Ionic conductivity and structure of M1−xPbxF2 (M = Ca, Sr, Ba) solid solutions prepared by ball milling

Abstract

Nanocrystalline M_1−xPb_xF₂ (M = Ca, Sr, Ba) solid solutions were prepared by ball milling mixtures of binary parent materials. The structure of the obtained materials was investigated by ¹⁹F MAS NMR spectroscopy and XRPD, and their ionic conductivity by impedance spectroscopy. It was found that fluorite-structured solid solutions over the whole range of compositions can be prepared by ball milling for all three systems, closing the broad miscibility gap of the CaF₂–PbF₂ system. The fluoride ion conductivity increased with increasing Pb content of the solid solutions, with Ba_0.10Pb_0.90F₂ showing the highest conductivity of all samples prepared, being 1.5 orders of magnitude smaller than the one of PbSnF₄. Ca_1−xPb_xF₂ showed a fluoride ion conductivity increase which can be assigned to geometric frustration induced disorder. Ball milling of pure PbF₂ revealed an increase of the fluoride ion conductivity by 2.5 orders of magnitude in the case of PbF₂ containing a large amount of β-PbF₂ compared to microcrystalline β-PbF₂. Its fluoride ion conductivity is also 2.5 orders of magnitude larger than the fluoride ion conductivity of ball milled PbF₂ consisting of a small amount of β-PbF₂ but a large amount of α-PbF₂ pointing to differently conducting and structured grain boundaries of α-PbF₂ and β-PbF₂.