  Sb-NMR/NQR study has revealed a formation of a pseudogap at the Fermi level
in the density of states in a valence fluctuating compound CeIrSb. The nuclear
spin-lattice relaxation rate divided by temperature, 1/T_1T has a maximum
around 300 K and decreases significantly as 1/T_1T ~ T^2, followed by a 1/T_1T
= const. relation at low temperature. This temperature dependence of 1/T_1T is
well reproduced by assuming a V-shaped energy gap with a residual density of
states at the Fermi level. The size of energy gap for CeIrSb is estimated to be
about 350 K, which is by one order of magnitude larger than those for the
isostructural Kondo semiconductors CeRhSb and CeNiSn. Despite the large
difference in the size of energy gap, CeIrSb, CeRhSb and CeNiSn are indicated
to be classified into the same group revealing a V-shaped gap due to c-f
hybridization. The temperature dependence of the Knight shift measured in a
high magnetic field agrees with the formation of this pseudogap.
