CRYSTAL AND MAGNETIC-STRUCTURE OF YBACUFEO5

The perovskite-like antiferromagnetic compound YBaCuFeO3 was studied by powder neutron diffraction, magnetic susceptibility measurements, and Mossbauer spectroscopy. Crystal structure is analysed in the centrosymmetric space group P4/mmm and compared to previous results in the noncentrosymmetric space group P4mn. Fe/Cu atoms occupy randomly the oxygen pyramids with iron displaced inside the pyramid with respect to the copper position. The magnitude and the distribution of the quadrupolar splitting obtained by electric field gradient calculations, taking into account the cationic disorder, are in qualitative agreement with those observed by Mossbauer spectroscopy in the paramagnetic range. Magnetic susceptibility and neutron powder diffraction reveal two magnetic transitions at T-N1 = 441(2) K and T-N2 approximate to 230 K. Between T-N1 and T-N2, the magnetic structure is based on a unit cell related to that of the nuclear structure by a(mag) = root 2a(nuc) and c(mag) = 2c(nuc). The magnetic moments are coupled antiferromagnetically within each [CuFeO5](infinity) layer with the stacking sequence along c: +--+. The magnetic structure is refined with moments tilted with respect to the c axis. The large reduction of magnetic moment observed by neutron powder diffraction is explained by the cationic disorder. Below T-N2 the magnetic structure is incommensurate with a short-range order. (C) 1995 Academic Press, Inc.