Transition Metal Diborides Research Articles

Electronic States of NbB2(0001), TaB2(0001) and ZrB2(0001) Surfaces Studied by Angle-resolved Ultraviolet Photoelectron Spectroscopy

We have investigated the surface electronic structures of transition-metal diboride (0001) surfaces by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and ab-initio calculations based on the slab model with 13 layers. In the spectra of NbB2 and TaB2, the peaks near -1 eV blow EF are derived from the π band of their surface graphitic boron layer. As compared with graphene, the dispersion curves of the π bonds are greatly modulated by the hybridization of B-π orbits with the metal d orbits. On the other hand, the similar peaks were not observed in spectra of ZrB2, of which the surface is terminated with Zr atoms. [DOI: 10.1380/ejssnt.2006.100]

CHARACTERIZATION OF 3D TRANSITION-METAL DIBORIDES BY X-RAY-ABSORPTION SPECTRA AT THE METAL K EDGE

The 3d transition-metal diborides TiB 2, VB 2 and CrB 2 have been characterized by X-ray absorption near-edge-structure (XANES) spectroscopy at the metal K edges. XANES spectra at the Ti , V and Cr K -edges of TiB 2, VB 2 and CrB 2, respectively, show similar spectral features up to an energy of about 70 eV above threshold. These features can be correlated to their specific atomic arrangement and electronic structure via multiple-scattering calculations as a function of the cluster size. Actually the second pre-edge feature is assigned to a transition from the metal 1s core state to the central metal 4p state hybridized with 3d band state of the higher-shell metal atoms. The first weak feature on the rising part of the edge is due to higher-shells multiple scattering contributions, suggesting that the transition features at the metal K -edge XANES of 3d transition-metal diborides are strongly affected by medium or long-range order contributions.

NMR study of B‐2p Fermi‐level density of states in the transition metal diborides

AbstractWe present a systematic study of the AlB2‐type transition metal diborides by measuring the 11B NMR spin‐lattice relaxation rate on TiB2, VB2, ZrB2, NbB2, HfB2, as well as TaB2. For all studied materials, the observed relaxation at B nuclei is mainly due to the p‐electrons. The comparison with theoretical calculations allows the experimental determination of the partial B‐2p Fermi‐level density of states (DOS). In addition, the extracted B‐2p Fermi‐level DOS values in TiB2, ZrB2, and HfB are consistently smaller than in VB2, NbB2, and TaB2. We connect this trend to the rigid‐band scenario raised by band structure calculations. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Advances in point-contact spectroscopy: two-band superconductor MgB2 (Review)

Analysis of the point-contact spectroscopy (PCS) data on the dramatic new high-Tc superconductor magnesium diboride MgB2 reveals quite different behavior of two disconnected σ and π electronic bands, deriving from their anisotropy, different dimensionality, and electron–phonon interaction. PCS allows direct registration of both the superconducting gaps and electron–phonon interaction spectral function of the two-dimensional σ and three-dimensional π band, establishing the correlation between the gap value and the intensity of the high-Tc driving force—the E2g boron vibrational mode. PCS data on some nonsuperconducting transition-metal diborides are surveyed for comparison.

Lattice dynamics and electron-phonon coupling in transition-metal diborides

The phonon density-of-states of transition metal diborides TMB2 with TM = Ti, V, Ta, Nb and Y has been measured using the technique of inelastic neutron scattering. The experimental data are compared with ab initio density functional calculations whereby an excellent agreement is registered. The calculations thus can be used to obtain electron-phonon spectral functions within the isotropic limit. A comparison to similar data for MgB2 and AlB2 which were subject of prior publications as well as parameters important for the superconducting properties are part of the discussion.

Acoustic-mode-driven electron-phonon coupling in transition-metal diborides

We show that the electron-phonon coupling in the transition-metal diborides NbB2 and TaB2 is dominated by the longitudinal acoustical (LA) mode, in contrast to the optical E_{2g} mode dominated coupling in MgB2. Our ab initio results, described in terms of phonon dispersion, linewidth, and partial electron-phonon coupling along Gamma to A, also show that (i) NbB2 and TaB2 have a relatively weak electron-phonon coupling, (ii) the E_{2g} linewidth is an order of magnitude larger in MgB2 than in NbB2 or TaB2, (iii) the E_{2g} frequency in NbB2 and TaB2 is considerably higher than in MgB2, and (iv) the LA frequency at A for TaB2 is almost half of that of MgB2 or NbB2.

Superconductivity and x-ray photoemission study of MgB 2 thin films

Highly c-axis oriented MgB2 thin films with T-c(onset) of 39.6K were fabricated by magnesium diffusing into pulsed-laser-deposited boron precursors. The estimation of critical current density J(c), using hysteresis loops and the Bean model, has given the value of 10(7) A/cm(2) (15K, 0T), which is one of the highest values ever reported. The x-ray photoemission study of the MgB2 thin films has revealed that the binding energies of Mg 2p and B 1s are at 49.4 eV and 186.9 eV, which are close to those of metallic Mg and transition-metal diborides, respectively.

