  The nickelate compound Y_{2}BaNiO_{5} is a spin-1 Haldane-gap
antiferromagnet. The compound is doped with holes on replacing the off-chain
Y^{3+} ions by Ca^{2+} ions. Inelastic neutron scattering (INS) experiments
reveal the existence of sub-gap states on doping. A recent INS experiment
provides evidence for an incommensurate double-peaked structure factor S(q)
corresponding to the sub-gap states. In this paper, we formulate a microscopic
theory for the origin of the incommensurate peak.
