Synthesis and Characterization of Single-Phase Metal Dodecaboride Solid Solutions: ZrY B and ZrU B.

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