Insights on the Stability and Cationic Nonstoichiometry of CuFeO₂ Delafossite

Abstract

CuFeO 2 , 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 CuFeO 2 stability region as well as the eutectic composition and temperature for the reaction L = CuFeO 2 + Cu 2 O. 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 CuFeO 2 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−y O 2−δ was unambiguous, as evidenced by two independent sets of experiments: (1) Electron probe microanalysis evidenced homogeneous micronic CuFe 1−y O 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 CuFeO 2 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−y O 2−δ delafossite in the Cu−Fe−O phase diagram, enabling its optimization for specific applications.

Thickness-dependent microstructural properties of heteroepitaxial (00.1) CuFeO2 thin films on (00.1) sapphire by pulsed laser deposition

“…[11][12][13][14][15][16][17][18] High-quality epitaxial Cu 1+ Fe 3+ O 2 thin films without the presence of impurities containing Cu 2+ ions would be a preferable platform to study the fundamentals of physical and chemical properties of this material. However, the heteroepitaxial growth of high-quality and impurity-free CuFeO 2 thin film turns out to be a formidable challenge owing to the complex thermodynamics and nonstoichiometry, and the coexistence of multiple phases in the Cu-Fe-O solid solution [19][20][21][22] as well as the metastable nature of the Cu 1+ cation in complex oxide compounds.…”

Microstructural and transport properties of Mg doped CuFeO2 thin films: A promising material for high accuracy miniaturized temperature sensors based on the Seebeck effect

“…Fig. 2 shows the phase diagram of the CueFeeO system calcu lated using Factsage software from the thermodynamic model published Shishin et al [49] and by Schorne Pinto et al [48] for an oxygen pressure pO 2 10 À5 atm which corresponds to the pO 2 measured in the furnace during the annealing in this work. Based on previous experimental tests done under air and argon atmo sphere, the copper solubility in the delafossite structure is extrap olated in this plot at x 0.47.…”

“…Trofimov [47] have already reported such copper non stoechiometry in undoped copper iron delafossite, but without any structural proof of the presence of the pure delafossite phase. Schorne Pinto et al [48] have recently demonstrated that the delafossite structure can accept a copper excess up to 14% in air, i.e. up to x 0.53 before any precipitation of copper single oxide.…”

“…Phase diagram of the Cu Fe O system calculated with Factsage software for an oxygen pressure of 8.6 Pa (pressure in the furnace) by using the data from Shishin et al[49] and Pinto et al[48] publications. GIXRD patterns of 300 nm thick CuFeO 2 :Mg thin films annealed at various temperature under vacuum.…”

Crystal structure and hyperfine interactions of delafossite (CuFeO2) synthesized hydrothermally

“…Moreover, above 673 K, the structure of delafossite CuFeO 2 decomposes according to the following reaction: 2Cu I Fe III O 2 + 1 2 O 2 ! Cu II Fe III 2 O 4 + CuO (Schorne-Pinto et al, 2019). In most cases, during the synthesis of copper ferrite by any method, the 3R polytype (rhombohedral phase), with more or less content of the 2H polytype (hexagonal phase), is formed.…”