Space group determination of the BaY(Cu_0.5Fe_0.5) ₂O_5+δ phase using a convergent-beam electron-diffraction technique

The space group of the BaY(Cu_0.5Fe_0.5) ₂O_5+δ (δ=0.03-0.17) phase was studied by selected-area electron diffraction and convergent-beam electron diffraction (CBED). The CBED patterns for BaY(Cu_0.5Fe_0.5) ₂O_5.03 grains taken from the zone axes of [111], [001] and [010] had the symmetries of m, 4mm and 2mm, respectively. Forbidden reflections were observed neither in selected-area electron diffraction nor in the CBED patterns. From these results, the space group of the BaY(Cu_0.5Fe _0.5)₂O_5.03 was determined to be P4/mmm. Since the presence of a mirror plane parallel to the (Cu,Fe)O₂ planes was confirmed, Cu and Fe were found to be randomly distributed in the (Cu,Fe)O ₂ planes. The same analyses were performed for BaY(Cu _0.5Fe_0.5)₂O_5.17 grains and the space group was also found to be P4/mmm. The change in the magnetic properties of BaY(Cu_0.5Fe_0.5)₂O_5+δ samples due to the high-pressure heat-treatment was concluded to be caused by excess of oxygen.