Physica B 345 (2004) 40–44 Precursor symmetry breaking in Cr doped V 2 O 3 A. Bombardi a, *, F. de Bergevin a , S. Di Matteo b,d , L. Paolasini a , P.A. Metcalf c , J.M. Honig c a European Synchrotron Radiation Facility, 38043 Grenoble, France b INFN Laboratori Nazionali di Frascati, c.p. 13, I-00044 Frascati, Italy c Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA d Dipartimento di Fisica ‘‘E Amaldi’’, Universit ! a di Roma III, via della Vasca Navale 84, 00146 Roma, Italy Abstract The results of X-ray resonant scattering experiments performed on a ðV 1x Cr x Þ 2 O 3 sample with x ¼ 0:028 are reported. This compound undergoes a first-order phase transition from paramagnetic insulator (PI) to antiferromag- neticinsulator(AFI)at T N C181K: AtthesametemperaturealatticedistortionreducesthesymmetryfromR % 3cinthe PIphasetoI2/aintheAFIphase.Togetmoreinsightintothetransitiondrivingmechanism,westudiedthebehaviour oftheBraggforbiddenpeak ð003Þ h ð10 % 1Þ m ; atVKedge,asafunctionoftemperature,polarization,azimuthalangle and X-ray energy. On the contrary to what observed in the pure system and to what expected theoretically a non- vanishing signal is observed already at room temperature in the dipolar channel. r 2003 Published by Elsevier B.V. PACS: 71.30.+h; 61.10.i Keywords: Vanadium sesquioxide; Templeton scattering; Resonant X-ray scattering 1. Introduction V 2 O 3 has been extensively investigated in the last 30 years. Despite the quantity of experimental data available and the amount of theoretical studies reported, its electronic and magnetic properties are still object of intense scientific debate [1]. At room temperature V 2 O 3 is in the a-Corun- dum structure (R % 3c; space group N. 167), which can be described by the pseudohexagonal space group. Cooling down the sample below T N ¼ 150Kthepuresystemundergoesatransitionfrom the paramagnetic metallic (PM) states to an antiferromagnetic insulating (AFI) state. At T N a strong lattice distortion, which is destructive for thecrystal,reducesthesymmetryfromR % 3ctoI2/a (space group N. 15). Three distinct monoclinic domains are formed and the hexagonal axis is tilted by B1  [2,3]. Chromium doping strongly modify the trans- port properties in the Corundum phase and 2.8% of Cr is enough to stabilize a paramagnetic insulating (PI) phase at room temperature [4]. The AFI state is not affected by the Cr presence apart for a change in T N : From the crystal- lographic point of view, the Cr presence does not ARTICLE IN PRESS *Corresponding author. E-mail address: bombardi@esrf.fr (A. Bombardi). 0921-4526/$-see front matter r 2003 Published by Elsevier B.V. doi:10.1016/j.physb.2003.11.016