ROOM-TEMPERATURE MOSSBAUER-SPECTRA OF FE-57-DOPED LA2SRCU2O6, LA2CACU2O6 AND LA1.6SR0.4CACU2O6 - STRUCTURAL IMPLICATIONS

In order to elucidate the properties of iron in the pyramidal Cu(2) sites of YBa2Cu3O7-y, 1% Fe-57 was substituted to copper in the simple bilayer compounds La2SrCu2O6, La2CaCu2O6 and La1.6Sr0.4CaCu2O6 (in their insulating forms). It appears that iron enters these systems as high-spin Fe3+. The room temperature Mossbauer spectra consist of paramagnetic quadrupole doublets, and a magnetic hyperfine structure appears below about 20 K. At RT, two quadrupolar doublets with \DELTA\ approximately 1 mm/s and \DELTA\ approximately 0.3 mm/s are observed in variable proportions in all three compounds. They have been interpreted by structural considerations involving, first a small displacement of the iron with respect to the copper position inside the oxygen pyramid, and second the possibility of additional oxygen insertion into the 2b vacancy site, leading in some cases to an octahedral oxygen neighbourhood.