Five-fold twinned β-PbF₂ nanocrystals in oxyfluoride glass ceramics

Oxyfluoride glass ceramics (GCs) doped with trivalent lanthanide ions (Ln³⁺) have been prepared using a conventional melting-quenching method and studied by X-ray diffraction (XRD). β-PbF₂ nanocrystals (NCs) doped with Ln³⁺ ions (β-PbF₂:Ln³⁺) in GCs were released from the GCs by etching off the glass matrix. β-PbF₂:Ln³⁺ NCs can be clearly observed by eliminating the influence of the glass matrix. The nanotwinned structures of β-PbF₂:Ln³⁺ NCs, including two-fold twinned NCs and five-fold twinned NCs, were examined using high-resolution transmission electron microscopy (HRTEM). The five-fold twinned phenomenon in metal fluorides with strong ionicity in oxyfluoride GCs is reported for the first time. Based on detailed analysis of the twinned NC structure, the twinning mechanism of β-PbF₂:Ln³⁺ NCs was proposed. Ln³⁺ ions and 'sublattice melting' of fluorine ions (F^-) in β-PbF₂ play extremely important roles in the formation of five-fold twinned β-PbF₂:Ln³⁺ NCs. The nanotwinned structures reported here may have far-reaching significance with respect to the further application of oxyfluoride glass ceramics doped with rare-earth elements and NC fabrication.