Thermodynamic and Structural Properties of CuCrO₂ and CuCr₂O₄: Experimental Investigation and Phase Equilibria Modeling of the Cu–Cr–O System

Abstract

This work considers the equilibria between the stable phases of the Cr–Cu–O ternary chemical system at atmospheric pressure and in a large temperature range that includes the high-temperature liquid phase. Based on a thorough evaluation of literature data, we performed complementary experimental investigations of solid-phase properties (especially the mixed oxides) by implementation of spark plasma sintering and microanalysis, high-temperature X-ray diffraction combined with Rietveld analysis, differential thermal analysis, and thermogravimetry. We show that neither the spinel CuCr2O4 nor the delafossite CuCrO2 phases exhibit significant cationic nonstoichiometry. We provide an assessment of the thermodynamic functions of the spinel up to 1200 K, taking into account the α/β phase transition. We propose, for the first time, a consistent thermodynamic description of the Cr–Cu–O system based on the Calphad method, allowing the computation of reactions and phase equilibria, including the high-temperature liquid phase described by the modified quasichemical model.

CuCr2O4 particle growth and evolution across sol–gel routes and calcination profiles

“…This mass gain points to reaction A3 [35] where delafossite, which was found in the ash XRD patterns, transforms into spinel. This reaction has been found to be thermodynamically favourable above 500°C [33]. An exothermic peak observed on cooling at 538°C (not shown in Figure A2), can be attributed to an undercooled transition from alpha phase to beta phase, which has been reported at 583°C [33].…”

“…The Pechini sample peaks are higher in intensity, likely due to the more complete combustion of the ash. Materials produced by both methods contained a phase with peaks at 31.32°, 62.25°; additionally, the Pechini sample displayed a peak at 71.32°. While these three peaks might be assigned to CuCrO 2 (ICDD PDF#00-039-0247), its formation is not thermodynamically favourable [33]. CuCrO 4 has been identified in other studies as an intermediate phase in spinel formation, but does not match these peaks [21,34].…”

“…This mass gain points to reaction A3 [35] where delafossite, which was found in the ash XRD patterns, transforms into spinel. This reaction has been found to be thermodynamically favourable above 500°C [33]. …”

Effect of Cr Addition on Cu–Mn Spinel/Delafossite Redox Couples for Medium-High Temperature Thermochemical Energy Storage

“…The mass change, reduction starting temperature, and oxidation ending temperature of each redoxable sample are summarized in Table 2. Importantly, the reduction starting temperatures of the Cr-doped material were slightly lower than those of pure CuMn 2 O 4 , which corresponds to the Cu−Cr−O (Figure S1) 27 and Cu−Mn−O (Figure S2) 15,28 phase diagrams that CuMn 2 O 4 requires a higher temperature (920 °C) for reduction than CuCr 2 O 4 (782 °C). However, the oxidation ending temperature exhibited a larger variation as the amount of Cr increased, which led to a larger temperature hysteresis.…”

Electrical properties of strained off-stoichiometric Cu–Cr–O delafossite thin films

“…The limited availability of previously reported values for elastic constants in the literature adds a further challenge to the present study. There are only two reports for the CTE of stoichiometric CCO (α CCO = 10.26 [26] and 7.5(1) ppm K −1 [27]). Both correspond to pellets of standard CCO stoichiometry.…”