On the preparation and characterization of REBa2F7-type compounds

YBCO superconductors have been fluorinated using ammonium hydrogen fluoride. Upon heating, the YBa2Cu3O3F obtained yields CuO and a new compound identified as YBa2F7. Seven of the rare earth analogues of YBa2F7 have also been prepared and characterised (RE = La, Ce, Pr, Nd, Sm, Gd and Tb), and results indicated that from Nd onwards there is an orthorhombic distortion of the cubic modification. The remaining six rare earth analogues (RE = Eu, Dy, Ho, Er, Tm and Yb) of YBa2F7 were prepared and characterised. The Pm analogue could not be prepared because of its radioactivity. The powder diffraction patterns and unit cell data of prepared samples were obtained. The XRD patterns were fitted based on an orthorhombic unit cell. The lattice parameters decrease upon going from Eu to Tm and from Dy to Yb. The unit cell volume decreases from Eu to Yb. These results agree with the trend in ionic radius of the rare earth cations. In addition, EuBa2F7 was studied by 151Eu-Mossbauer spectroscopy. It was concluded that the double fluorides of rare earths and Ba of general formula REBa2F7 exist for all the rare earths. Compounds up to Nd are cubic and show an orthorhombic distortion beyond. 9 refs.