Metal Borocarbides Research Articles

Novel hard and unusual superconducting monoclinic phase of FeB2C2: An ab initio evolutionary study

This study focuses on conducting an ab initio evolutionary investigation to search for stable polymorphs of iron diborocarbides with the formula FeB2C2. We also examined other forms of C contents, including FeB3C and FeBC3. Our findings reveal that the lowest energetic structure of FeB2C2 is a semimetallic monoclinic phase with a space group (s.g.) of C2/m and a metastable metallic phase of FeB2C2 is an orthorhombic structure with s.g. of Pmmm. In addition, structural and relative properties of FeB3C and FeBC3 are performed and discussed to compare with FeB2C2. All predicted structures are dynamically and elastically stable, verified without negative phonon frequency and Born criteria, respectively. We also analyzed the energetic stability through calculated cohesive and formation energies, which showed that C2/m-FeB2C2 is stable at low pressure. Interestingly, the C2/m and Pmmm phases of FeB2C2 are hard materials with Vickers hardness (Hv) of 22.40 and 27.52 GPa, respectively. Additionally, we examined the electron–phonon coupling of both FeB2C2 phases. Unexpectedly, we found that the semimetallic C2/m-FeB2C2 phase is a superconductor with a significant superconducting temperature (Tc) exceeding 6 K. These findings provide some novel results for the Fe–B–C system and pave the way for investigating other metal borocarbides and related ternary compounds.

Robust half metallic properties in two-dimensional transition metal borocarbides: TMBC (TM = 3d transition metals)

Two-dimensional transition metal borocarbides with intrinsic magnetism have garnered significant research attention due to their potential applications in spintronics. Using density functional theory calculations, we designed a type of transition metal borocarbides with two distinct configurations, TMBC-Is and TMBC-IIs (TM = V–Co), and explored their electronic and magnetic properties. Our results demonstrate that all the studied systems exhibit both thermal and kinetic stability. Notably, four systems of MnBC-I/MnBC-II and FeBC-I/FeBC-II are robust ferromagnetic (FM) half metals (HMs) with Curie temperatures of 145, 180, 108, and 315 K. Expect FeBC-II monolayer, FM to antiferromagnetic transition occurs for three other FM HMs under 8%–10% compressive strains, while FM HM to FM semiconductor transition is found for MnBC-II monolayer under 8% tensile strain. These findings provide a promising way to design two-dimensional FM HMs, which hold potential applications in spintronics.

Prospect of high-temperature superconductivity in layered metal borocarbides

A large-scale ab initio search for conventional superconductors has revealed new thermodynamically stable and metastable layered metal borocarbides expected to form under ambient pressure and display critical temperatures exceeding 70 K.

Ferromagnetic TM2BC (TM = Cr, Mn) monolayers for spintronic devices with high Curie temperature.

Transition metal boro-carbide (TM2BC) structures crystallize in the layered orthorhombic structure in their bulk phases. In this study, however, we find that TM2BC (TM = Cr, Mn) prefer a tetragonal (t) crystal structure in their monolayer phases due to the occurrence of strong sp2 bonds between the metal and B/C atoms instead of sp3 + sp2 bonds which exist in the orthorhombic phase. The calculated energy difference between the orthorhombic and the tetragonal structures based on density functional theory (DFT) is more than 1 eV per unit cell. In addition, t-Cr2BC and t-Mn2BC monolayers are dynamically and thermally stable with their magnetic metal electronic structures. For further investigations, we combine our DFT calculations with the Monte Carlo simulations and find that both t-TM2BC monolayers show ferromagnetic properties. The calculated Curie temperatures are 846 K and 128 K for t-Cr2BC and t-MnBC sheets, respectively. In addition, we examine the magnetic anisotropy energies (MAE) of t-TM2BC monolayers and find that both structures prefer out-of-plane as the easy axis magnetization direction and the applied electric field can easily modulate the MAE of the monolayers. Our theoretical calculations reveal that t-TM2BC (TM = Cr, Mn) sheets have great potential for the future design of controllable spintronic devices with their tunable MAE properties.

