Study of local environment in a series of amorphous ferric fluorides

Amorphous samples of Fe${\mathrm{F}}_{3}$, KFe${\mathrm{F}}_{4}$, and RbFe${\mathrm{F}}_{4}$ have been prepared by vapor deposition at liquid-nitrogen temperature, and room-temperature M\"ossbauer quadrupole spectra are reported for each. The spectra are broad and indicate the existence of a distribution of electric-field-gradient (EFG) magnitudes at the iron sites. Using a least-squares computer fit to the data determined from a Lorentz broadened EFG distribution function, we are able to make separate determinations of the Lorentz widths of the individual transitions and of the form of the EFG distribution. We find a natural Lorentz width of 0.20 mm/s and an EFG distribution function of asymmetric Gaussian form. The latter has the same shape for all three materials and is both significantly broader and more asymmetric about its peak than predicted by random-packing models using hard spheres and point-charge summations. An analytic shell-model distribution function is shown to be an accurate description of EFG histograms for random-packing models but is unable to provide a quantitative description of the measured quadrupole spectra.