X-ray-Induced Disordering of the Dimerization Pattern and Apparent Low-Temperature Enhancement of Lattice Symmetry in Spinel CuIr 2 S 4

At low temperatures, spinel ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ is a charge-ordered spin-dimerized insulator with triclinic lattice symmetry. We find that x rays induce a structural transition in which the local triclinic structure is preserved, but the average lattice symmetry becomes tetragonal. These structural changes are accompanied by a thousandfold reduction in the electrical resistivity. The transition is persistent, but the original state can be restored by thermal annealing. We argue that x-ray irradiation disorders the lattice dimerization pattern, producing a state in which the orientation of the dimers is preserved, but the translational long-range order is destroyed.