Insights on the Stability and Cationic Nonstoichiometry of CuFeO₂ Delafossite

CuFeO₂, the structure prototype of the delafossite family, has received renewed interest in recent years. Thermodynamic modeling and several experimental Cu–Fe–O system investigations did not focus specifically on the possible nonstoichiometry of this compound, which is, nevertheless, a very important optimization factor for its physicochemical properties. In this work, through a complete set of analytical and thermostructural techniques from 50 to 1100 °C, a fine reinvestigation of some specific regions of the Cu–Fe–O phase diagram under air was carried out to clarify discrepancies concerning the delafossite CuFeO₂ stability region as well as the eutectic composition and temperature for the reaction L = CuFeO₂ + Cu₂O. Differential thermal analysis and Tammann’s triangle method were used to measure the liquidus temperature at 1050 ± 2 °C with a eutectic composition at Fe/(Cu + Fe) = 0.105 mol %. The quantification of all of the present phases during heating and cooling using Rietveld refinement of the high-temperature X-ray diffraction patterns coupled with thermogravimetric and differential thermal analyses revealed the mechanism of formation of delafossite CuFeO₂ from stable CuO and spinel phases at 1022 ± 2 °C and its incongruent decomposition into liquid and spinel phases at 1070 ± 2 °C. For the first time, a cationic off-stoichiometry of cuprous ferrite CuFe_1–yO_2−δ was unambiguous, as evidenced by two independent sets of experiments: (1) Electron probe microanalysis evidenced homogeneous micronic CuFe_1–yO_2−δ areas with a maximum y value of 0.12 [i.e., Fe/(Cu + Fe) = 0.47] on Cu/Fe gradient generated by diffusion from a perfect spark plasma sintering pristine interface. Micro-Raman provided structural proof of the existence of the delafossite structure in these areas. (2) Standard Cu additions from the stoichiometric compound CuFeO₂ coupled with high-temperature X-ray diffraction corroborated the possibility of obtaining a pure Cu-excess delafossite phase with y = 0.12. No evidence of an Fe-rich delafossite was found, and complementary analysis under a neutral atmosphere shows narrow lattice parameter variation with an increase of Cu in the delafossite structure. The consistent new data set is summarized in an updated experimental Cu–Fe–O phase diagram. These results provide an improved understanding of the stability region and possible nonstoichiometry value of the CuFe_1–yO_2−δ delafossite in the Cu–Fe–O phase diagram, enabling its optimization for specific applications.