A structural interpretation of the vibrational spectra of binary fluorohafnate glasses

Abstract

Binary glass samples have been prepared in the HfF4–BaF2 system, containing between 62–73 mol % HfF4. Their infrared absorption and polarized Raman spectra have been studied. The glass transition and crystallization temperatures have also been measured. The vibrational modes have been assigned and the results were compared with those previously obtained for ZrF4–BaF2 glasses. The cation isomorphic substitution of Zr by Hf allowed us to make a more definite vibrational assignment. The structures of the two glass systems were found to be similar, though not identical. The structural interpretation of the vibrational spectra was consistent with the occurrence of sixfold coordinated Hf atoms, probably forming a network of octahedral chains cross linked by Ba–F ionic bonds, with the degree of bridging increasing with the Hf content of the glasses. Substitution of Ba by Sr did not affect the structure. Due to the large F/Ba ratio of ∼10, there is an overlap of neighboring chains that may lead to sharing of a single nonbridging fluorine atom between two Hf atoms belonging to neighboring chains. The existence, on the average, of two such F atoms for each Hf may raise its effective coordination number from 6 to ∼8, when determined from an x-ray radial distribution function analysis, as recent results show. Vibrational spectroscopy, however, appears to be able to effectively distinguish which is the parent Hf atom to which every fluorine is bonded by a predominantly covalent, directional and shorter bond, as part of a highly symmetric octahedral coordination shell. In this respect, vibrational spectroscopy (particularly Raman) appears to be a more sensitive structural tool than x-ray diffraction.