Conclusion
The short and medium range order govern diffraction patterns
of glasses. From this study, expectations for a unique structure representation
for the NaPbM2F9 (M = Fe, V) fluoride glasses from
the RDM or RMC methods are disappointed. Several models which may seem
very different to crystallographer eyes have led by the RDM method to similar
diffraction data fits because they are in fact characterized by similar
mean short and medium range orders. The RMC model may be considered as
combining the various special arrangements (plus others) characterizing
the crystalline-derived models tested by the RDM method. A tridimensional
network built up from corner sharing [MF6] octahedra was expected
from crystal chemistry considerations before to start the study. Such a
network characterizes the NaPbFe2F9 crystal structure
which is quite different from that of KPbCr2F9 and
of the other related enneafluorides having a monodimensional character
drawn by infinite isolated staircase double chains of corner sharing octahedra.
In fact, RDM as well as RMC results tend to show that locally the KPbCr2F9
configuration cannot be ruled out. The contrast between the 1950x3 free
atomic coordinates implicit in the RMC model and the 14 coordinates refined
by the RDM method for the NaPbFe2F9 crystalline structure-derived
model is not reflected by a proportionally large difference in fit quality.
This suggests that the RDM method could be improved maybe by the introduction
of constraints on interatomic distances and of a Monte Carlo process which
would allow to describe small cells in the P1 space group. With such a
modification, the 14 parameters of the model designed here as the best
would extend to 52x3 by a method combining RMC and RDM, leading very probably
to some R factor improvement. On another hand, the presence of [MF6]
trigonal prisms in the RMC result is theoretically a nonsense regarding
fluoride crystal chemistry (but who knows really glass structures ?). A
new RMC modelling with a more drastic constraint in order to build exclusively
octahedra should be undertaken. Application of RMC and RDM methods to TMFG
fluoride glasses evenmore concentrated in [MF6] octahedra is
in preparation for the selected typical compositions BaMnMF7
and PbMnMF7 (M = Fe,V) (66% of the cations in sixfold coordination
against 50% in the title glasses). More edge sharing is to be expected
in these glasses as suggested by the corresponding crystal structures in
which up to 50-100% octahedra share at least one edge with another octahedron.
Acknowledgments. Many thanks are due to A-M. Mercier who prepared
the glass samples, to the Institut Laue-Langevin
for providing neutrons during experiment 6-08-189 (invaluable help in measuring
data was ensured by P. Chieux) and to Prof. C. Jacoboni for his constructive
criticism of the ILL proposal. Drawings of RDM and RMC models are respectively
by the STRUVIR [38]
and GLASSVIR softwares [39].
-/0/+
Armel Le Bail
- June 1997