Molecular-based mixed valency ferrimagnets (XR 4 )Fe II Fe III (C 2 O 4 ) 3 (X = N, P; R =,n-propyl, n-butyl, phenyl): anomalous negative magnetisation in the tetra-n-butylammonium derivative

The phases AFeIIFeIII(C2O4)3(A = NPrn4; NBun4; PPh4) have been synthesised and characterised chemically, structurally and magnetically; they behave as ferrimagnets, but the tetrabutylammonium salt shows a highly unusual negative magnetisation at low temperature.