Superhard and superconducting boron clathrates in the prediction of U-B compounds

The binary metal borides provide a promising platform for searching for unique materials with superconductivity and superhardness under high pressure, owing to the distinctive bonding characteristics of boron. In this work, we combined the first-principles calculations and crystal structure predictions, and we predicted four exotic stoichiometries and eight unique U-B compounds under high pressure. The predicted compounds have layered or caged structure units, and four of them host high hardness under ambient pressure. By removal of the U atoms, we predicted three metastable boron clathrates at ambient pressure. Remarkably, the Vickers hardness of the predicted $C2/m\text{\ensuremath{-}}{\mathrm{B}}_{6}$ is estimated to be 49--53 GPa, and the $C2/m\text{\ensuremath{-}}{\mathrm{B}}_{12}$ is superconducting with the ${T}_{\mathrm{c}}$ value of 16.12 K. Our calculations enrich the phase diagram of binary metal borides and boron allotropes, providing insights for the future theoretical and experimental studies on unique materials.