Cation inversion in inverse spinel NiAl2O4 couples local coordination to macroscopic compressibility, yet its pressure evolution remains poorly constrained. Here, we quantify the inversion parameter i and oxygen positional parameter u by in situ X-ray diffraction in a diamond anvil cell. Rietveld refinements show that i increases from 0.753 at ambient pressure to 0.917 at 9.06 GPa, accompanied by a decrease of u over the same hydrostatic range. The unit-cell volume is well described by the equation of state fitted below ∼10 GPa, yielding V0 = 514.7(7)Å3, B0 = 238.53(15) GPa, and B0′ = 5.31(14). The average tetrahedral A–O bond length decreases overall with pressure, whereas the octahedral B–O bond length changes only modestly. The normalized average tetrahedral polyhedral volume decreases systematically, while the normalized octahedral polyhedral volume remains close to its ambient-pressure value within uncertainty. These results indicate that pressure-driven densification in NiAl2O4 involves coupled evolution of lattice compression, oxygen-framework relaxation, and cation redistribution, with a stronger relative response of the tetrahedral coordination environment over the investigated hydrostatic range.
Zheng et al. (Sat,) studied this question.