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

Single-phase metal dodecaboride solid solutions, Zr 0.5 Y 0.5 B 12 and Zr 0.5 U 0.5 B 12 , 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 10 B and 11 B solid-state nuclear magnetic resonance (NMR) spectroscopy. The effects of carbon addition to Zr 1- x Y x B 12 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 Zr 0.5 Y 0.5 B 12 phase is 47.6 ± 1.7 GPa at 0.49 N load, which is ∼17% higher than that of its parent compounds, ZrB 12 and YB 12 , with hardness values of 41.6 ± 2.6 and 37.5 ± 4.3 GPa, respectively. The hardness of Zr 0.5 U 0.5 B 12 is ∼54% higher than that of its UB 12 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 ZrB 12 and blue for YB 12 -can be attributed to charge-transfer. XPS indicates that the metals are in the following oxidation states: Y 3+ , Zr 4+ , and U 5+/6+ . The superconducting transition temperatures ( T c ) of the dodecaborides were determined to be 4.5 and 6.0 K for YB 12 and ZrB 12 , respectively, as shown by resistivity and superconducting quantum interference device (SQUID) measurements. The T c of the Zr 0.5 Y 0.5 B 12 solid solution was suppressed to 2.5 K.