  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 K 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.
