X-ray-induced disordering of the dimerization pattern and apparent low-temperature enhancement of lattice symmetry in spinelCuIr2S4

Abstract

At low temperatures, spinel CuIr2S4 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.PACS numbers: 61.10.Nz, 72.80.Ga, 61.80.Cb Spinel compounds AB 2 X 4 (X is O, S, Se, or Cl) have attracted much attention over the last decade because they exhibit a large variety of interesting ground states, including superconductivity, cooperative antiferromagnetism, heavy fermion, and charge-ordered and spin-dimerized states [1,2]. The panoply of different properties exhibited by spinels results from the interplay of Coulomb interactions, effects of frustrated magnetism, and electron-lattice interaction. The corner sharing tetrahedral network of B sites in the spinel structure can accommodate a large number of different charge ordering patterns. In fact, some of the most complex charge-ordering patterns reported to date are found in spinel compounds [3,4]. The same tetrahedral network gives rise to geometric magnetic frustration when the ions occupying the B site are magnetic. The electronic and magnetic states realized in such a complex environment are often multi-degenerate and strongly fluctuating. Because of these complexity, a number of properties of the spinels remain poorly understood. Spinels, therefore, are important subjects of research in the physics of strongly correlated materials. In addition, some of these compounds, such as the lithium manganese spinels used in battery cathodes and the ferrites used in microwave applications, are of substantial technological importance.Chalcogenide spinel CuIr 2 S 4 has attracted attention because this compound undergoes a sharp metalinsulator transition at T MI ≈230 K [5]. The lowtemperature insulating state is nonmagnetic. NMR and photoemission experiments have shown that the Cu ion is monovalent in the insulating phase, and therefore the nominal valence of the iridium atoms is 3.5 [6]. It was proposed that charge ordering of Ir 3+ (S=0) and Ir 4+(S=1/2) ions is the origin of the metal-insulator transition in CuIr 2 S 4 [5], and that some kind of spin dimerization is responsible for the nonmagnetic nature of the insulating phase. Recent experimental determination of the lowtemperature structure [3] has provided very strong evidence that this scenario is correct. Specifically, these experiments have shown that the Ir sublattice consists of two types of Ir bi-capped hexagonal rings, which were described as Ir

X-ray-induced electrical conduction in the insulating phase of thiospinel CuIr2S4

“…In addition, the result shows clearly the negative temperature coefficient for the resistance after the irradiation. Thus, the tetragonal phase cannot be still called metallic, but it is semiconducting.The obtained resistivity of the x-ray-induced phase, 65 Ω·cm, is somewhat larger than the value 2 Ω·cm reported by Ishibashi et al [9]. However, the decrease of the resistance by irradiation, more than five decades, is much larger in this work.…”

“…Ishibashi et al [9] concluded that the observed tetragonal phase intrinsically has the triclinic lattice distortion in the short-range scale from the analysis of the diffuse reflections.…”

“…According to Ishibashi et al [9], a considerable number of Ir 4+ ions still form dimers in the x-ray-induced phase. However, the presence of non-dimerized Ir 4+ is not excluded.…”

“…Apparently, the difference could be attributed to the quality of the sample. Although it is not readily concluded which is better, it is to be mentioned that the resistivity of the insulating phase at low temperatures decreases by introducing air while the preparation process [10].Ishibashi et al [9] concluded that the observed tetragonal phase intrinsically has the triclinic lattice distortion in the short-range scale from the analysis of the diffuse reflections.They attributed the electrical conductance in the irradiated state to charge disorder at the boundaries of the triclinic grains. It is unclear, however, how such disordered state leads to the electric conduction.…”

“…Ishibashi et al [9] showed that the transition to tetragonal with the distinction of the superstructure spots is induced by irradiation with x-ray at low temperatures. In addition, electrical resistance was found to decrease at the same time.…”