Solid-State Chemistry of Cuprous Delafossites: Synthesisand Stability Aspects.

Cuprous delafossites exhibit exceptionalelectrical, magnetic,optical, and catalytic properties. Through the application of a batteryof in situand ex situcharacterizationmethods 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 M2O3and of their stability againstoxidative disproportionation to CuM2O4and CuO.TGA-DTA and XRD studies of the synthesis revealed that the natureof the M3+cation strongly impacts (i)the formation temperature of the delafossite phase, which occurredat a much lower temperature for CuCrO2than for the othermetals (1073 versus 1273–1423 K), (ii) themechanism of formation of the CuMO2in different atmospheres,which was found to comprise up to four steps in air and a single stepin N2, and (iii) the kinetics of the process,which could be significantly accelerated upon mechanochemical activationof the precursors by ball milling. The identification of unstableintermediate phases and, thus, a proper description of the synthesismechanism was only possible by the application of in situXRD. Electron microscopy, nitrogen sorption, and mercury porosimetryanalyses of the precursor oxide mixtures at different stages of thesynthesis in air revealed that particle agglomeration took place priorto the solid-state reactions forming the intermediate spinel phaseand the delafossite, respectively, and that these led to a substantialdrop in porosity and specific surface area. On the basis of XRD andHe pycnometry, the resulting CuMO2samples exhibit puredelafossite phase with rhombohedral structure (R3̅m), except for CuMnO2which features a monoclinicstructure (C2/m). Upon heating inair, CuCrO2retained its structure up to 1373 K, whileall other delafossites decomposed, CuAlO2at 1073 K, CuGaO2at 873 K, CuFeO2at 773 K, and CuMnO2at 673 K. The DFT-calculated surface phase diagram of CuCrO2and CuAlO2indicated that, at elevated oxygenpressures, the terminations with 1/2 and 0 ML of Cu are the most stablefor the (0001) facet. The formation enthalpy for interstitial oxygenspecies in the bulk is endothermic for both delafossites, while thatfor oxygen insertion in subsurface layers of these terminations isstill endothermic for CuCrO2but slightly exothermic forCuAlO2. These results provide an improved understandingof the chemistry of these mixed oxides, enabling their optimizationfor specific applications. [ABSTRACT FROM AUTHOR]