On a qualitative model for the incorporation of fluoride nano-crystals within an oxide glass network in oxy-fluoride glass-ceramics

Abstract We report on a Raman scattering study of oxy-fluoride glass-ceramics with a typical composition 32SiO 2 :9AlO 1.5 :31.5CdF 2 :18.5PbF 2 :5.5ZnF 2 :3.5ErF 3 (in mol%), which indicates a narrow size distribution of β-PbF 2 fluoride nano-crystals, typically of 13 ± 1 nm, in the silica-based glass network and gives insight on how then nano-crystals are incorporated into the glassy network. The Raman spectra indicate the presence of Q 0 , Q 1 and Q 2 -like units, which are SiO x F 4− x tetrahedra with zero, one and two bridging oxygens, respectively. The frequency, width and depolarization ratio of Raman bands corresponding to these Q n -like units ( n =0, 1, 2) indicate that Q 0 units are mostly symmetric SiO 4 tetrahedra, Q 1 are SiO 4 tetrahedra where one F may substitute for O and Q 2 are SiO 4 tetrahedra where one or two F may substitute for O. We argue that the non-bridging O atoms belonging to Q n tetrahedra, mostly to Q 0 tetrahedra, are located near the interface between the nano-crystalline and glassy phases, allowing an easy accommodation of fluoride nano-crystals with a spherical shape, in the oxide glass network. This was in agreement with the very low-frequency Raman features found between ∼4 and 8 cm −1 , due to acoustic vibrations of the nano-crystals embedded in the glassy matrix.