Effect of temperature and X-ray illumination on the oxygen ordering in La2CuO4.1 superconductor.

Structural properties of La2CuO4.1 single crystal are studied by high-resolution synchrotron X-ray diffraction as a function of temperature and of X-ray fluence. Superstructures with a periodicity 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 transitions, 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-photoinduced phase transition has been found by placing the sample under high X-ray flux at 300 and 220 K.