A SYSTEMATIC NEUTRON-DIFFRACTION STUDY OF RE(2)CUO(4-DELTA) (RE=PR, ND, EU)

The influence of the rare earth (RE) on the structure of the T' phase has been investigated by single crystal neutron diffraction. There is an intrinsic instability of the T' phase against a deformation of the CuO2 planes which is characterised by the rotation of the O-squares ar ound the c-axis. As it can be seen from the atomic displacement parame ters of the in plane oxygen, this instability increases with decreasin g ionic radius of the RE. The evolution of structural parameters as a function of temperature has been measured for RE = Pr and Nd in order to analyse statical contributions to the Debye-Waller factors. The oxy gen loss in the Cu-planes of Nd2CuO4-delta is derived (delta=0.046(13) ) from the structural parameters obtained for the same single crystal before and after reduction. The other oxygen site remains unchanged wi th respect to thermal parameters and occupancy. The existance of an ex tra apical oxygen claimed in the literature has not been observed.