Low-frequency Raman investigation of lead-containing fluorozirconate glasses and melts

Raman measurements of fluorozirconate glasses and melts were conducted from −265 (8.5 K) to ∼800 °C for numerous compositions, e.g., from 30 mol % BaF2 (no PbF2) to 35 mol % PbF2 (no BaF2), at nearly constant ZrF4 content. The normalized amplitude of the nominal 45 cm−1 peak was found to increase linearly with increasing PbF2 from 0–35 mol %, as the peak frequency decreased slightly, from ∼47 to ∼43 cm−1. From these observations, depolarization ratios, Pb–F force constants, etc., it was concluded that the 45 cm−1 peak involves a correlated motion of second-neighbor Pb2++Zr4+ or Ba2++Zr4+ cations vibrating against their edge- (or corner-) shared coordination cages of F− anions. Martin–Brenig-type analysis of the band shape below ∼50 cm−1 (at −265 °C) yielded a structural correlation length, SCL=2σ=6.2±0.2 Å (0–15 mol % PbF2). This SCL corresponds to a repeat distance, e.g., the Ba–Zr separation of the Ba(F2)Zr edge-sharing, 4.1–4.2 Å, plus the Zr–F bond length, 2.1 Å, of the ZrF4−nn polyhedra, 5<n<7, and n=CN. Normalized Bose–Einstein corrected difference spectra, I35%–I0% (mol % PbF2), and for 5 mol % PbF2, I635–I25 (°C), show common frequencies between ∼200–500 cm−1 which refer mainly to edge or corner sharing between the ZrF4−nn , PbF2−ll , and BaF2−mm polyhedra, l, m=CN. The 45 cm−1 peak frequency decreases sigmoidally with temperature rise, dropping steeply near TG, and reaching a ∼5 cm−1 limit in the melt. This and the rough density modulation observed for the mode frequency are indicative of viscoelasticity. The virtual absence of the low-frequency mode in the melt due to the Pb+Zr and Ba+Zr vs F cage motions occurs simultaneously with the disappearance of the F sharing modes. For a total stoichiometric F/Zr mole ratio of 5.5 for the melt, nearly free ZrF−5 , ZrF2−6 , and ZrF3−7 anions are indicated.