The electronic and mechanical properties of tetragonal YB2C as explored by first-principles methods

By means of the first-principles calculations, the lattice parameters, electronic structures, phonon dispersions, and mechanical properties of the rare earth metal borocarbide YB2C have been theoretically investigated. The dynamically stability of the layered tetragonal YB2C has been evidenced based on the frozen phonon method. We have found that the covalent bonding between B-2p, C-2p and Y-5d orbitals are responsible for the strong interlayer interactions based on the calculated electronic structures and ELF images. The estimated hardness of P42/mbc-YB2C is around 23.46 GPa which is comparable with the well-known ultra-incompressible oP6-OsB2. Additionally, the analysis of the ideal shear and tensile strength of YB2C reveals the importance of covalent bonds between Y and B/C layer which help to enhance the resistance under deformation.

Layered phases in the composites prepared using ZrC and LnB6 (Ln = La, Ce, and Gd)

We found some layered phases in the composite prepared by reactive hot pressing at 1800°C for 30 mins, using ZrC and LnB6 (Ln = La, Ce, and Gd) as raw materials. The layered phases are related to rare-earth metal borocarbides, but they are relatively and chemically stable in moisture environment. XRD patterns of these layered phases cannot be identified far away, indicating that the layered phases are possibly some new compounds.

Infinite and Finite Boron Carbon Branched Chains: The Crystal Structures of New Ternary Boride Carbides RE10B7C10 and RE4B3C4*

AbstractNew ternary rare earth metal boride carbides RE10B7C10 (RE = Gd, Tb, Dy Ho, Er) and RE4B3C4 (RE = Tb, Dy, Ho, Er, Tm, Yb, Lu) were prepared from the elements by arc‐melting followed by annealing in silica tubes at 1000°C for one month. The crystal structures were investigated by means of single‐crystal X‐ray diffraction data. In the structures of both compounds the boron and carbon atoms form two different types of nonmetal arrangements: one‐dimensional (BC)∞ branched chains and finite CBC units. The branched chains and finite units are differently oriented in both structures. The new ternary rare earth metal boride carbides RE4B3C4 (R = Tb, Dy, Ho, Er, Tm, Yb, Lu) are isotypic to Gd4B3C4, the first characterized member of the rare earth metal borocarbide series in which both one dimentional (BC)∞ branched chains and finite CBC units coexist. Tb10B7C10 is the second characterized member of this family. The compounds RE10B7C10 (RE = Gd, Tb, Dy Ho, Er) are isotypic.

Crystal growth of rare earth-transition metal borocarbides and silicides

Large high quality crystals of various classes of novel rare earth-transition metal intermetallic compounds like RNi2B2C, R11−xR2xNi2B2C, R2PdSi3 and RPd2Si2 (R, R1, R2=Y, La, Sc and rare earth elements) were grown by floating zone (FZ) methods. Careful selection and handling of high-purity starting materials and the control of oxygen impurities during the whole preparation process is required. The process parameters crucially depend on the melting temperature and the solidification mode of the compound. After growth heat treatments can improve the real structure of as grown crystals. The high perfection of single crystals enabled the discovery of some novel physical and crystallographic features of these compounds.

Structural arrangements of the ternary metal boride carbide compounds MB 2C 4 ( M=Mg, Ca, La and Ce) from first-principles theory

The structural arrangements of the ternary metal borocarbides MB 2C 4 ( M=Mg, Ca; La and Ce) are investigated using density-functional theory (DFT) calculations within the generalized gradient approximation (GGA). Results indicate that these compounds adopt a layered structure consisting of graphite-like B 2C 4 layers alternating with metal sheets. Within the hexagonal layers, the coloring with the –C–C–C–B–C–B– sequence is energetically more stable than that with the –C–C–C–C–B–B– one. The electronic structures of these compounds, mainly determined by the B 2C 4 sheets, can be rationalized with the simple valence electron distribution M 2+[B 2C 4] 2− xe −, with the metals essentially acting as two-electron donors with respect to the boron–carbon network, the other x electrons remaining in the relatively narrow d and/or f bands of the metals. Accordingly, MB 2C 4 are narrow band-gap semiconductors (Δ E≈0.2–0.4 eV) with M=Mg and Ca. On the other hand, with M=La and Ce, the compounds are conducting with a relatively high density of states at the Fermi level predominantly metal in character with substantial B/C π* antibonding state admixture.

First-principles prediction of low shear-strain resistance of Al3BC3: A metal borocarbide containing short linear BC2 units

The elastic stiffness and shear deformation mode of Al3BC3, a metal borocarbide containing linear C–B–C units, are studied based on the first-principles total energy calculations. The predominant effect of C–B–C units on mechanical properties is reported by leading to low shear modulus. The low shear-strain resistance originates from the deformation mode as follows: the rigid linear C–B–C units tilt with respect to the c direction easily, and the corner-sharing Al5C bipyramid slabs simultaneously slide along the basal plane with low resistance. The proposed deformation mode may be universal for the ternary metal borocarbides family containing short linear C–B–C units.

