Superconductivity with Tc 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 Nb1-xB2, and the lattice parameters also changed abruptly at x = 0.04. As x increased, the transition temperature Tc 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 Tc-value changed in the range from 7 to 9 K, depending on the sintering pressure. A series of Ta1-xB2 (0 ≤ x ≤ 0.24) was also synthesized under high pressure to examine a special effect of high-pressure synthesis.
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