Short-range order quantification in transition metal fluoride glasses (TMFG) through EPR spectra simulation

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Abstract
Cr3+ and Fe3+ EPR spectra in transition metal fluoride glasses (TMFG), pyrochlore CsZnGaF6 and amorphous GaF3 are found to be very similar. Then, it is inferred that the constituent octahedra of all these disordered fluoride compounds are characterized by closely related distortions. Two short-range structural models, based on CsZnGaF6 and GaF3, are worked out with the aim of characterizing octahedra in TMFG. The CsZnGaF6 model is developed using structural information on this compound. Amorphous GaF3 structure is simulated by using molecular dynamics calculations starting from rhombohedral GaF3. From the M-F distance and bonding angle distributions obtained from these two models, the Cr3+ and Fe3+ fine-structure parameter distributions are calculated with the help of the superposition model and finally the corresponding EPR spectra are computed. A good agreement is obtained simultaneously for the two local paramagnetic probes, which leads to a quantification of the short-range order in TMFG: constituent octahedra are only slightly distorted. It is interesting to note that the distributions of fine-structure parameters are very similar to the Czjzek ones previously used to simulate Cr3+ and Fe3+ spectra in TMFG.