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

The band structure of nonstoichiometric layered diborides Me_0.75B₂ (Me=Nb, Zr, or Y) is calculated for the first time in the framework of the self-consistent full-potential linearized muffin-tin orbital (FLMTO) method, and the energies of formation of Me vacancies are estimated numerically. It is established that metal vacancies affect the electronic properties and energy states of Group III–V metal diborides in radically different ways. The superconducting properties of these diborides are discussed with due regard for the results obtained.     

Ab initio study of Os0.5W0.5B2, Re0.5W0.5B2, and Os0.5Re0.5B2 with high shear modulus

By using first‐principles calculations, the authors investigate the structural, mechanical, and electronic properties of experimentally synthesized Os_0.5W_0.5B₂. The calculated structural parameters and elastic properties are in good agreement with the experimental results. In addition, two new 5d transition‐metal diborides (Re_0.5W_0.5B₂ and Os_0.5Re_0.5B₂) are predicted to have promising large shear moduli. The latter mainly come from the non‐uniform distribution of valence charge density, which raises the value of the shear moduli. We discuss potentially high hardness in these materials. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Band structure of superconducting dodecaborides YB<Subscript>12</Subscript> and ZrB<Subscript>12</Subscript>

The band structure of superconducting UB₁₂-like cubic dodecaborides, namely, YB₁₂ and ZrB₁₂, is calculated in the framework of the self-consistent full-potential linearized muffin-tin orbital (FLMTO) method. The calculated parameters of the electronic subsystems of YB₁₂ and ZrB₁₂ dodecaborides are analyzed and compared with the relevant parameters of the hypothetical dodecaborides □B₁₂ (□ is a metal vacancy) and BB₁₂; nonsuperconducting AlB₂-like layered diborides, namely, YB₂ and ZrB₂; and a new superconductor, MgB₂.     

Superconductivity and x—ray photoemission study of MgB2 thin films

Highly c-axis oriented MgB2 thin films with Tc^onset of 39.6K were fabricated by magnesium diffusing into pulsedlaser-deposited boron precursors.The estimation of critical current density Jc,using hysteresis loops and the Bean model,has given the value of 10^7A/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 49.4eV and 186.9eV,which are close to those of metallic Mg and transition-metal diborides,respectively.     

Electron transport in diborides: Observation of superconductivity in ZrB2

We report on syntheses and electron transport properties of polycrystalline samples of diborides (AB₂) with different transition metals atoms (A=Zr, Nb, Ta). The temperature dependence of resistivity, ρ(T), and ac susceptibility of these samples reveal a superconducting transition of ZrB₂ with T _c =5.5 K, while NbB₂ and TaB₂ have been observed to be nonsuperconducting up to 0.37K. H _c2(T) is linear in temperature below T _c , leading to a rather low H _c2(0)=0.1 T. At T close to T _c , H _c2(T) demonstrates a downward curvature. We conclude that these diborides, as well as MgB₂ samples, behave like simple metals in the normal state with usual Bloch-Grüneisen temperature dependence of resistivity and with Debye temperatures 280, 460, and 440 K for ZrB₂, NbB₂, and MgB₂, respectively, rather than T ² and T ³, as previously reported for MgB₂.     

5d过渡金属二硼化物的结构和热、力学性质的第一性原理计算

利用基于密度泛函理论的第一性原理系统地研究了5d过渡金属二硼化物TMB₂ (TM=Hf–Au) 的结构、热学、力学和电学性质. 我们考虑了三种结构, 分别为AlB₂, ReB₂和WB₂结构. 计算得到的晶格常数与先前的理论和实验研究符合得很好. 通过计算生成焓预测了化合物的热力学稳定性; 从HfB₂到AuB₂, 生成焓的整体趋势是逐渐增加的. 在所考虑的结构中, 对HfB₂和TaB₂, AlB₂结构是最稳定的; 对WB₂, ReB₂, OsB₂, IrB₂和AuB₂, ReB₂结构是最稳定的; 对PtB₂, WB₂结构是最稳定的. 在所考虑的化合物中, ReB₂结构的ReB₂具有最大的剪切模量(295 GPa), 是最硬的化合物, 与先前的理论和实验结果相符. 计算得到的总态密度显示所有结构都具有金属特性. 讨论了系列化合物的变化趋势. 关键词： 弹性性质 热力学性质 第一性原理 5d过渡金属二硼化物     

Low-temperature X-ray studies of TlInS2, TlGaS2, and TlGaSe2 single crystals

The results of X-ray diffraction studies of the unit-cell parameters and thermal-expansion coefficients of TlInS₂, TlGaS₂, and TlGaSe₂ crystals in the temperature range 100–300 K are described. It is shown that the unit-cell parameters of all the studied crystals gradually increase with increasing temperature. The temperature dependences of these parameters exhibit anomalies in the form of bends and kinks at temperatures corresponding to phase transitions in the crystals. The thermal-expansion coefficients along the [001] crystallographic direction of the crystals under study are determined. It is found that their values slightly change with increasing temperature.     

