From 1D to 3D fluorine octahedron networks in transition metal fluoride glasses:: a 19F MAS NMR study

The sensitivity of the F-19 isotropic chemical shift to the environment of the fluorine atom is used to investigate transition metal fluoride glass (TMFG) networks. The glass compositions were chosen in order to vary the initial constituents and check their influence on the octahedron network connectivity: PbF2-ZnF2-InF3 (PZI), PbF2-BaF2InF3 (PBI) glasses and alkali fluoride glasses such as PbF2-LiF-GaF3, PbF2-NaF-GAF(3), PbF2-KF-GaF3. NMR experiments give evidence for different fluorine atom sites. From the chemical shift values, it is sl;own that these fluorines may be classified into three categories:shared and unshared fluorines between two MF6 octahedra and free fluorines which are not implied in these MF6 octahedra, From. the relative intensities of the corresponding NMR lines, the proportions of these different fluorines in the glasses are obtained and the connectivity of the MF6, octahedra which constitute the networks is deduced. It is shown that the octahedra are corner shared and the dimensionality of the network varies from nearly ID to 3D according to the glass composition. Furthermore, it appears that the structure of the alkali fluoride glasses is quite different from the other ones: the transition metal ions Zn2+ and Ca3+ or Zn2+ and In3+ are statistically distributed in the PZG (previously investigated) and PZI glasses, respectively when disconnected 3D domains of LiF6/NaF6/KF6 and GaF6 octahedra with Pb2+ in interstitial positions are found in alkali fluoride glasses. (C) 1999 Elsevier Science B.V. All rights reserved.