Thermal and magnetic properties of DyB62 at low temperatures

Temperature dependences of heat capacity C_P(T) and magnetization M(T) of an icosahedral dysprosium boride (DyB₆₂) single crystal have been experimentally investigated in the temperature range of 2-300 K. The magnetic susceptibility χ(T) of DyB₆₂ follows Curie-Weiss law with a paramagnetic Curie temperature of −3.7 K, which implies that the antiferromagnetic interactions are dominant in this material and suggests the possibility of magnetic ordering at low temperatures. This conjecture is supported by the temperature dependence of heat capacity C_P(T), which decreases upon heating from 2 to 7 K. The heat capacity of DyB₆₂ at 2 K is analyzed as a sum of magnetic, Debye, two-level system and soft atomic potential components.     

Synthesis of titanium boride nanoparticles and fabrication of flexible material for radiation shielding

A numerical simulation for a radio frequency (RF) thermal plasma torch was carried out to identify the temperature and velocity distributions and determine the optimal design of a plasma system based on the coil turn and torch inner diameter parameters. Then, the optimized conditions of five coil turns and a torch inner diameter of 64 mm were applied to synthesize titanium boride nanoparticles. The major crystal structure of the synthesized titanium boride nanoparticles was TiB₂, with TiB as a minor phase and an average particle size of 42 nm. Subsequently, the gamma-ray shielding performance was analyzed using synthesized titanium boride nanoparticles. Flexible shielding materials were fabricated using a hydrogel to load the synthesized nanoparticles. The attenuation coefficient of the titanium boride nanoparticle flexible material was 0.238 cm⁻¹, and the half-value layer was 2.91 cm.     

The band structures of BSb and B<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sb alloys

The band structures of BSb and B _x Ga_1−x Sb alloys are studied using first-principles calculations in the generalized gradient approximation. By SQS-8 supercells to model a random alloy, the direct transition energy-gap (Γ _15v−Γ _1c) bowing of 3.0 eV is obtained for B _x Ga_1−x Sb alloys in x = 0–50%, in x = 0–11% the energy-gap is the band-gap and increases by 7 meV/%B with boron composition increasing; by SQS-16 supercells the bowing parameter is about 1.9 eV in x = 0–12.5%. The formation enthalpies of mixing, ΔH, are calculated for B _x Ga_1−x As and B _x Ga_1−x Sb alloys. A comparison of enthalpies indicates that B _x Ga_1−x Sb films with boron composition of 7% may be possible. Supported by the National Natural Science Foundation of China (Grant No. 10774031) and the Guangdong Provincial Natural Science Foundation (Grant No. 07001790)     

First-principles investigation of BAs and BxGa1-xAs alloys

First-principles investigation of BAs and BxGa1-xAs alloys Using first-principles calculations in the generalized gradient approximation, the electronic properties of BAs and BxGa1-xAs alloys are studied. At the Brillouin-zone centre, the lowest conduction band is the three-degenerate p-like Г15c state rather than s-like Г1c state, and the conduction band minimum （CBM） is along the A line between the Г and X points-at approximately 11/14（1,0,0）2π/a. With boron content at 0%-18.75%, BxGa1-xAs alloys have a small （2.6 eV） and relatively composition-independent band-gap bowing parameter, the band-gap increases monotonically by -18meV/B% with increasing boron content. In addition, the formation enthalpies of mixing for BxGa1-xAs alloys with boron content at 6.25% and 12.5% are calculated, and the large formation enthalpies may explain the difficulty in alloying boron to GaAs.     

Synthesis, crystal structure, and properties of two modifications of MgB(12)C(2)

Single crystals of two modifications of the new magnesium boride carbide MgB(12)C(2) were synthesized from the elements in a metallic melt by using tantalum ampoules. Crystals were characterized by single-crystal X-ray diffraction and electron microprobe analysis (energy-dispersive (EDX) and wavelength-dispersive (WDX) X-ray spectroscopy). Orthorhombic MgB(12)C(2) is formed in a Cu/Mg melt at 1873 K. The crystal structure of o-MgB(12)C(2) (Imma, Z=4, a=5.6133(10), b=9.828(2), c=7.9329(15) A, 574 reflections, 42 variables, R(1)(F)=0.0208, wR(2)(I)=0.0540) consists of a hexagonal primitive array of B(12) icosahedra with Mg atoms and C(2) units in trigonal-prismatic voids. Each icosahedron has six exohedral B--B and six B--C bonds. Carbon is tetrahedrally coordinated by three boron atoms and one carbon atom with a remarkably long C--C distance of 1.727 A. Monoclinic MgB(12)C(2) is formed in an Al/Mg melt at 1573 K. The structure of m-MgB(12)C(2) (C2/c, Z=4, a=7.2736(11), b=8.7768(13), c=7.2817(11) A, beta=105.33(3) degrees , 1585 reflections, 71 variables, R(1)(F)=0.0228, wR(2)(I)=0.0610) may be described as a distorted cubic close arrangement of B(12) icosahedra. Tetrahedral voids are filled by C atoms and octahedral voids are occupied by Mg atoms. The icosahedra are interconnected by four exohedral B--B bonds to linear chains and by eight interstitial C atoms to form a three-dimensional covalent network. Both compounds fulfill the electron-counting rules of Wade and Longuet-Higgins.     

