Intermediate-valence behavior of the transition-metal oxide CaCu${}_3$Ru${}_4$O${}_{12}$

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.