Intermediate-valence behavior of the transition-metal oxide CaCu₃Ru₄O₁₂

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The transition--metal oxide CaCu₃Ru₄O₁₂ 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 CaCu₃Ru₄O₁₂ as an intermediate--valent system in the classical sense of having low--energy charge fluctuations.