Effect of metal vacancies on the band structure of Nb, Zr, and Y diborides

Energy band structures of metal-deficient hexagonal diborides M$_{0.75}$B$_2$ (M = Nb, Zr and Y) were calculated using the full-potential LMTO method. The metal vacancies change the density of states near the Fermi level and this effect is quite different for III-V group transition metal diborides. Contradictory data on superconductivity in diborides may be supposed to be connected with nonstoichiometry of samples. Vacancy formation energies are estimated and analyzed.

Electron-phonon interaction in transition-metal diboridesTB2(T=Zr,Nb,Ta)studied by point-contact spectroscopy

The electron-phonon interaction (EPI) in transition metal diborides TB_2 (T=Zr, Nb, Ta) is investigated by point-contact (PC) spectroscopy. The PC EPI functions were recovered and the EPI parameters lambda<0.1 were estimated for all three compounds. Common and distinctive features between the EPI functions for those diborides are discussed also in connection with the superconductivity in MgB_2.

Diffusion studies in non-oxide ceramics: analytical aspects of the use of ion implanted stable tracers and SIMS.

The physical and chemical properties of complex non-oxide ceramic materials require advanced methods of diffusivity determination. In this study, we present a method based on the high-dose ion implantation of stable tracers in combination with secondary ion mass spectroscopy for depth profiling. The analytical basics, advantages and problems of the method are discussed for two examples of complex materials, the Si-B-C-N precursor ceramics and the Ti-based transition metal diborides. We demonstrate that is possible to measure the temperature dependence of diffusivities, especially for ceramic systems with low diffusivities, for systems that contain elements for which no suitable radioactive tracers exist for extended measurements.

Synthesis and photoemission study of as-grown superconducting MgB 2 thin films

As-grown superconducting thin films of MgB 2 were prepared by molecular beam epitaxy (MBE), and studied by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). Only films prepared at temperatures between 150 and 320 °C showed superconductivity. The best T c onset of 36 K was obtained with a sharp transition width of ∼1 K although the film crystallinity was poor. The in situ photoelectron spectra obtained on the surfaces of the MBE grown MgB 2 films were free from dirt peaks. The XP spectra revealed the binding energy of the Mg 2p levels in MgB 2 is close to that of metallic Mg and the binding energy of B 1s is close to that of transition-metal diborides. The valence UP spectra showed a clear Fermi edge although the density of states at E F is low and the major components of the valence band are located between 5 and 11 eV.

X-ray photoemission study ofMgB2

A c-axis oriented thin film and a high density sintered pellet of MgB2 have been studied by x-ray photoemission spectroscopy, and compared to measurements from MgO and MgF2 single crystals. The as-grown surface has a layer which is Mg-rich and oxidized, which is effectively removed by a nonaqueous etchant. The subsurface region of the pellet is Mg-deficient. This nonideal near-surface region may explain varied scanning tunneling spectroscopy results. The MgB2 core level and Auger signals are similar to measurements from metallic Mg and transition metal diborides, and the measured valence band is consistent with the calculated density of states.

Electronic structure, bonding, and ground-state properties of AlB_{2}-type transition-metal diborides

The electronic structure and ground state properties of ${\mathrm{AlB}}_{2}$ type transition metal diborides ${\mathrm{TMB}}_{2}$ (TM=Sc, Ti, V, Cr, Mn, Fe, Y, Zr, Nb, Mo, Hf, Ta) have been calculated using the self consistent tight-binding linear muffin-tin orbital method. The equilibrium volume, bulk moduli ${(B}_{0}),$ pressure derivative of bulk moduli ${(B}_{0}^{\ensuremath{'}}),$ cohesive energy ${(E}_{\mathrm{coh}}),$ heat of formation $(\ensuremath{\Delta}H),$ and electronic specific heat coefficient $(\ensuremath{\gamma})$ are calculated for these systems and compared with the available experimental and other theoretical results. The bonding nature of these diborides is analyzed via the density of states (DOS) histogram as well as the charge density plots, and the chemical stability is analyzed using the band filling principle. The variation in the calculated cohesive properties of these materials is correlated with the band filling effect. The existence of a pseudogap in the total density of states is found to be a common feature for all these compounds. The reason for the creation of the pseudogap is found to be due to the strong covalent interaction between boron p states. We have made spin polarized calculations for ${\mathrm{CrB}}_{2},$ ${\mathrm{MnB}}_{2},$ and ${\mathrm{FeB}}_{2}$ and found that finite magnetic moments exist for ${\mathrm{MnB}}_{2}$ and ${\mathrm{CrB}}_{2}$ whereas ${\mathrm{FeB}}_{2}$ is nonmagnetic.

A Thermodynamic Estimation of the Chemical Vapor Deposition of Some Borides

The temperature dependencies of the change in Gibbs energy for a series of probable reactions of chemical vapor deposition (CVD) of TiB2, ZrB2, NbB2, TaB2, and LaB6 using BCl3, BBr3, B2H6, B5H9, and B10H14 as boron sources have been plotted on the basis of thermodynamic data from the literature. It has been shown that from a thermodynamic point of view the deposition of these borides should proceed in all cases under milder conditions when boron hydrides are the boron sources. Is has been established that of the two boron halides BBr3 is the more suitable boron precursor in the CVD of the borides under consideration. Due to its properties this boron halide would probably prove especially appropriate for technological processes of transition metal diboride deposition.

