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Letters to Nature

Nature 416, 155-158 (14 March 2002) | doi:10.1038/416155a; Received 16 November 2001; Accepted 29 January 2002

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Formation of isomorphic Ir3+ and Ir4+ octamers and spin dimerization in the spinel CuIr2S4

Paolo G. Radaelli1, Y. Horibe2,3,4, Matthias J. Gutmann1, Hiroki Ishibashi4,5, C. H. Chen2, Richard M. Ibberson1, Y. Koyama3, Yew-San Hor4, Valery Kiryukhin4 & Sang-Wook Cheong4

  1. ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK
  2. Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974, USA
  3. Department of Materials Science and Engineering and Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Shinjuku-ku, Tokyo 169, Japan
  4. Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
  5. Department of Materials Science, Osaka Prefecture University, Sakai 599, Osaka, Japan

Correspondence to: Paolo G. Radaelli1 Correspondence and requests for materials should be addressed to P.G.R. (e-mail: Email: p.g.radaelli@rl.ac.uk).

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Inorganic compounds with the AB2X4 spinel structure have been studied for many years, because of their unusual physical properties. The spinel crystallographic structure, first solved by Bragg in 19151, has cations occupying both tetrahedral (A) and octahedral (B) sites. Interesting physics arises when the B-site cations become mixed in valence. Magnetite (Fe3O4) is a classic and still unresolved example, where the tendency to form ordered arrays of Fe2+ and Fe3+ ions competes with the topological frustration of the B-site network2. The CuIr2S4 thiospinel is another example, well known for the presence of a metal–insulator transition at 230 K with an abrupt decrease of the electrical conductivity on cooling accompanied by the loss of localized magnetic moments3, 4, 5, 6, 7. Here, we report the determination of the crystallographic structure of CuIr2S4 below the metal–insulator transition. Our results indicate that CuIr2S4 undergoes a simultaneous charge-ordering and spin-dimerization transition—a rare phenomenon in three-dimensional compounds. Remarkably, the charge-ordering pattern consists of isomorphic octamers of Ir83+S24 and Ir84+S24 (as isovalent bi-capped hexagonal rings). This extraordinary arrangement leads to an elegant description of the spinel structure, but represents an increase in complexity with respect to all the known charge-ordered structures, which are typically based on stripes, slabs or chequerboard patterns.