The
structure of glasses in the systems (100 – x)B2O3–xPbF2 (x = 30, 40, and 50) and 50B2O3–(50 – x)PbO–xPbF2 (x = 5, 10, 15, 20, 25, 30, 35,
40, and 45) has been studied by solid state NMR and EPR spectroscopies.
On the basis of 11B and 19F high resolution
solid state NMR as well as on 11B/19F double
resonance results, we develop a quantitative structural description
on the atomic scale. 19F NMR results indicate a systematic
dependence of the fluoride speciation on PbF2 content:
At low x-values, F– ions are predominantly
found on BO3/2F– units, whereas, at higher x-values, fluoride tends to be sequestrated into amorphous
domains rich in PbF2. In addition, both pulsed EPR studies
of Yb3+ doped glasses and photophysical studies of Eu3+ doped samples indicate a mixed fluoride/borate coordination
of the rare-earth ions and the absence of nanophase segregation effects.
A series of heavy metal oxide (HMO) glasses with composition 26.66B 2 O 3 -16GeO 2 -4 Bi 2 O 3 -(53.33-x)PbO-xPbF 2 (0 ≤ x ≤ 40) were prepared and characterized with respect to their bulk (glass transition and crystallization temperatures, densities, molar volumes) and spectroscopic properties. Homogeneous glasses are formed up to x = 30, while crystallization of β-PbF 2 takes place at higher contents. Substitution of PbO by PbF 2 shifts the optical band gap toward higher energies, thereby extending the UV transmission window significantly toward higher frequencies. Raman and infrared absorption spectra can be interpreted in conjunction with published reference data. Using 11 B and 19 F high-resolution solid state NMR as well as 11 B/ 19 F double resonance methodologies, we develop a quantitative structural description of this material. The fraction of four-coordinate boron is found to be moderately higher compared to that in glasses with the same PbO/B 2 O 3 ratios, suggesting some participation of PbF 2 in the network transformation process. This suggestion is confirmed by the 19 F NMR spectra. While the majority of the fluoride ions is present as ionic fluoride, ∼20% of the fluorine inventory acts as a network modifier, resulting in the formation of four-coordinate BO 3/2 F − units. These units can be identified by 19 F{ 11 B} rotational echo double resonance and 11 B{ 19 F} cross-polarization magic angle spinning (CPMAS) data. These results provide the first unambiguous evidence of B−F bonding in a PbF 2 -modified glass system. The majority of the fluoride ions are found in a lead-dominated environment. 19 F− 19 F homonuclear dipolar second moments measured by spin echo decay spectroscopy are quantitatively consistent with a model in which these ions are randomly distributed within the network modifier subdomain consisting of PbO, Bi 2 O 3 , and PbF 2 . This model, which implies both the features of atomic scale mixing with the network former borate species and some degree of fluoride ion clustering, is consistent with all of the experimental data obtained on these glasses.
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