Nanocrystalline solid solutions of alkaline earth metal fluorides Ma1-xMbxF2 (M: Ca, Sr, Ba) can be prepared by high energy ball milling of metal acetates and metal hydroxides, respectively, with ammonium fluoride as fluorinating agent. In Ca1-xSrxF2 and Ba1-xSrxF2 samples, a mutual substitution of cations on their lattice position occurs, even by mechanochemical syntheses starting from non-fluorinated reactants. Furthermore, the probability of formation of local structural units [FCa4-xSrx] or [FBa4-xSrx] can be adjusted with the supplied molar ratio of the two cations Ca:Sr and Ba:Sr, respectively. The latter was unambiguously evidenced by 19F MAS NMR experiments, where the expected five signals of the five possible fluoride ion environments were recorded. Calculations of 19F chemical shifts with the superposition model along with probability calculations for the intensity of the individual lines are in full agreement with the experimentally found and simulated parameters. Based on these results and the possible mechanochemical synthesis of PbF2, nanocrystalline solid solutions of M1-xPbxF2 (M = Ca, Sr, Ba) with fluorite structure can also be prepared by ball-milling using Pb(OAc)2.3H2O and M(OH)2 as metal precursors with NH4F as fluorinating agent. As before, the mutual substitution of the cations on their lattice positions and therefore the formation of solid solutions was proven by XRD and solid state NMR spectroscopy. The 19F MAS NMR spectra allow not only the direct observation of different structural units [FM4-xPbx] (0 &lt; x &lt; 4), but also the partial resolution of the scalar Pb-F coupling in mainly lead containing structural units. MAS NMR measurements revealed the existence of few byproducts like M(OAc)F and pure fluorides MF2 not detectable by XRD. DC conductivity measurements of Ba1-xPbxF2 samples showed an increasing fluoride ion conductivity compared to (i) the microcrystalline pure binary fluorides, (ii) to the binary fluorides prepared themselves by ball milling, and (iii) also compared to solid solutions without lead.

Mechanochemical synthesis, structure and properties of solid solutions of alkaline earth metal fluorides MaxMb1-xF2 (M = Ca, Sr, Ba, Pb)

255th National Meeting (2018)

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