene's electronic structure. Atomically-resolved scanning tunnelling microscopy images and density functional th…
Ab initio calculations within the density-functional theory formalism are performed to investigate the chemical functionalization of a graphene-like monolayer of silicon - silicene - with B, N, Al or P atoms. The structural, electronic, magnetic and vibrational properties are reported. The most preferable adsorption sites are found to be valley, bridge, valley and hill site for B, N, Al and P adatoms, respectively. All the relaxed systems with adsorbed/substituted atoms exhibit metallic behaviour with strongly bonded B, N, Al, and P atoms accompanied by an appreciable electron transfer from si…
Periodically corrugated epitaxial graphene and hexagonal boron nitride (h-BN) on metallic substrates are considered as perspective templates for the self-assembly of nanoparticles arrays. By using first-principles calculations, we determine binding energies and diffusion activation barriers of metal adatoms on graphene and h-BN. The observed chemical trends can be understood in terms of the interplay between charge transfer and covalent bonding involving the adatom d electrons. We further investigate the electronic effects of the metallic substrate and find that periodically corrugated templat…
Usually the ions of a particular element in solids are in positive or negative oxidation states, depending upon the chemical environment. It is highly unusual for an atom having both positive as well as negative oxidation states simultaneously within the same structural framework in a particular compound. Our structural and chemical bonding analyses show that the hydrogen ions in the transition metal ammine borohydrides (TMABHs) with the chemical formula M(BH4)2(NH3)2 (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) have both positive and negative oxidation states. Our detailed analyses confirm …
Inspired by the successful synthesis of several allotropes, boron sheets have been one of the hottest spot areas of focus in various fields. Here, we study phonon transport in three types of boron nanoribbons with zigzag and armchair edges by using a non-equilibrium Green's function combined with first principles methods. Diverse transport properties are found in the nanoribbons. At the room temperature, their highest thermal conductance can be comparable with that of graphene, while the lowest thermal conductance is less than half of graphene's. The three boron sheets exhibit different anisot…
Dielectric breakdown has historically been of great interest from the perspectives of fundamental physics and electrical reliability. However, to date, the anisotropy in the dielectric breakdown has not been discussed. Here, we report an anisotropic dielectric breakdown strength (EBD) for h-BN, which is used as an ideal substrate for two-dimensional (2D) material devices. Under a well-controlled relative humidity, EBD values in the directions both normal and parallel to the c axis (EBD+c & EBD//c) were measured to be 3 and 12 MV/cm, respectively. When the crystal structure is changed from sp3 …
Despite several first principles studies, the relative stability of BN polymorphs remains controversial. The stable polymorph varies between the cubic (c-BN) and hexagonal (h-BN) depending on the van der Waals (vdW) dispersion approximation used. These studies are unable to explain the main experimental results, c-BN is stable, the relative stability order and the large energy difference between h-BN and c-BN (greater than 150 meV/formula unit). In this study, we introduce contributions from electron-phonon interactions (EPI) to the total energy of BN polymorphs. This clearly establishes c-BN …
We study the mechanical properties of two-dimensional (2D) boron, borophenes, by first-principles calculations. The recently synthesized borophene with 1/6 concentration of hollow hexagons (HH) is shown to have in-plane modulus C up to 210 N/m and bending stiffness as low as D = 0.39 eV. Thus, its Foppl-von Karman number per unit area, defined as C/D, reaches 568 nm-2, over twofold higher than graphene's value, establishing the borophene as one of the most flexible materials. Yet, the borophene has a specific modulus of 346 m2/s2 and ideal strengths of 16 N/m, rivaling those (453 m2/s2 and 34 …
This Chapter deals with single layers of carbon (graphene) and hexagonal boron nitride on transition metal surfaces. The transition metal substrates take the role of the support and allow due to their catalytic activity the growth of perfect layers by means of chemical vapor deposition. The layers are sp2 hybridized honeycomb networks with strong in plane sigma and weaker pi bonds to the substrate and to the adsorbates. This hierarchy in bond strength causes anisotropic elastic properties, where the sp2 layers are stiff in plane and soft out of plane. A corrugation of these layers imposes a th…
Both the basic electronic structure of tetraborides, and the changes across the lanthanide series in $R$B$_4$ ($R$ = rare earth) compounds, are studied using the correlated band theory LDA+U method in the all-electron Full Potential Local Orbital (FPLO) code. A set of boron bonding bands can be identified that are well separated from the antibonding bands. Separately, the ``dimer B'' $2p_z$ orbital is non-bonding ({\it viz.} graphite and MgB$_2$), and mixes strongly with the metal 4d or $5d$ states that form the conduction states. The bonding bands are not entirely filled even for the trivalen…
In the present study, the structural and hitherto uninvestigated mechanical (elastic stiffness constants, machinability index, Cauchy pressure, anisotropy indices, brittleness/ductility, Poissons ratio), electronic, optical, and thermodynamic properties of novel boron-rich compounds B6X (X = S, Se) have been explored using density functional theory. The estimated structural lattice parameters were consistent with the prior report. The mechanical and dynamical stability of these compounds have been established theoretically. The materials are brittle in nature and elastically anisotropic. The v…
Modification of graphene to open a robust gap in its electronic spectrum is essential for its use in field effect transistors and photochemistry applications. Inspired by recent experimental success in the preparation of homogeneous alloys of graphene and boron nitride (BN), we consider here engineering the electronic structure and bandgap of C$_{2x}$B$_{1-x}$N$_{1-x}$ alloys via both compositional and configurational modification. We start from the BN end-member, which already has a large bandgap, and then show that (a) the bandgap can in principle be reduced to about 2 eV with moderate subst…
Two-dimensional van der Waals (2D-vdW) layered hexagonal boron nitride (h-BN) has gained tremendous research interest over recent years due to its unconventional domain growth morphology, fascinating properties and application potentials as an excellent dielectric layer for 2D-based nano-electronics. However, the unidirectional domain growth of h-BN thin films directly on insulating substrates remains significantly challenging because of high-bonding anisotropicity and complex growth kinetics than the conventional thin films growth, thus resulting in the formation of randomly oriented domains …
Boron carbide is a ceramic material with unique properties widely used in numerous, including armor, applications. Its mechanical properties, mechanism of compression, and limits of stability are of both scientific and practical value. Here, we report the behavior of the stoichiometric boron carbide B13C2 studied on single crystals up to 68 GPa. As revealed by synchrotron X-ray diffraction, B13C2 maintains its crystal structure and does not undergo phase transitions. Accurate measurements of the unit cell and B12 icosahedra volumes as a function of pressure led to conclusion that they reduce s…
From Amorphous to Amorphous-Crystalline Mixed-Phase Boron Nitride: Evolution of the Thermal and Elastic Properties
Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST（加泰罗尼亚纳米科学与纳米技术研究所）；Faculty of Physics, Autonomous University of Barcelona（巴塞罗那自治大学物理系）；Universite Claude Bernard Lyon 1, CNRS, LMI, UMR 5615（里昂第一大学）；Faculty of Physics and Astronomy, Adam Mickiewicz University（亚当密茨凯维奇大学物理与天文学院）；International Iberian Nanotechnology Laboratory（伊比利亚纳米技术国际实验室）；ICREA（加泰罗尼亚高级研究院）
