Low-temperature phase transitions and magnetic susceptibility of low-dimension uranium fluorides

Magnetic susceptibility of quasi-one-dimensional magnetic α-UF5 and cluster (four-nuclear) magnetic U2F9 in the temperature range 4.2-300 K is studied. For α-UF5 above 200 K Curie-Weiss law, the parameters being μef=2.37, μB and Θ=-118 K, is followed. Character of antiferromagnetic ordering in one-dimensional chains of α-UF5 and phase transition detected in U2F9 at 6 K are discussed. It is suggested that U4+ and U5+ valence is stabilized below phase transition and only in that case U2F9 may be considered as UF4xUF2 compound