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

Abstract

The sensitivity of the 19F 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: PbF 2–ZnF 2–InF 3 (PZI), PbF 2–BaF 2–InF 3 (PBI) glasses and alkali fluoride glasses such as PbF 2–LiF–GaF 3, PbF 2–NaF–GaF 3, PbF 2–KF–GaF 3. NMR experiments give evidence for different fluorine atom sites. From the chemical shift values, it is shown that these fluorines may be classified into three categories: shared and unshared fluorines between two MF 6 octahedra and free fluorines which are not implied in these MF 6 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 MF 6 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 1D 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 Zn 2+ and Ga 3+ or Zn 2+ and In 3+ are statistically distributed in the PZG (previously investigated) and PZI glasses, respectively when disconnected 3D domains of LiF 6/NaF 6/KF 6 and GaF 6 octahedra with Pb 2+ in interstitial positions are found in alkali fluoride glasses.