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 CuIr(2)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.