Correlation between local vibrations and metal mass in AlB2‐type transition‐metal diborides

Lattice vibrations have been investigated in TiB₂, ZrB₂ and HfB₂ by temperature‐dependent extended X‐ray absorption fine structure (EXAFS) experiments. Data clearly show that the EXAFS oscillations are characterized by an anomalous behavior of the Debye–Waller factor of the transition‐metal–boron pair, which is suggested to be associated with a superposition of an optical mode corresponding to phonon vibrations induced by the B sublattice and an acoustic mode corresponding to the transition‐metal (TM) sublattice. Data can be interpreted as a decoupling of the metal and boron vibrations observed in these transition‐metal diborides (TMB₂), a mechanism that may be responsible for the significant reduction of the superconducting transition temperature observed in these systems with respect to the parent MgB₂ compound. The vibrational behavior of TM–TM bonds has also been investigated to study the occurrence of anisotropy and anomalies in the lattice vibrational behavior of TM–TM bonds.     

Foreign gas broadening of difluoromethane by N2, O2 and CO2

The linewidth parameters of difluoromethane broadened by N₂, O₂, and CO₂ have been measured at microwave frequencies and are compared with calculated values for dipole-quadrupole interactions by using the Murphy-Boggs theory. The observed linewidths yield reasonable values of molecular quadrupole moments.     

Superconducting NbB2: An ab initio study of elastic constants

The five different elastic constants of the superconducting NbB₂ are calculated for the first time by ab initio density functional method with both correlation and exchange potentials. In the absence of experimental data, the results are compared with those of other related diborides. The fully relaxed and isotropic bulk moduli are also estimated and the implication of their comparison is made.     

Raman line positions in molecular hydrogen: H2, HD,HT, D2, DT,and T2

Spontaneous Raman spectroscopy is used to determine line positions of the six isotopomers of molecular hydrogen: H₂, HD, HT, D₂, DT, and T₂. State populations as low as 1.3 × 10⁸ are detected with the present experimental apparatus. This sensitivity makes possible measurement of the first overtone Q-branch line positions for H₂ and D₂ and of higher rotational transitions than previous investigations. The line positions for D₂, DT, and T₂ indicate that literature values for molecular parameters do not predict accurately line positions of transitions at J values above the observed transitions from which they were determined. The results for the six molecular isotopomers show that ab initio energy levels restricted to the adiabatic approximation do not yield line positions within the experimental uncertainty whereas recent nonadiabatic calculations reproduce the present observations. Reexamination of literature results at high energies indicates discrepancies between the theoretical calculations and experimental vibrational band origins for all vibrational levels in HT, DT, and T₂. No experimental measurements are currently available that test the accuracy of nonadiabatic ab initio rotational levels at high energies.     

The absorption cross sections of N2, O2, CO,NO, CO2, N2O,CH4, C2H4, C2H6 and C4H10 from 180 to 700Å

The absorption cross sections of N₂, O₂, CO, NO, CO₂, N₂O, CH₄, C₂H₄, C₂H₆, C₄H₁₀ have been measured photoelectrically in the 180–700 Å region using synchrotron radiation. The absorption cross sections in the region λ ≥ 500 Å was found to be structureless and to increase monotonically with wavelength for all gases. The positions of the structure observed in the 520–720 Å region for N₂, O₂, CO₂ and N₂O are consistent with the various Rydberg series reported by previous authors.     

Diboride bifullerenes and binanotubes

The structures and energy characteristics of a new class of nanotubes and fullerenes formed by binary layers of a trigonal network of metal atoms M and a graphite-like network of boron atoms are considered by the example of magnesium and zirconium diborides. It is shown that, contrary to the familiar carbon monolayer nanotubes, the dependence of the strain energy of diboride bitubulene on its diameter D deviates from the 1/D ² law because of the “crustlike” shape of a free fragment of the MB₂ bilayer, while bitubulenes with axis parallel to the M-M bonds have the optimum shape at a given diameter. Such bitubulenes are expected to be metallic because of the specific features of the band structure of bilayered diborides.     

The band structures of superconducting MgB2 and the isostructural compounds CaGa2, AgB2, AuB2, ZrBe2, and HfBe2

This paper reports on a self-consistent, full-potential LMTO calculation of the band structure of the medium-T _c superconductor MgB₂ and of the isostructural hexagonal phases of CaGa₂, ZrBe₂, HfBe₂, AgB₂, and AuB₂. The factors responsible for the superconducting properties of magnesium diboride are considered. The results obtained are compared with previous calculations and available experimental data.     

