Phase Transitions in Lead(II) Fluoride Upon Milling

ABSTRACTPbF2 is known to exist under two different polymorphic structures. Orthorhombic α-PbF 2 is stable at ambient temperature. It has the PbC12 structure. Cubic β-PbF2 is obtained by heating α-PbF2. It does not transform back to α-PbF2 on cooling, and it seems to be infinitely stable in the metastable state under ambient conditions. β-PbF2 crystallizes in the fluorite (CaF2) type. Owing to the large number of potential interstitial sites, many F- Frenkel defects can be formed, which make β-PbF2 the highest performance fluoride ion conductor among binary fluorides. In this work, both phases of PbF2 have been ball milled. Milling α-PbF2 results in a partial transformation to microcrystalline β-PbF2. The energy required for obtaining the high temperature phase is probably provided in the mechanical form. Milling β-PbF2 leads to partial amorphization and formation of α-PbF2. In this case, milling transforms the high temperature polymorph to the low temperature form, by providing the energy required to overcome the activation energy that keeps PbF2 trapped in the high temperature β-form after cooling.