Coupled phase transitions in crystalline solids with extreme chemical disorder
Structural phase transitions often couple to magnetic and electronic degrees of freedom, enabling emergent phenomena in solids. In high-entropy oxides (HEOs), which typically stabilize in highly symmetric cubic phases, such transitions are considered rare due to the extreme chemical disorder-analogous to the behavior observed in high-entropy alloys. This raises a fundamental question: can the rich physics of coupled phase transitions persist in such disordered systems? Here, we show that targeted design of compositionally complex oxides (CCOs) can trigger symmetry-lowering transitions, with spinel-type materials serving as a representative case. For instance, [Mn$_{0.2}$Co$_{0.2}$Ni$_{0.2}$Cu$_{0.2}$Zn$_{0.2}$]Cr$_2$O$_4$, having two Jahn-Teller (J-T) active ions, undergoes two successive coupled structural transitions upon cooling: an orbital-driven transition at 100 K and a magnetism-driven transition at 40 K. Systematic substitution of $A$-site cations reveals that both Ni and Cu are essential for these transitions. Element specific local structure investigations uncover distinct and opposing local distortions around Ni and Cu, while Mn, Co, and Zn remain largely undistorted. These results establish that CCOs can host coupled phase transitions through `cooperation via competition' among local distortions in a chemically disordered lattice. This discovery expands the design principles for complex oxides, introducing a new paradigm for tuning structural and functional properties in high-entropy systems beyond conventional symmetry constraints.