Near zero thermal expansion and electrocatalysis for quadruple perovskite oxides CaCu₃Fe_4−xMn_xO₁₂

Abstract

We report structural and electrochemical properties of Fe–Mn-mixed quadruple perovskite oxides, CaCu₃Fe_4−xMn_xO₁₂ (x = 0–4). Polycrystalline samples of CaCu₃Fe_4−xMn_xO₁₂ in the entire composition range were successfully obtained by utilizing high-pressure and high-temperature conditions of 12 GPa and 1273 K, respectively. X-ray diffraction study demonstrated that all the samples crystallized in the AA′₃B₄O₁₂-type cubic quadruple perovskite structure at room temperature, and the lattice constant a changed nonlinearly with Mn content x; a slightly increased from ∼7.31 Å (x = 0) to ∼7.32 Å (x = 0.5), almost retained at ∼7.32 Å (x = 1), and monotonically decreased to ∼7.21 Å (x = 4). X-ray absorption spectroscopy revealed the valence state transitions with Mn⁴⁺ doping from 3Cu^2.4+ + 4Fe^3.7+ (x = 0) to 3Cu^2.67+ + 3Fe^3.33+ + Mn⁴⁺ (x = 1), causing the elongations of (Fe,Mn)–O bonds and unusual lattice expansions between x = 0 and 1 at room temperature. The temperature-variable X-ray diffraction data displayed a negative thermal expansion (NTE) at temperatures between 230 and 330 K for x = 1, as previously reported for x = 0.5. In contrast, a near-zero thermal expansion was observed in a wide temperature range between 90 and 350 K for x = 1.5, which originates from the suppression of the intermetallic charge transfer between Cu and Fe with further Mn doping. An electrochemical study demonstrated that the catalytic activity for oxygen evolution reaction roughly decreased with x but its trend was nonmonotonic; the activity was heavily suppressed in the Fe-rich side (x = 0.5–1) but retained at almost constant level in the Mn-rich side (x = 2–3.5). The nonsystematic composition dependence of properties observed in CaCu₃Fe_4−xMn_xO₁₂ was attributed to the unusual valence state transformations with Mn doping, proposing a rich variety of properties of solid-solution in transition metal oxides.