  The transition--metal oxide CaCu$_3$Ru$_4$O$_{12}$ with perovskite--type
structure shows characteristic properties of an intermediate--valence system.
The temperature--dependent susceptibility exhibits a broad maximum around $150
- 160$ K. At this temperature, neutron powder diffraction reveals a small but
significant volume change whereby the crystal structure is preserved. Moreover,
the temperature--dependent resistivity changes its slope. NMR Knight shift
measurements of Ru reveal a cross--over from high temperature paramagnetic
behavior of localized moments to itinerant band states at low temperatures.
Additional density--functional theory calculations can relate the structural
anomaly with the $d$--electron number. The different experimental and
calculational methods result in a mutually consistent description of
CaCu$_3$Ru$_4$O$_{12}$ as an intermediate--valent system in the classical sense
of having low--energy charge fluctuations.
