  Neutron diffraction measurements have been performed on the cubic compound
PrPb3 in a [001] magnetic field to examine the quadrupolar ordering.
Antiferromagnetic components with q=(1/2+-d 1/2 0), (1/2 1/2+-d 0) (d~1/8) are
observed below the transition temperature TQ (0.4 K at H=0) whose amplitudes
vary linear with H and vanish at zero field, providing the first evidence for a
modulated quadrupolar phase. For H<1 T, a non-square modulated state persists
even below 100 mK suggesting quadrupole moments associated with a Gamma3
doublet ground state to be partially quenched by hybridization with conduction
electrons.
