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The authors investigate how copper concentration and Jahn-Teller active sites govern out-of-plane lattice parameters in coherent MgCoNiCuZnO-derived high-entropy oxide thin films grown on MgO. Understanding these structural drivers is crucial because lattice strain dictates electronic, magnetic, and catalytic properties in complex multicomponent oxides. They show that while lattice parameters scale linearly with Cu content at 500 °C, they decrease nonlinearly at 300 °C—driving up to a 2.6% lattice divergence in equimolar films through coupled Jahn-Teller distortions and cobalt valence shifts.
Lattice tuning in high-entropy oxides is not dictated by cobalt valence alone: growth temperature-dependent Jahn-Teller distortions from copper drive a massive 2.6% swing in lattice parameters.
Here, we isolate how Cu concentration and Jahn-Teller distortions control the out-of-plane lattice parameter of pseudomorphic MgCoNiCuZnO-derived high-entropy oxide films on MgO. We prepare nine compositions spanning 0-20% Cu, verify target composition and phase by X-ray fluorescence and diffraction, and grow films by pulsed laser deposition across multiple substrate temperatures. At 500 {\deg}C, the lattice parameter increases nearly linearly with Cu content; at 300{\deg}C, it decreases nonlinearly, producing a 2.6% difference between temperature regimes for equimolar MgCoNiCuZnO at our growth conditions. We attribute this change in lattice parameter to temperature-dependent coupling between Cu-driven Jahn-Teller distortions and Co valence change. While prior work identifies Co valence as the primary driver of the MgCoNiCuZnO lattice response with temperature; our results establish Cu Jahn-Teller distortion as an equally important structural control parameter.