High-pressure synthesis of superconducting Nb1−xB2 (x=0–0.48) with the maximum Tc=9.2 K

Abstract Superconductivity with T c above 9 K was found in metal-deficient NbB 2 prepared under 5 GPa, while no clear superconductivity was observed down to 3 K in stoichiometric NbB 2 . The superconductivity was observed above x =0.04 in Nb 1− x B 2 , and the lattice parameters also changed abruptly at x =0.04. As x increased, the transition temperature T c slightly rose and fell with the maximum value of 9.2 K at x =0.24 for the samples sintered at 5 GPa and 1200 °C. The T c -value changed in the range from 7 to 9 K, depending on the sintering pressure. A series of Ta 1− x B 2 (0⩽ x ⩽0.24) was also synthesized under high pressure to examine a special effect of high-pressure synthesis.