FLUORINATION OF YBA2CU3O6.7 - POWDER NEUTRON-DIFFRACTION DETERMINATION OF FLUORINE SITES AND THEIR INFLUENCE ON THE SUPERCONDUCTING PROPERTIES

The crystal structures of YBa2Cu3O6.7 before and after 30 minutes or 75 minutes of fluorination under diluted NF3 at 300°C, were determined by powder neutron diffraction experiments. For short time of fluorination, the occupancy of the (0, 1 28:48 AM 6/6/02, 0) site increases while the one of the ( 1 2, 0, 0) position does not vary too much. For higher fluorine concentration, the (0, 1 2, 0) occupancy seems to reach a limit, while the ( 1 2, 0, 0) one further increases. These structural results are discussed in relation with the two different evolutions of the superconducting behaviour previously observed : the enhancement of Tc from 65 K upto 90 K is related to the occupancy of the 0 (4) sites in the CuO chains along the b axis, while filling the 0(5) sites along the a axis mainly affects the a.c. susceptibility amplitude which decreases suddenly from 100% down to about 30%. © 1990.