Phase Selectivity and Stability in Compositionally Complex Nano (nA_1/n )Co₂O₄

A family of compositionally complex spinels with the formula(nA(1/n))Co2O4 (A = combinations of Mg, Cr, Mn, Fe, Co, Ni, Cu, and Zn) was synthesized using a low-temperature softtemplating method. The phase selectivity and the temperature stability window for the series were found to depend strongly upon the A-site composition and only modestly on the number of elements (n) present on the A-site. Select control reactions and in situ high-temperature X-ray diffraction (XRD) uncovered a propensity for temperature-activated de-mixing for compositions containing Mg, Ni, Mn, and Fe. The A-site cations exhibit spatially heterogeneous distributions in the as-formed spinels, which diminish with intermediate thermal annealing, as shown by scanning transmission electron microscopy (STEM)/energy dispersive spectroscopy (EDS) and X-ray line profile analysis. The single spinel phases obtained are metastable, separating into a mix of impurity phases and multiple spinel phases with higher temperature annealing. Furthermore, we demonstrate that a '' continuous lattice '' parameterization of the compositionally complex oxide structure provides a rapid means by which to examine the heterogeneity of the cation distribution through full profile refinement. The demonstrated tunability of the cation distribution or clustering in these compositionally complex spinels via thermodynamic levers affords interesting opportunities for rational design of functional materials.