New heteronuclear trimers in fluorides with usovite-type structure

Abstract

The Ba 2CaCuCr 2F 14 and Ba 2CaCuFe 2F 14 fluorides with usovite-type structure have been prepared. The cationic distribution has been determined by neutron diffraction. The Cu 2+ ions occupy elongated octahedra characterizing the d 9 electronic configuration of copper (II). The magnetic properties are typical of a low-dimensional magnetic behavior. The magnetic properties of the two compounds have been analyzed in terms of linear antiferromagnetic (M 3+Cu 2+M 3+) trimers resulting from the anisotropy of the electronic configuration of copper (II). The exchange interactions are antiferromagnetic. No 3D-magnetic ordering has been observed down to 1.5K in the case of Ba 2CaCuCr 2F 14. For Ba 2CaCuFe 2F 14 the Néel temperature is T 2 N=2.6 K. At low temperature long-range interactions are taken into account to fit the experimental results. The sign of the couplings between trimers has been explained qualitatively by a simple exchange mechanism between the d z2 orbital of Cu 2+ and an e g orbital of the M 3+ ions (M = Cr or Fe).