Valence properties of Cu and Ru in titanium-substituted LnCu3Ru4O12+δ (Ln = La, Pr, Nd) investigated by XANES and TGA

Abstract

In the solid-solution series La_yCu₃Ru_xTi_4−xO_12+δ (0 ≤ x ≤ 4) the Cu and Ru electronic states are highly correlated. With increasing Ru content x the system properties change from a paramagnetic insulator with colossal dielectric constant to a heavy-fermion metal. To further elucidate the occurring phase transitions, the valences of Cu and Ru have been investigated utilizing XANES measurements at the Cu-K and the Ru-K absorption edges. It was found that the Ru oxidation number is close to +4 in all samples, while the Cu valence linearly decreases from +2 for the titanate (x = 0) to +1.6 for the ruthenate (x = 4). Additional thermogravimetric measurements have been used to determine the oxygen content and rather high oxygen excesses up to δ ≈ 0.7 (for x = 0.5) were obtained. The additional oxygen for x < 2 is required to compensate the constant Ru +4 valence. Our findings are in accordance with the reported phase transitions of the magnetic and transport properties. Both the valence shift and the shapes of the absorption edges suggest a change from localized to itinerant character of the Cu electronic states with increasing x, while the Ru electrons remain localized. Analogous results concerning the valences were found for the Pr_yCu₃Ru_xTi_4−xO_12+δ and Nd_yCu₃Ru_xTi_4−xO_12+δ solid-solution series.