  A pronounced Curie-like upturn of the magnetic susceptibility chi(T) of the
quasi one-dimensional spin chain compound Ba2V3O9 has been found recently.
Frequently this is taken as a signature for a staggered field mechanism due to
the presence of g-factor anisotropy and Dzyaloshinskii-Moriya interaction. We
calculate this contribution within a realistic structure of vanadium 3d- and
oxygen 2p-orbitals and conclude that this mechanism is far too small to explain
experimental results. We propose that the Curie term is rather due to a
segmentation of spin chains caused by broken magnetic bonds which leads to
uncompensated S=1/2 spins of segments with odd numbers of spins. Using a
finite-temperature Lanczos method we calculate their effective moment and show
that ~1% of broken magnetic bonds is sufficient to reproduce the anomalous
low-T behavior of chi(T) in Ba2V3O9.
