Glasses in the NaF/PbF2/MF3 systems
with high content of 3d cations (MIII = Cr3+, Fe3+,
V3+, Ga3+) were prepared first by Miranday et al.
[1,2]. These 3d cations are well known for presenting
exclusively a sixfold coordination in crystallized fluoride compounds.
Optical absorption spectra early confirmed the [MF6] octahedra
presence in the glasses. EXAFS measurements suggested that M-F mean distances
and polyhedra regularity were quite similar either in crystallized fluorides
or in glasses . There is little doubt that
the Transition Metal Fluoride Glasses (TMFG) are structurally related to
the fluoroaluminate ones (with AlF6 octahedra) but they should
not be confused with the fluoroberyllates glasses (BeF4 tetrahedra)
or the Heavy Metal Fluoride Glasses (HMFG) like fluorozirconates 
(with ZrF6 to ZrF9 possible polyhedra). Previous
structural studies of TMFG which represents now a quite large family of
fluoride glasses, have concerned mainly the AIIF2/MIIF2/MIIIF3
systems (AII = Ba2+, Pb2+ ; MII
= Mn2+, Zn2+, Cu2+ ; MIII as
above) by EXAFS , neutron magnetic and nuclear
diffraction [6-8], Raman [9-10]
and EPR [11-14]. Few structural modelling were
undertaken, which is understandable because the study of these multicomponent
glasses is a real challenge. However modelling was first attempted by applying
a "Rietveld for disordered materials" (RDM) method on a series
of Pb2MIIMIIIF9 glasses ,
assuming isomorphous substitution for the transition metals (MII
= Mn/Zn and MIII = Fe/V), a practice relatively well accepted
at least when 3d elements are concerned .
Isotopic substitution in TMFG is unfortunately hardly possible because
one of the best 3d candidate (Ni) does not lead to wide glassy domains.
The atom pair Fe/V is particularly convenient for isomorphous replacement
in fluoride materials due to a large difference in the neutron scattering
lengths (0.954/-0.0382 x10-12 cm), and because crystal chemistry
arguments (see below) well support this choice. Molecular dynamics (MD)
simulation on amorphous fluoride samples (GaF3 and BaGaF5)
prepared by chemical vapour deposition (CVD) were not found very convincing
when the resulting X-ray distribution functions were compared with the
observed ones  ; MD studies on fluoroaluminates
glasses were also published , leading in
one case to 2/3 [AlF6] octahedra and 1/3 [AlF7] polyhedra,
a hardly believable result regarding the usual sixfold coordination behaviour
of aluminum in fluorides.
The aim of the present study is to perform the first application of
the Reverse Monte Carlo (RMC) method [19-22]
to the octahedral network-based TMFG glasses, selecting the nominal NaPbM2F9
compositions (M = Fe, V). The results will be compared to those obtained
by the RDM method (recently applied to glassy SiO2 ).
We will examine how the statement "RMC is no different in principle
to the method of Rietveld refinement" 
has to be understood when considering glass modelling.
Armel Le Bail
- June 1997