Shock Compression of Lead Fluoride at Room Temperature

Abstract

Lead difluoride (PbF₂) is known to occur in two distinct crystalline forms; namely, fluorite and PbCl₂ structures. The fluorite structure has a cubic symmetry (Fm3m - O⁵ _h) ancd PbCl₂ has an orthorhomic symmetry (Pbnm — V¹⁶). The fluorite and orthorhombic forms of PbF₂ are identified as β- and α-PbF₂, respectively. The prevalent information about the stability of these two forms of PbF₂ may be summarized as follows: (1) α-PbF₂ is the stable form of PbF₂ at ambient conditions; (2) α-PbF₂ transforms to β-PbF₂ at 583 ± 1K; (3) The reverse transformation of β;-PbF₂ to α-PbF₂ has not yet been observed; (4) The crystals of PbF₂ grown from its melt are invariably in the form of β;-PbF₂; (5) Even though unstable at ambient conditions, β-PbF₂ needs to be subjected to a hydrostatic pressure of around 0.4 + 0.1 GPa to transform it into α-PbF₂ form. This transformation has also been reported to be rather sluggish; (6) The rate of transformation of β- to α-PbF₂ increases with an increase in temperature; (7) The pressure of transformation P_t of β- to α-PbF₂ decreases with an increase in temperature T. The magnitude of dP_t /dT is approximately -3.6 MPa/K; (8) There is some indication that the β- to α-PbF₂ transformation proceeds readily under shearing stress conditions at room temperature.