Journal of Superconductivity: Incorporating Novel Magnetism, Vol. 17, No. 1, February 2004 ( C  2004) Effect of Temperature and X-Ray Illumination on the Oxygen Ordering in La 2 CuO 4.1 Superconductor G. Campi, 1 C. Dell’omo, 1 D. Di Castro, 1 S. Agrestini, 1 M. Filippi, 1 G. Bianconi, 2 L. Barba, 3 A. Cassetta, 3 M. Colapietro, 4 N. L. Saini, 1 and A. Bianconi 1 Received 10 August 2003 Structural properties of La 2 CuO 4.1 single crystal are studied by high-resolution synchrotron X-ray diffraction as a function of temperature and of X-ray fluence. Superstructures with a pe- riodicity 2, along the c axis, due to a 3D long-range oxygen ordering, have been observed. The temperature-dependent study has allowed us to distinguish two order–disorder phase transi- tions, at 350 and 375 K for two different ordered phases, respectively. After rapid quenching from 380 to 100 K we were able to induce disorder-to-order-like transition because of a 2D rearrangement of the excess oxygen atoms in the temperature range of 130–180 K. The oxygen ordering could also be produced by X-ray beam illumination; a clear signature of X-ray-photo- induced phase transition has been found by placing the sample under high X-ray flux at 300 and 220 K. KEY WORDS: La 2 CuO 4.1 ; X-ray diffraction; oxygen ordering. 1. INTRODUCTION Transition metal oxides posses peculiar prop- erties [1] with novel electronic state as high-T c superconductivity in the cuprates. It is now widely accepted that these copper oxides are intrinsically inhomogeneous because of interplaying spin, charge, and lattice excitations, with strong tendencies toward a mesoscopic phase separation [2,3]. The strength of this inhomogeneous state seems to be related with strain fields because of the sublattice mismatch between the bcc CuO 2 layers with the neighboring rocksalt fcc blocks. This has been further underlined by recent experimental and theoretical efforts [4,5]. Oxygen-doped La 2 CuO 4+x superconducting system, where the hole doping is realized by introducing 1 INFM Research Unity, Universit` a di Roma “La Sapienza,” P. le A. Moro 2, 00185 Rome, Italy. 2 Department of Physics, University of Fribourg, Chemin du Mus´ ee 3, CH-1700 Fribourg, Switzerland. 3 Istituto di Strutturistica Chimica del CNR, sezione Trieste, I-34012 Trieste, Italy. 4 Dipartimento di Chimica, Universit ` a di Roma “La Sapienza,” P. le Aldo Moro 2, 00185 Rome, Italy. mobile interstitial oxygen ions as acceptors in the rocksalt layers, is one of the simplest systems among the copper oxide superconductors to study interplay of various electronic degrees of freedom [6–10]. The mobility of intersitial oxygen ions and high strain fields on the CuO 2 plane makes La 2 CuO 4+x a model system for studying interplay of dopants and charge ordering in the complex oxides [4,5,8]. In fact, oxygen ions remain mobile even below room temperatures [8] and macroscopic phase separation occurs for oxygen doping at 0.01 < x < 0.06, with the formation of oxygen-poor (insulating La 2 CuO 4.01 ) and oxygen- rich (superconducting La 2 CuO 4.06 ) domains [11–16]. On the other hand, at high oxygen doping x > 0.06 the macroscopic phase separations are suppressed, however, the system remains inhomogeneous at a mesoscopic length scale with oxygen and charge ordering. The doped oxygen ions, in fact, present a staging behavior in La 2 CuO 4+x forming modulation structure along c axis and the modulation period is n times the spacing between two CuO 2 layers, as it has been demonstrated by neutron diffraction [8,17–21]. Moreover, the microstrain on the CuO 2 plane in the La 2 CuO 4+x is much larger than the 137 0896-1107/04/0200-0137/0 C  2004 Plenum Publishing Corporation