High-pressure synthesis of superconducting Nb_1-xB₂ (x = 0-0.48) with the maximum T_c = 9.2 K

Superconductivity with T_c above 9 K was found in metal-deficient NbB₂ prepared under 5 GPa, while no clear superconductivity was observed down to 3 K in stoichiometric NbB₂. The superconductivity was observed above x = 0.04 in Nb_1-xB₂, 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-xB₂ (0 ≤ x ≤ 0.24) was also synthesized under high pressure to examine a special effect of high-pressure synthesis.