Comparing the Pressure-Induced Structural Behavior of CuCr2O4 and CuCr2Se4 Spinets

We have conducted high-pressure measurements on the CuCr2O4 and CuCr2Se4 spinels to unravel the structural systematics of these materials under compression. Our studies have revealed diverse structural behavior in these two compounds. In particular, CuCr2O4 retains its ambient-pressure I4(I)/amd structure up to 50 GPa. Close inspection of the lattice and interatomic parameters reveals a compressibility change near 23 GPa, which is accompanied by an expansion of the apical Cr-O bond distances. We speculate that an outer Cr3+ 3d orbital reorientation might be at play in this system, manifesting as the change in compressibility at that pressure point. On the other hand, CuCr2Se4 undergoes a structural transformation from the starting Fd (3) over barm phase toward a monoclinic structure initiated at similar to 8 GPa and completed at similar to 20 GPa. This high-pressure behavior resembles that of ZnCr2Se4, and it appears that, unlike similar chalcogenide Cr spinels, steric effects take a leading role in this pressure-induced Fd (3) over barm -> monoclinic transition. Close comparison of our results with the reported literature yields significant insights behind the pressure-induced structural systematics of this important family of materials, thus both allowing for the careful manipulation of the structural/physical properties of these systems by strain and promoting our understanding of similar pressure-induced effects in relevant systems.