Synthesis and Characterization of Single-Phase Metal Dodecaboride Solid Solutions: Zr1–xYxB12 and Zr1–xUxB12

Abstract

<p>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–<em>x</em>Y<em>x</em>B12 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 charge-transfer. XPS indicates that the metals are in the following oxidation states: Y3+, Zr4+, and U5+/6+. The superconducting transition temperatures (<em>T</em>c) 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 <em>T</em>c of the Zr0.5Y0.5B12 solid solution was suppressed to 2.5 K.</p>