Crystallographic origin of pressure-induced bond order in the frustrated spinel CuIr₂S₄

CuIr₂S₄ is a frustrated pyrochlore spinel in which pressure reshapes competing charge- and bond-ordered states. Here we determine the crystallographic nature of its pressure-induced ordered phases up to about 25 GPa by high-pressure synchrotron powder X-ray diffraction combined with first-principles-guided structural analysis. Pressure first stabilizes the ambient-pressure low-temperature ordered state to room temperature and then induces a distinct high-pressure phase. Using single-phase datasets collected at 19.8 and 25.7 GPa, we perform full-atom refinements that consistently establish the Phase-IV symmetry and Ir-Ir short-bond topology. The resulting Phase IV pattern differs from Phase II and is not captured by a simple orbitally induced Peierls picture. Instead, it is consistent with an Anderson-type local tetrahedron rule on the pyrochlore lattice. Total-energy calculations reproduce a pressure-induced reversal in the relative stability of the Phase-II- and Phase-IV-type structural models.