  At ambient temperatures, CeRuSn exhibits an extraordinary structure with a
coexistence of two types of Ce ions in a metallic environment, namely trivalent
Ce3+ and intermediate valent Ce(4-x)+. Charge ordering produces a doubling of
the unit cell along the c-axis with respect to the basic monoclinic CeCoAl type
structure. Below room temperature, a phase transition with very broad
hysteresis has been observed in various bulk properties like electrical
resistivity, magnetic susceptibility, and specific heat. The present x-ray
diffraction results show that at low temperatures the doubling of the CeCoAl
type structure is replaced by an ill-defined modulated ground state. In this
state, at least three different modulation periods compete, with the dominant
mode close to a tripling of the basic cell. The transition is accompanied by a
significant contraction of the c axis. XANES data suggest that the average Ce
valence remains constant, thus the observed c axis contraction is not due to
any valence transition. We propose a qualitative structure model with modified
stacking sequences of Ce3+ and Ce(4-x)+ layers in the various modulated phases.
Surprisingly, far below 100 K the modulated state is sensitive to x-ray
irradiation at photon fluxes available at a synchrotron. With photon fluxes of
order 10E12/s, the modulated ground state can be destroyed on a timescale of
minutes and the doubling of the CeCoAl cell observed at room temperature is
recovered. The final state is metastable at 10 K. Heating the sample above 60 K
again leads to a recovery of the modulated state. Thus, CeRuSn exhibits both
thermally and x-ray induced reversible transformations of the Ce3+/Ce(4-x)+
charge ordering pattern. Such a behavior is unique among any know intermetallic
compound.
