Boron-Rich Molybdenum Boride with Unusual Short-Range Vacancy Ordering, Anisotropic Hardness, and Superconductivity

Determination of the structures of materials involving more light elements such as boron-rich compounds is challenging and technically important in understanding their varied compositions and superior functionalities. Here we resolve the long-standing uncertainties in structure and composition about the highest boride (termed MoB4, Mo1–xB3, or MoB3) through the rapid formation of large-sized boron-rich molybdenum boride under pressure. Using high-quality single-crystal X-ray diffraction analysis and aberration-corrected scanning transmission electron microscopy, we reveal that boron-rich molybdenum boride with a composition of Mo0.757B3 exhibits P63/mmc symmetry with a partial occupancy of 0.514 in 2b Mo sites (Mo1), and direct observations reveal the short-range ordering of cation vacancies in (010) crystal planes. Large anisotropic Young’s moduli and Vickers hardness are seen for Mo0.757B3, which may be attributed by its two-dimensional boron distributions. Mo0.757B3 is also found to be superconducting with a transition temperature (Tc) of ∼2.4 K, which was confirmed by measurements of resistivity and magnetic susceptibility. Theoretical calculations suggest that the partial occupancy of Mo atoms plays a crucial role in the emergence of superconductivity.