VOLUME 87, NUMBER 14 PHYSICAL REVIEW LETTERS 1OCTOBER 2001 Structural Transition and Pair Formation in Fe 3 O 2 BO 3 M. Mir, R. B. Guimarães, J. C. Fernandes, and M. A. Continentino Instituto de Física, Universidade Federal Fluminense, Campus da Praia Vermelha, Niterói, 24.210-340, RJ, Brazil A. C. Doriguetto, Y.P. Mascarenhas, J. Ellena, and E.E. Castellano Instituto de Física de São Carlos–USP, Caixa Postal 369, São Carlos, SP, Brazil, 13560-970 R. S. Freitas and L. Ghivelder Instituto de Física, Universidade Federal do Rio de Janeiro, CP 68528, Rio de Janeiro, 21945-970 RJ, Brazil (Received 23 May 2001; published 14 September 2001) We observe for the first time a structural phase transition in the oxyborate Fe 3 O 2 BO 3 which occurs along three leg ladders present in this material. X-ray diffraction shows that this transition at 283 K is associated with a new phase where atomic displacements occur in alternate directions perpendicular to the axis and within the plane of the ladders. Magnetic data show that these displacements lead to the formation of singlet pairs which dissociate close to the structural transition. Anomalies in the transport properties also occur close to 283 K showing that the structural transition is related to a charge ordering phenomenon in a low dimensional structure. DOI: 10.1103/PhysRevLett.87.147201 PACS numbers: 75.20.Ck, 61.10.Nz, 73.50.Fq Recently, there is an enormous interest in strongly cor- related systems in low dimensional structures [1]. Among these is the oxyborate Fe 21 2 Fe 31 O 2 BO 3 which has in its structure low dimensional units in the form of three leg lad- ders (3LL) with sites occupied by Fe ions in intermediate valence states. These ladders are responsible for its singu- lar physical properties. In this system, antiferromagnetism and paramagnetism coexist between 70 and 112 K as ob- served by Mössbauer spectroscopy [2]. The ac-magnetic susceptibility curve does not present any visible anomaly at 112 K but a sharp peak appears at 70 K [2]. The spe- cific heat curve presents a peak at each temperature and, above 112 K, it is rigorously linear with temperature up to 190 K [3]. EPR measurements show, between 10 and 300 K, only a wide single line due to Fe 21 ions [4]. The current vs tension isotherms present strong nonlinear- ity above room temperature [5]. The present Letter reports new measurements of x-ray diffraction, magnetic susceptibility, and resistivity on single crystals of Fe 3 O 2 BO 3 . Using a single crystal four circle diffractometer Enraf-Nonius, equipped with a Kappa charge-coupled device detector, we were able to observe for the first time, at 283 K, a structural transition for which the low temperature phase shows alternate displacements of the metallic ions within the columns corresponding to the crystallographic site 2 (see Figs. 1 and 2). This implies a doubling of the lattice period along the c axis and the pairing of these ions with those in adjacent columns (sites 4). The magnetization measure- ments show that the pairs form in singlet states which are progressively decoupled as temperature increases close to the structural transition. The transport measurements show an anomaly on the derivative of the resistivity close to the structural transition at T C  283 K as expected for a charge ordering phenomenon in a low dimensional system. These results indicate the formation of singlet dimers in the 3LL associated with the columns of sites 4-2-4, as discussed below. Although the structure of Fe 3 O 2 BO 3 has been reported earlier to be orthorhombic with space group Pbam [6,7], there is no mention to any structural transition. The synthesis of crystalline needles is described in Ref. [2]. A black cylindrically shaped crystal of di- mensions 0.04 3 0.04 3 0.60 mm was used for data collection from x-ray diffraction. The radiation em- ployed was a graphite monochromated Mo Ka radiation (l  0.710 73 Å). Data were collected up to 60 0 in 2u, with a redundancy of 4. The final unit cell parameters FIG. 1. The structure of Fe 3 O 2 BO 3 projected along the c axis. Along the ladder the sites are occupied by Fe 31 ions with one extra electron per triad. The dashed lines indicate sides a and b of the unit cell. 147201-1 0031-9007 01 87(14) 147201(4)$15.00 © 2001 The American Physical Society 147201-1