Solid-State Chemistry of Cuprous Delafossites: Synthesis and Stability Aspects

Abstract

Cuprous delafossites exhibit exceptional electrical, magnetic, optical, and catalytic properties. Through the application of a battery of in situ and ex situ characterization methods complemented by density functional theory (DFT) calculations, we gathered an in-depth understanding of the synthesis of CuMO2 (M = Al, Cr, Fe, Ga, Mn) by the solid-state reaction of Cu2O and M2O3 and of their stability against oxidative disproportionation to CuM2O4 and CuO. TGA-DTA and XRD studies of the synthesis revealed that the nature of the M3+ cation strongly impacts (i) the formation temperature of the delafossite phase, which occurred at a much lower temperature for CuCrO2 than for the other metals (1073 versus 1273–1423 K), (ii) the mechanism of formation of the CuMO2 in different atmospheres, which was found to comprise up to four steps in air and a single step in N2, and (iii) the kinetics of the process, which could be significantly accelerated upon mechanochemical activation of the precursors by ball milling. The identification of unstable intermediate phases and, thus, a proper description of the synthesis mechanism was only possible by the application of in situ XRD. Electron microscopy, nitrogen sorption, and mercury porosimetry analyses of the precursor oxide mixtures at different stages of the synthesis in air revealed that particle agglomeration took place prior to the solid-state reactions forming the intermediate spinel phase and the delafossite, respectively, and that these led to a substantial drop in porosity and specific surface area. On the basis of XRD and He pycnometry, the resulting CuMO2 samples exhibit pure delafossite phase with rhombohedral structure (R3̅m), except for CuMnO2 which features a monoclinic structure (C2/m). Upon heating in air, CuCrO2 retained its structure up to 1373 K, while all other delafossites decomposed, CuAlO2 at 1073 K, CuGaO2 at 873 K, CuFeO2 at 773 K, and CuMnO2 at 673 K. The DFT-calculated surface phase diagram of CuCrO2 and CuAlO2 indicated that, at elevated oxygen pressures, the terminations with 1/2 and 0 ML of Cu are the most stable for the (0001) facet. The formation enthalpy for interstitial oxygen species in the bulk is endothermic for both delafossites, while that for oxygen insertion in subsurface layers of these terminations is still endothermic for CuCrO2 but slightly exothermic for CuAlO2. These results provide an improved understanding of the chemistry of these mixed oxides, enabling their optimization for specific applications.