Electronic structure and anisotropic superconductivity in diborides and borocarbides

We compare calculated Fermi surface sheet (FSS) areas F and masses of MgB 2 and ZrB 2 with dHvA data. Deviations in F in MgB 2 can be removed by a small mutual shift of σ- and π bands. The dHvA masses lead to orbit averaged el–ph coupling constants λ ̄ σ ≈1.03 and λ ̄ π ≈0.32 , while for ZrB 2 only λ ̄ <0.1 holds. The anisotropy of the Fermi velocities and the orbital character of various FSS of rare earth (R) transition metal (T) borocarbides RT 2B 2C, their relationship to the H c2-anisotropy, and the coexistence of magnetism and superconductivity are discussed.

First-principles study of ternary metal borocarbide compounds containing finite linear BC 2 units

Electronic structures of the ternary metal borocarbide compounds Sc 2BC 2, Al 3BC 3 and Lu 3BC 3 containing linear BC 2 units are compared using density functional calculations. Results reveal a covalent bonding between the metallic matrix and the formally BC 2 5− nonmetal anions which is stronger for the aluminum compound than for the two others.

Anisotropic electronic structure and orbital analysis of borocarbides

The anisotropy of the Fermi velocities and the orbital character of various parts of the Fermi surface of rare earth (R) transition metal (T) borocarbides RT 2 B 2 C ( T = Ni,Pd,Pt , etc.) are investigated by full-potential relativistic band structure calculations. Their relationship to the anisotropy of the upper critical field and the coexistence of magnetism and superconductivity is discussed on a qualitative level within the multiband Eliashberg theory.

A brief comparison of superconductivity in borocarbides and cuprates

The challenge of the present knowledge of the electronic structure and selected thermodynamic properties on the mechanism of superconductivity in rare-earth transition metal borocarbides (nitrides) is discussed with respect to somewhat unexpected similarities with as well as to clear differences from the cuprate superconductors. The effect of substitutional nonmagnetic and magnetic disorder in the rare-earth intermediate layer in between the transition metal boron networks as well as in the network itself upon various thermodynamic properties in the superconducting state of Y x R 1− x C(Ni 1− y Pt y B) 2 R= Lu or Tb is studied theoretically as well as experimentally. The suppression of the upper critical field and its positive curvature near T c can be used as highly sensitive measures of the degree of disorder.

Synthesis, characterization, and structural and theoretical analysis of Gd4B3C4: a novel rare earth metal borocarbide containing two different boron-carbon arrangements.

The synthesis by arc-melting techniques, the single-crystal X-ray structure, and the theoretical analysis of Gd4B3C4 are reported. It crystallizes in the triclinic space group P1 with a = 3.637(2) A, b = 3.674(2) A, c = 11.859(5) A, alpha = 93.34(5) degrees, beta = 96.77(5) degrees, gamma = 90.24(5) degrees, and Z = 1. In this structure, the boron and carbon atoms form two different types of nonmetal arrangements: 1-D (BC)infinity branched chains and finite (0-D) linear CBC "molecular" units. Gd4B3C4 is the first characterized member of the rare earth metal borocarbide series in which both 1-D and "molecular" 0-D nonmetal atom systems coexist. From the structural and theoretical analysis, the following formal charge distribution can be proposed within the ionic limit: (Gd3+)4(BC2(5-)(BC3-)2.e-. Tight-binding calculations suggest that the excess electron in the ionic limit is mainly localized on the Gd atoms (at the bottom of the 5d band), while LAPW calculations favor its localization on the (BC)infinity chain. The bonding within this compound is fully analyzed and compared to other members of the rare earth metal borocarbide series.

