Discovery of Metastable W 3 P Single Crystals with High Hardness and Superconductivity.

Hard and superconducting materials play significant roles in their respective application areas and are also crucial research fields in condensed matter physics. Materials with the key properties of both hard and superconducting properties could lead to technology development, but it is also full of challenges. Herein, we report the synthesis of high-quality metastable W 3 P single crystals with superconductivity and excellent mechanical properties. The synergistic effect of temperature and pressure was effective in suppressing further decomposition of metastable W 3 P as-synthesized by our synthesis technique (high-pressure and high-temperature method). The transport and magnetic measurements indicate that W 3 P is a typical type-II BCS superconductor, displaying a superconducting transition temperature of 5.9 K and an impressive critical magnetic field of 4.35 T. Theory calculations reveal a metallic property in W 3 P, and the phonon modes of the vibration of W atoms are important for electron-phonon interaction. Meanwhile, W 3 P shows excellent mechanical properties with a high fracture toughness of 8 MPa m 1/2 and an impressive asymptotic hardness of 22 GPa, which is currently reported as being the hardest among transition metal phosphides. It opens up a new class of advanced materials that combine excellent mechanical properties with superconductivity.