Boron-Rich\nMolybdenum Boride with Unusual Short-Range\nVacancy Ordering, Anisotropic Hardness, and Superconductivity

Determination of the structures of materials involving\nmore light\nelements such as boron-rich compounds is challenging and technically\nimportant in understanding their varied compositions and superior\nfunctionalities. Here we resolve the long-standing uncertainties in\nstructure and composition about the highest boride (termed MoB<sub>4</sub>, Mo<sub>1–<i>x</i></sub>B<sub>3</sub>, or\nMoB<sub>3</sub>) through the rapid formation of large-sized boron-rich\nmolybdenum boride under pressure. Using high-quality single-crystal\nX-ray diffraction analysis and aberration-corrected scanning transmission\nelectron microscopy, we reveal that boron-rich molybdenum boride with\na composition of Mo<sub>0.757</sub>B<sub>3</sub> exhibits <i>P</i>6<sub>3</sub>/<i>mmc</i> symmetry with a partial\noccupancy of 0.514 in <i>2b</i> Mo sites (Mo1), and direct\nobservations reveal the short-range ordering of cation vacancies in\n(010) crystal planes. Large anisotropic Young’s moduli and\nVickers hardness are seen for Mo<sub>0.757</sub>B<sub>3</sub>, which\nmay be attributed by its two-dimensional boron distributions. Mo<sub>0.757</sub>B<sub>3</sub> is also found to be superconducting with\na transition temperature (<i>T</i><sub>c</sub>) of ∼2.4\nK, which was confirmed by measurements of resistivity and magnetic\nsusceptibility. Theoretical calculations suggest that the partial\noccupancy of Mo atoms plays a crucial role in the emergence of superconductivity.