Nanocubes and Cuboctahedra: Entropic Stabilization Versus Thermal Segregation in Rock Salt‐ and Spinel‐Type Multinary Oxides from Spray‐Flame Synthesis

Multinary oxides with ≥5 cations, often termed high‐entropy oxides (HEOs), are attractive for energy applications for their compositional tunability. HEOs are frequently associated with entropy‐driven stabilization, where high temperature and configurational entropy offset mixing enthalpies. Preserving entropy‐stabilized structures requires rapid cooling, making synthesis challenging, especially for nanomaterials. To investigate whether spray‐flame synthesis (SFS), providing flame temperatures >2300 K and cooling rates up to 10 6 K s −1 , meets these requirements and to critically assess the entropy‐stabilization concept, three model systems were synthesized: the rock salt‐type Co 0.2 Cu 0.2 Mg 0.2 Ni 0.2 Zn 0.2 O (RS1) and two spinels featuring tetrahedral and octahedral sites, constraining the effective entropy, the known (Co 0.2 Cu 0.2 Fe 0.2 Mn 0.2 Ni 0.2 ) 3 O 4 (SP1) and the novel seven‐cation (Co 0.2 Cu 0.2 Fe 0.2 Mn 0.2 Ni 0.1 Mg 0.05 Al 0.05 ) 3 O 4 (SP2). All samples formed faceted nanocubes and cuboctahedra (~10 nm), a morphology rarely observed for materials from gas‐phase syntheses. Hot‐stage X‐ray diffraction revealed entropy‐stabilization for RS1: the single‐phase rock salt transiently segregated CuO, and re‐formed a solid solution above 1120 K. In contrast, as‐synthesized SP1 contained segregated NiO, whereas as‐synthesized SP2 was phase‐pure; upon heating, NiO dissolved into the spinel lattice while CuO segregated continuously in both spinels. The Ni 2+ /Cu 2+ substitution reflects site competition and the enthalpic penalty of Jahn–Teller‐active Cu 2+ , while its incorporation in the as‐synthesized products as observed for SP2 highlights that SFS can access phases otherwise unstable at room temperature. Electrochemical measurements reveal that RS1 exhibits promising oxygen evolution reaction activity under alkaline conditions.