A computer model for amorphous FeF3

A computer model has been constructed for amorphous Fe${\mathrm{F}}_3$ using a method involving the sequential addition of hard spheres of two different radii. A recognition of the effects of Coulomb forces is included by separating the ions of like charge as far as possible within the basic building scheme. A cluster sample of 858 ions has been produced and used to compute iron-iron, iron-fluorine, and fluorine-fluorine pair correlation functions, ion density, packing fractions, and such details of local configurational structure as the distribution of Fe-F-Fe bond angles $\theta$. The electric field gradient distribution at the iron sites has been computed using a point-charge model and used to interpret the measured Mössbauer quadrupole spectra. Comparisons are made with the earlier model presented for amorphous yttrium iron garnet (YIG) and reveal subtle differences in the iron coordination between the two. It is suggested that whereas the dominant exchange energy in amorphous YIG comes via essentially contact Fe-anion-Fe nearest-neighbor bonds, well over half the magnetic energy in amorphous Fe${\mathrm{F}}_3$ may come from "noncontact" bonds.