CRYSTAL AND MAGNETIC-STRUCTURE OF YBACUFEO5

The perovskite-like antiferromagnetic compound YBaCuFeO3 was studied b y powder neutron diffraction, magnetic susceptibility measurements, an d Mossbauer spectroscopy. Crystal structure is analysed in the centros ymmetric space group P4/mmm and compared to previous results in the no ncentrosymmetric space group P4mn. Fe/Cu atoms occupy randomly the oxy gen pyramids with iron displaced inside the pyramid with respect to th e copper position. The magnitude and the distribution of the quadrupol ar 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. Ma gnetic susceptibility and neutron powder diffraction reveal two magnet ic transitions at T-N1 = 441(2) K and T-N2 approximate to 230 K. Betwe en T-N1 and T-N2, the magnetic structure is based on a unit cell relat ed to that of the nuclear structure by a(mag) = root 2a(nuc) and c(mag ) = 2c(nuc). The magnetic moments are coupled antiferromagnetically wi thin each [CuFeO5](infinity) layer with the stacking sequence along c: +--+. The magnetic structure is refined with moments tilted with resp ect to the c axis. The large reduction of magnetic moment observed by neutron powder diffraction is explained by the cationic disorder. Belo w T-N2 the magnetic structure is incommensurate with a short-range ord er. (C) 1995 Academic Press, Inc.