  We present a mixed spin-(1/2, 5/2) chain composed of a charge-transfer salt
(4-Br-$o$-MePy-V)FeCl$_4$. We observe the entire magnetization curve up to
saturation, which exhibits a clear Lieb-Mattis magnetization plateau and
subsequent quantum phase transition towards the gapless Luttinger-liquid phase.
The observed magnetic behavior is quantitatively explained by a mixed
spin-(1/2, 5/2) chain model. The present results demonstrate a quantum
many-body effect based on quantum topology and provide a new stage in the
search for topological properties in condensed matter physics.