Structural, Electronic, and Magnetic Properties of Ternary Rare-Earth Metal Borocarbides R5B2C5 (R=Y, Ce–Tm) Containing BC2 “Molecules”

The borocarbides R5B2C5 (R=Y, Ce–Tm) were prepared by arc melting from the pure rare-earth metals, boron and carbon. The structural arrangement of these compounds, which crystallize in the tetragonal space group P4/ncc, consists of a three-dimensional framework of rare-earth atoms resulting from the stacking of slightly corrugated two-dimensional squares, which lead to the formation of octahedral holes and distorted bicapped square antiprismatic cavities filled with isolated carbon atoms and C–B–C chains, respectively. In contrast to the heavy rare-earth metal (Gd–Tm)-containing compounds which melt congruently, those with the early rare-earth elements (Ce–Sm) are formed peritectically. The electronic structure and chemical bonding of Sm5B2C5 and Gd5B2C5 are analyzed using extended Hückel tight-binding and density-functional theory calculations. Results reveal a rather strong covalency between the metallic matrix and the BC2 groups and the isolated carbon atoms, respectively, similar to what is generally computed in related rare-earth metal borocarbides. Magnetic susceptibility measurements indicate that all samples, which were investigated, undergo ferromagnetic transitions in the temperature region below T=130 K. The heavy rare-earth metal (Tb–Tm) borocarbides exhibit a magnetic behavior typical of narrow-domain-wall ferromagnets. Both the Curie temperatures, TC, as well as the paramagnetic Curie temperatures, ΘP, scale approximately with the de Gennes factor. Hence the indirect exchange interaction via conduction electrons (RKKY-interacting) is the dominating force of the R–R coupling mechanism.

Chemical and Superconducting Properties of the Quaternary Borocarbides Ln–M–B–C (Ln=rare earths, Y; M=Ni, Pd)

We present a status report on the quaternary rare-earth transition metal borocarbides. We discuss in particular the superconducting and magnetic properties of LnNi2B2C (Ln=rare earths, Y) in connection with sample preparation and composition. We also report new results on the analysis of the chemical composition of the superconducting phase at 23 K in the Y–Pd–B–C system, using electron microprobe and, for the first time in these materials, nuclear microprobe. We will briefly comment on new borocarbides and other related systems.

Metal Borocarbides as Novel π-Electronic Systems

Electronic structures and Fermi surfaces of RB2C2 (R=La, Ce) were investigated by transport and the de Haas van Alphen (dHvA) oscillation measurements. The transport properties indicate the presence of multi-carriers in RB2C2, as suggested by recent band calculation. The angle dependence of the cross sectional area of the Fermi surfaces observed by dHvA oscillation suggests the strong hybridization between the π-electron in the BC layer and the metal 5d electron resulting in the presence of three dimensional Fermi surfaces.

Superconducting rare earth transition metal borocarbides

We present an overview of selected properties of quaternary intermetallic rare earth transition metal borocarbides and related boronitride compounds, as well as of theoretical calculations with possible relevance to the mechanism of superconductivity. The interplay of superconductivity and magnetism for compounds with pure and mixed rare earth components is considered. We suggest that the incommensurate magnetic structure modulated along the a-axis is responsible for the pair breaking in Ho x R 1− x Ni 2B 2C; R=Y, Lu samples. The effect of doping (Cu, Co) at the transition metal site is considered experimentally and theoretically. The possible role of correlation effects due to the presence of the transition metal component in determining the electronic structure is discussed comparing the band structure calculation results with various electronic spectroscopies as well as de Haas–van Alphen data. Important thermodynamic properties of these systems are analyzed within multiband Eliashberg theory with special emphasis on the upper critical field H c2( T) and the specific heat. In particular, the unusual positive curvature of H c2( T) near T c observed for high-quality single crystals, polycrystalline samples of YNi 2B 2C, LuNi 2B 2C as well as to a somewhat reduced extent also for the mixed system Y 1− x Lu x Ni 2B 2C is explained microscopically. It is shown that in these well-defined samples the clean limit of type II superconductors is achieved. The values of H c2( T) and of its positive curvature near T c (as determined both resistively and from magnetization as well as from specific heat measurements is an intrinsic quantity generic for such samples) decrease with growing impurity content. Both quantities thus provide a direct measure of the sample quality.

Growth of RENi 2B 2C single crystals by RF-zone melting

Single crystals of rare earth–transition metal–borocarbide intermetallic compounds, which exhibit superconductivity (YNi 2B 2C), reentrant superconductivity (HoNi 2B 2C) and low-temperature magnetic ordering (TbNi 2B 2C), have been grown by a computer aided vertical floating zone melting technique. Selected parts of the equilibrium phase diagrams, which are relevant for solidification and crystal growth processes have been determined. Fluctuations of the physical properties along the rod axis, such as the superconducting transition temperature and Neél temperature were corroborated by local resistance ratio measurements and accounted for segregation effects, which were investigated by electron probe microanalysis.