Synthesis and Characterization of Single-Phase Metal Dodecaboride Solid Solutions: Zr_1–xY_xB₁₂ and Zr_1–xU_xB₁₂

Abstract

Single-phase metal dodecaboride solid solutions, Zr0.5Y0.5B12 and Zr0.5U0.5B12, were prepared by arc melting from pure elements. The phase purity and composition were established by powder X-ray diffraction (PXRD), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and 10B and 11B solid-state nuclear magnetic resonance (NMR) spectroscopy. The effects of carbon addition to Zr1-xYxB12 were studied and it was found that carbon causes fast cooling and as a result rapid nucleation of grains, as well as "templating" and patterning effects of the surface morphology. The hardness of the Zr0.5Y0.5B12 phase is 47.6 ± 1.7 GPa at 0.49 N load, which is ∼17% higher than that of its parent compounds, ZrB12 and YB12, with hardness values of 41.6 ± 2.6 and 37.5 ± 4.3 GPa, respectively. The hardness of Zr0.5U0.5B12 is ∼54% higher than that of its UB12 parent. The dodecaborides were confirmed to be metallic by band structure calculations, diffuse reflectance UV-vis, and solid-state NMR spectroscopies. The nature of the dodecaboride colors-violet for ZrB12 and blue for YB12-can be attributed to chargetransfer. XPS indicates that the metals are in the following oxidation states: Y3+, Zr4+, and U5+/6+. The superconducting transition temperatures (Tc) of the dodecaborides were determined to be 4.5 and 6.0 K for YB12 and ZrB12, respectively, as shown by resistivity and superconducting quantum interference device (SQUID) measurements. The Tc of the Zr0.5Y0.5B12 solid solution was suppressed to 2.5 K.

Insight into the Crystal Structures and Physical Properties of the Uranium Borides UB1.78±0.02, UB3.61±0.041 and UB11.19±0.13

“…Despite the close stoichiometry between our sample and UB 1.79 , the lattice parameters are slightly different, probably due to the impurities indicated by the previous authors (U and UB 4 ), which might modify the overall UB 2−x stoichiometry. For UB 11.78 , the only reported UB 12−x stoichiometry in the open literature was UB 16 [18] (a = 7.475 Å), which indeed differs from ours. Finally, the UB 3.98 (a = 7.0764 Å, c = 3.9811 Å) sample synthesized by Menovsky et al [19] is the closest to UB 4 stoichiometry that we could find in the open literature.…”

“…In fact, the P4/mbm structures B2 has the Wyckoff Symbol 4g and B3 8i. [18] −10 ± 5 1.08 0 ZrB 2 [18] −29 --MgB 2 [45] 40 [18] 10 1.083 0.98 YB 12 [18] 25 1.08 0.93…”

“…We compared in Table 5 the NMR parameters of the UB X with their isostructural MB X : UB 1.78 with the Pauli paramagnets AlB 2 and ZrB 2 [18] and the superconductor (T c = 39 K [46]) MgB 2 [48]; UB 3.61 with the diamagnetic YB 4 [44,46], the diamagnetic at room temperature [49] LaB 4 [37,47], and the antiferromagnetic (at 7 K) [49] NdB 4 [37,46]; finally, UB 11.19 with the metallic ZrB 12 [18] and YB 12 [18,50]. All the UB X possess higher shifts than their MB X counterpart, most probably linked to the hybridization between the U5f and B2p orbitals.…”

Design and Synthesis of Clathrate LaB8 with Superconductivity

“…Elemental boron exhibits extraordinary structural and chemical complexity due to different arrangements of icosahedral B 12 cages with three-center bonds within icosahedra, and covalent two-and three-center bonds between icosahedra [1][2][3]. Through metal doping, a more B-rich B 24 cage can be obtained in a B-based clathrate metal (M) dodecaboride, MB 12 , which has broadly tunable properties with different guest atoms [4][5][6]. To date, B-based clathrate structures have existed only in two types of MB 12 compound, namely I4/mmm ScB 12 and Fm-3m UB 12 , which include sodalite-like B 24 cages with metal atoms at the center of each B cage [4].…”

“…To date, B-based clathrate structures have existed only in two types of MB 12 compound, namely I4/mmm ScB 12 and Fm-3m UB 12 , which include sodalite-like B 24 cages with metal atoms at the center of each B cage [4]. With different guest atoms, MB 12 exhibits properties such as super-hardness (above 40 GPa for ZrB 12 [5] and Zr 0.5 Y 0.5 B 12 [6]), superconductivity (at 4.7 K for YB 12 and 5.8 K for ZrB 12 [7]), and oxidation resistance (at ∼695 • C for Y 0.5 Sc 0.5 B 12 [8]), making them of broad interest for industrial application. Studies of B-based clathrate materials seem to have focused mainly on guest-atom substitution [6,9].…”