Absolute Raman intensities of CH4, CH3D,CH2D2, CHD3, and CD4

Measurements of the absolute vibrational Raman intensities and depolarization ratios for the fundamental and some overtone and combination bands of CH₄, CH₃D, CH₂D₂, CHD₃, and CD₄ are reported. Experimental aspects of these measurements are discussed. The experimental data conform satisfactorily to all isotope intensity sum rules. The measured intensities and depolarization ratios, together with the vibrational potential function for CH₄, make possible the calculation of the four independent parameters of the isotopic invariant quantities $\text{α}{}^{\mathrm{S}}\text{= |}\text{?α}\text{?S}\text{|}$. The results deduced from these agree with all 36 experimentally observed values. Values of electro-optical parameters for the CH bond are calculated and discussed.     

Magnetic properties of UT2Si2 and UT2Ge2 (T = Co,Ni, Cu) intermetallic systems

Neutron diffraction and magnetization measurements indicate that, at low temperatures, long-range magnetic order is present in UCO₂Si₂, UNi₂Si₂, UCu₂Si₂, UNi₂Ge₂, and UCo₂Ge₂. UCo₂Si₂ and UNi₂Ge₂ are simple collinear antiferromagnets of +-+- type, UCu₂Si₂ a simple collinear ferromagnet. In UNi₂Si₂, a magnetic phase transition from a LSW type structure to collinear antiferromagnetism of +-+- type was found, while in UCu₂Ge₂, the antiferromagnetic structure of ++-- transforms into collinear ferromagnetism. Crystal structure and magnetic parameters are given. No magnetic moment on transition metal ions was found within the accuracy of a powder neutron diffraction experiment. The stability of particular magnetic ordering schemes is discussed in terms of an isotropic RKKY mechanism.     

Crystal structures,elastic, and lattice dynamical properties of BeB2, NaB2, and CaB2 from the first principles

Based on density functional theory, we have studied the structural stability, elastic, mechanical, and lattice dynamical properties of BeB₂, NaB₂, and CaB₂ compounds in AlB₂, OsB₂, and ReB₂ structures, respectively. Generalized gradient approximation has been used for modeling exchange-correlation effects. Our calculations indicate that ReB₂, AlB₂, and OsB₂ structures are energetically the most stable for BeB₂, NaB₂, and CaB₂ compounds, respectively. The results show that these structures are both mechanically and dynamically stable. The bulk modulus, elastic constants, shear modulus, Young’s modulus, Poisson’s ratio, Debye temperature, sound velocities, and anisotropic factors are also calculated and discussed. Furthermore, the phonon dispersion curves and corresponding phonon density of states are presented. Our structural and some other results are in agreement with the available experimental and theoretical data.     

Electric Field Gradients in Metal Diborides

Systematic calculations of the electric field gradient V _zz with the FLAPW code WIEN97 have been performed for the metal diborides MB₂ (M = Mg–Al, Sc–Fe, Y–Mo, Lu–W) with the hexagonal C32 structure, where with knowledge of the lattice constants a, c all atomic positions are fixed. For V _zz (B) good agreement with the experimental result is found in all cases. The case of the recently discovered superconductor MgB₂ is discussed as representative example. This revised version was published online in September 2006 with corrections to the Cover Date.     

The integrated number of vibrational states in acetylene (12C2H2, 13C2H2, 12C2D2)

A calculated exhaustive set of vibrational state energies in ¹²C₂H₂, ¹³C₂H₂ and ¹²C₂D₂ has been used to analyse the evolution of the integrated number of states with increasing vibrational energy N(E) up to 15000 cm^?1, 12000cm^?1 and 10000 cm^?1 in each isotopomer, respectively. The regular contribution to N(E) was modelled analytically and numerical parameters were fitted. The other expected contribution to N(E), which is of oscillatory nature, was quantified and is discussed using energyand time-dependent theories. Related periods of oscillation and temporal recurrences are interpreted consistently in terms of the constant of the motion N_r = 5v₂ + 3v₂ + 5v₃ + v₄ + v₅ and of an average vibrational quantum. More pragmatically, the vibrational dynamics appear to be dominated by the bending vibrations, i.e., by the slowest oscillators.     

Line profile study from diode laser spectroscopy in the CH4 2ν3 band perturbed by N2, O2, Ar, and He

We present a line profile study for two lines in the 2ν₃ band of CH₄ recorded with a frequency stabilized tunable diode laser spectrometer. The broadening and narrowing (Dicke effect) parameters of the R(0) line perturbed by N₂, O₂, and He are derived from a simultaneous fitting of spectra at pressures from 20 to 300 Torr by using the soft and hard collision models. These parameters are determined for the A and F components of the unresolved R(3) manifold perturbed by N₂, Ar, and He from the line profile analysis of spectra at pressures between 50 and 500 Torr. The line mixing effect between the two F components is also taken into account and the absorber speed dependent effect on broadening is estimated for N₂ and Ar.