Das Kupfer–Iridiumborid Cu2Ir4B3 mit einer vom ZnIr4B3‐Typ abgeleiteten Schichtstruktur

The Copper Iridium Boride Cu₂Ir₄B₃ with a Layer Structure Derived from the ZnIr₄B₃ Type The new compound Cu₂Ir₄B₃ (orthorhombic, Cmcm, a = 283.21(2) pm, b = 2540.6(1) pm, c = 281.06(2) pm, Z = 2, 209 reflexions, 18 parameters, R = 0.043) was prepared by reaction of the elements. The structure is related to the ZnIr₄B₃ type. It contains slabs composed of Ir₆B‐ und Ir₆‐prisms which alternate with copper double layers.     

Über die Kristallstruktur der Phase La5B2C6

On the Crystal Structure of the Phase La₅B₂C₆ A homogeneous sample of La₅B₂C₆ annealed at 1000 °C was investigated by X‐ray powder and single‐crystal as well as electron diffraction. The crystal structure was refined in space group P4/ncc (a = 8.590(1) Å, b = 12.398(1) Å, Z = 4, ρ_calc = 5.723 g/cm, ρ_exp = 5.70 g/cm) allowing for anharmonic displacement parameters for the metal atoms. The structure contains twofold disordered CBCC units in bicapped tetragonal antiprismatic coordination in contrast to C₂ and CBC units in earlier investigations.     

Zr2Ir6B with an eightfold superstructure of the cubic perovskite-like boride ZrIr3B0.5: Synthesis, crystal structure and bonding analysis

Single phase powder samples and single crystals of Zr₂Ir₆B were successfully synthesized by arc-melting the elements in a water-cooled copper crucible under an argon atmosphere. Superstructure reflections were observed both on powder and on single crystal diffraction data, leading to an eightfold superstructure of ZrIr₃B_x phase. The new phase, which has a metallic luster, crystallizes in space group (no. 225) with the lattice parameters a=7.9903(4) Å, V=510.14(4) Å³. Its crystal structure was refined on the basis of powder as well as single crystal data. The single crystal refinement converged to R₁=0.0239 and wR₂=0.0624 for all 88 unique reflections and 6 parameters. Zr₂Ir₆B is isotypic to Ti₂Rh₆B and its structure can be described as a defect double perovskite, A₂BB′O₆, where the A site is occupied by zirconium, the B site by boron, the O site by iridium but the B′ site is vacant, leading to the formation of empty and boron-filled octahedral Ir₆ clusters. According to the result of tight-binding electronic structure calculations, Ir-B and Ir-Zr interactions are mainly responsible for the structural stability of the phase. According to COHP bonding analysis, the strongest bonding occurs for the Ir-B contacts, and the Ir-Ir bonding within the empty clusters is two times stronger than that in the BIr₆ octahedra.     

Synthesis, crystal growth and structure of Mg containing β-rhombohedral boron: MgB17.4

For the first time, single crystals of Mg containing β-rhombohedral boron MgB_17.4 were synthesised from the elements in a Mg/Cu melt at 1600 °C. The crystal structure determined by the refinement of single crystal data (space group R-3m, , , 890 reflections, 123 variables, R₁(F)=0.049, wR₂(I)=0.122) improves and modifies the former structure model derived from earlier investigations on powder samples. Mg is located on four different positions with partial occupation. While the occupation of the sites D (53.3%), E (91%) and F (7.2%) is already known from other boron-rich borides related to β-rhombohedral boron, the occupation of the fourth position (18h, 6.7%) is observed for the first time. Two boron positions show partial occupation. The summation reveals the composition MgB_17.4 and Mg_5.85B_101.9, respectively, confirmed by WDX measurements. The single crystals of MgB_17.4 show the highest Mg content ever found. Preliminary measurements indicate no superconductivity.     

Synthesis and crystal structure of MgB12

Single crystals of MgB₁₂ were synthesized from the elements in a Mg/Cu melt at 1600 °C. MgB₁₂ crystallizes orthorhombic in space group Pnma with , and . The crystal structure (Z=30, 5796 reflections, 510 variables, R₁(F)=0.049, wR₂(I)=0.134) consists of a three dimensional net of B₁₂ icosahedra and B₂₁ units in a ratio 2:1. The B₂₁ units are observed for the first time in a solid compound. Mg is on positions with partial occupation. The summation reveals the composition MgB_12.35 or Mg_0.97B₁₂ , respectively. This is in good agreement with the value of MgB_11.25 as expected by electronic reasons to stabilize the boron polyhedra and .