Valence properties of Cu and Ru in titanium-substituted LnCu₃Ru₄O_12+δ (Ln = La, Pr, Nd) investigated by XANES and TGA

Abstract

In the solid-solution series La(y)Cu(3)RuxTi(4-x)O(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(y)Cu(3)RuxTi(4-x)O(12+δ) and Nd(y)Cu(3)RuxTi(4-x)O(12+δ) solid-solution series.

A Porous Pyrochlore Y2[Ru1.6Y0.4]O7–δ Electrocatalyst for Enhanced Performance towards the Oxygen Evolution Reaction in Acidic Media

“…The absorption energy values were determined to be 22 131.5 eV for Y 2 [Ru 1.6 Y 0.4 ]O 7− δ and 22 130.1 eV for Y 2 Ru 2 O 7− δ , in good agreement with those reported previously for Ru‐based pyrochlore oxides . The average Ru oxidation states were calculated to be +4.1 for Y 2 [Ru 1.6 Y 0.4 ]O 7− δ and +3.3 for Y 2 Ru 2 O 7− δ (Figure b), based on the linear relationship between the absorption energy and the valence state of Ru, where energy increment is about 1.9 eV per valence state . Based on the XANES data, we obtained the number of lattice vacancy for oxygen to be δ =0.12.…”

Fabricating Single‐Atom Catalysts from Chelating Metal in Open Frameworks

“…As shown in Figure a, the XANES curve of Fe SAC/N‐C exhibits near‐edge absorption energy close to those of Fe 2 O 3 and FeOOH, implying that Fe single atoms carry positive charges close to +3. To identify the oxidation state of Fe atom in Fe SAC/N‐C, we analyzed the absorption energy ( E 0 ), which was determined from the first maximum in the first‐order derivative . As shown in Table S1 in the Supporting Information, the E 0 value of Fe SAC/N‐C is 7126.4 eV, very close to the E 0 values of Fe 2 O 3 (7126.9 eV) and FeOOH (7128.0 eV), indicating the predominance of Fe 3+ in the Fe SAC/N‐C.…”

A Porous Pyrochlore Y2[Ru1.6Y0.4]O7–δ Electrocatalyst for Enhanced Performance towards the Oxygen Evolution Reaction in Acidic Media

“…[7,10] Thea verage Ru oxidation states were calculated to be + 4.1 for Y 2 [Ru 1.6 Y 0.4 ]O 7Àd and + 3.3 for Y 2 Ru 2 O 7Àd (Figure 4b), based on the linear relationship between the absorption energy and the valence state of Ru, where energy increment is about 1.9 eV per valence state. [10,24] Based on the XANES data, we obtained the number of lattice vacancy for oxygen to be d = 0.12. This vacancy is the result of mixed valances of Ru 4+/5+ in the Y 2 [Ru 1.6 Y 0.4 ]O 7Àd structure,r esulting in the aforementioned OER performance that can be explained by the LOM model.…”