Fluoride Glasses - Introduction

Introduction

Glasses in the NaF/PbF₂/MF₃ systems with high content of 3d cations (M^III = 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 [3]. 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 [4] (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 A^IIF₂/M^IIF₂/M^IIIF₃ systems (A^II = Ba²⁺, Pb²⁺ ; M^II = Mn²⁺, Zn²⁺, Cu²⁺ ; M^III as above) by EXAFS [5], 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 Pb₂M^IIM^IIIF₉ glasses [15], assuming isomorphous substitution for the transition metals (M^II = Mn/Zn and M^III = Fe/V), a practice relatively well accepted at least when 3d elements are concerned [16]. 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 [17] ; MD studies on fluoroaluminates glasses were also published [18], 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 [23]). We will examine how the statement "RMC is no different in principle to the method of Rietveld refinement" [19] has to be understood when considering glass modelling.

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Armel Le Bail - June 1997