Dependence of the magnetic ordering temperature on dilution in a 3D XY insulating ferromagnet

Ferromagnetic polycrystalline Fe[Se2CN(C2H5)2]2Cl is examined, as are several dilute versions of it containing diamagnetic Zn[S2CN(C2H5)2]2, present to the extents 3.6, 7.9, 13.7 and 20.2%. The magnetic ordering temperature decreases much less rapidly with dilution than predicted by any standard model such as the nearest neighbor 3D Ising, XY and Heisenberg ones, and the initial slope −(dTc/dx)/Tc is well below unity. A curvature change of Tc(x) appears at approximately 14% dilution. Extensions of simple models do not account for the results. It is possible, perhaps, that molecular shape factors lead to effectively nonrandom dilution, unexpectedly surviving ferromagnetic interactions, and thus to the unprecedented Tc(x).