Directional crystallization of MoSi 2 and some compositions based on it

It has been shown that by directional crystallization of pseudobinary compositions of molybdenum disilicide (MoSi 2) with some other refractory compounds, particularly with diborides of transition metals (composite materials having a disilicide matrix and extended in crystallization direction diboride inclusions) can be obtained that may improve the mechanical properties of the matrix material.

Borides in Thin Film Technology

The borides of transition and rare-earth metals are considered for application as wear- and corrosion-resistant, decorative or thermionic coatings. After a review of physical vapor deposition (PVD) techniques used for the deposition of these coatings, a survey of investigations to apply these coatings is given. As a result of the strong directionality of covalent boron–boron bonds, boride coatings show an increasing tendency to amorphous film growth with increasing B/Meatomic ratio and, for rare-earth hexaborides, with decreasing metallic radius of the rare-earth metal. Mechanical and optical properties are strongly influenced by the crystallographic structure of the boride phase. Because of their high hardness combined with good adhesion, crystalline films based on the diborides of transition metals seem to be promising candidates for wear resistant coatings on cutting tools. Alloying of these films with nitrogen by reactive PVD processes results in the formation of extremely fine-grained multiphase hard coatings with excellent tribological and corrosion behavior, thus offering new applications in the coating of engineering components. Because of their distinct colorations, some of the hexaborides of rare-earth elements may be used as decorative coatings on consumer products like wristwatch casings or eyeglass frames. Another promising field is the development of thermionic coatings based on rare-earth hexaborides, which may offer the possibility of the production of inexpensive and simple high emission filaments.

Phase Formation during Anneal of Supersaturated TiB2–CrB2–WB2Solid Solutions

Numerous transition metal diborides of AlB2structure type reveal large mutual solid solubility. Temperature-depending homogeneity ranges bordering immiscibility gaps within the system TiB2–CrB2–WB2provide the opportunity to tailor anin situgrown typical platelet-shaped phase emerging from a supersaturated host matrix. Results are presented where variations in composition, manufacturing parameters, and heat treatment lead to precipitates of a “W2B5” structure accompanied by the formation of another tungsten boride less rich in boron in a (Ti, W, Cr)B2matrix. All phases are characterized by high melting points and hardness. The enhanced formation of the matrix as well as the precipitation mechanism were activated by the addition of 6 to 20 mole% CrB2to the system TiB2–WB2. Within the ternary solid solution, a particular composition range exists exhibiting a thermal expansion behavior similar to boron carbide. Attempts are made to toughen boron and silicon carbide ceramics by processing thisin situgrowth of platelets out of dispersed transition metal diboride particles within a carbide matrix.

The electronic structure and chemical stability of the AlB2-type transition-metal diborides

The electronic structures of the AlB2-type transition-metal diborides TMB2 (TM=Sc,Ti,V,Cr,Mn,Y,Zr,Nb,Mo,Hf,Ta) have been calculated by using the self-consistent LMTO-ASA method. The binding mechanism is discussed and the rigid-band model is shown to provide a fairly good description of their electronic structures. The existence of a pseudogap in the total density of states is found to be a common feature of these compounds. The variation of the chemical stabilities of these diborides is analysed and we find that the trends can be understood in terms of the band-filling concept of the bonding states. The results are compared with other theoretical and experimental work.

PHASE DIAGRAMS AND BAND STRUCTURE OF TRANSITION METAL COMPOUNDS

Phase diagrams and phase stabilities have in the past been dealt with using different approaches. In the Calphad method one seeks a description of the Gibbs energy function G(T) which is accurate enough to be used in calculations of phase diagrams for complex alloys. Ab initio electron band structure calculations, on the other hand, have been used to predict the cohesive properties of solids but such calculations are usually restricted to 0 K. We have combined these methods to get a better understanding of the phase stabilities. In particular we have critically analysed available thermodynamic information and used that together with band structure calculations to systematically discuss trends in the cohesive energy at 0 K and in the vibrational entropy. We have considered transition-metal carbides and nitrides, both in the NaCl and in more complex structures, and also transition-metal diborides.

X‐ray Photoelectron Spectroscopy Studies of Metallic Diborides

The binding energies of core electrons in d‐type stoichiometric diborides of several transition metals were measured by X‐ray photoelectron spectroscopy. Metal core lines in the diborides shifted either to higher or to lower binding energies compared with those appertaining to corresponding elemental states and varied with the metal. The shifts were &lt; 0.6 eV. Carbides, oxides, and oxycarbides of variable stoichiometries were the major impurities detected in the diborides. The binding energy shifts showed a progressive change as the electronegativity difference between the and the metalloid (boron, carbon, nitrogen, oxygen) increased. This indicated that the bond in boro‐oxycarbides may be partially ionic.