  We report on the spin densities in CeRuSn determined at elevated and at low
temperatures using polarized neutron diffraction. At 285 K, where the CeRuSn
crystal structure, commensurate with the CeCoAl type, contains two different
crystallographic Ce sites, we observe that one Ce site is clearly more
susceptible to the applied magnetic field whereas the other is hardly
polarizable. This finding clearly documents that distnictly different local
environment of the two Ce sites causes the Ce ions to split into magnetic Ce3+
and non-magnetic Ce(4-delta)+ valence states. With lowering the temperature,
the crystal structure transforms to a structure incommensurately modulated
along the c axis. This leads to new inequivalent crystallographic Ce sites
resulting in a re-distribution of spin densities. Our analysis using the
simplest structural approximant shows that in this metallic system Ce ions
co-exist in different valence states. Localized 4f states that fulfill the
third Hund's rule are found to be close to the ideal Ce3+ state (at sites with
the largest Ce-Ru interatomic distances) whereas Ce(4-delta)+ valence states
are found to be itinerant and situated at Ce sites with much shorter Ce-Ru
distances. The similarity to the famous alpha-gamma transition in elemental
cerium is discussed.
