On the Structure of Lithium and Strontium Borate Glasses Modified with Yttrium and Rare-Earth Cations Investigated by Vibrational Spectroscopy
New York State College of Ceramics at Alfred University. Inamori School of Engineering.
The thesis begins with a comprehensive review of the structure and properties of borate glasses. This is followed by a predominantly qualitative assessment of highly modified lithium and strontium borates containing yttrium and rare-earth oxide additions that have been prepared by the traditional melt-quenching technique. To the author's knowledge, reports on the ternary glasses studied here are not available in the literature. The feasibility of glass formation for these new compositions is discussed and the structures of the resulting materials have been studied, primarily, with vibrational spectroscopy as well as selectively with differential scanning calorimetry, X-Ray diffraction, and time-resolved fluorescence spectroscopy. Raman and Infrared spectroscopies suggest the glasses formed in the vicinity of the orthoborate stoichiometry are structurally similar to the high temperature phase of the related yttrium or rare-earth orthoborate crystals, regardless of whether the glass transition temperature lies above or below the corresponding phase transformation temperature. A relationship between the crystal phase transformation temperature and the glass transition temperature has been observed through the partial devitrification of borate melts containing relatively high yttrium content. That is, for glasses where the glass transition temperature is above the phase transformation temperature, only short-range order structural units akin to high temperature phase will be present in both the glass and any crystallization products. For glasses where the glass transition temperature is below the phase transformation temperature, the bulk glass is also made up of the short-range order structural units found in the high temperature phase; however, low temperature phase crystallites are detected with XRD. These crystallites, comprised only of tetrahedral orthoborate units arranged n B3O9^9- rings, are seen also as well-defined structures approximately 30 microns in diameter using confocal Raman microscopy. The addition of oxides with the high field strength trivalent yttrium ion to strontium borate glasses was found to depolymerize the borate network into ionic species while simultaneously increasing the glass transition temperature. In this series, the increase of the cation motion band frequency from 180 cm^-1 to 323 cm^-1 indicates the trivalent yttrium ions form stronger bonds with network oxygen than the divalent strontium ions. This correlates with the onset of the glass transition temperature increasing non-linearly from 630.7 °C to 652.2 °C for glasses containing 5 mol% and 25 mol% Y2O3, respectively. Lithium yttrium/rare-earth orthoborate glasses were found to consist solely of isolated trigonal borate units and to be insensitive to the use of either yttrium or other rare-earth elements, all of which have a well-known tetrahedral orthoborate crystalline phase. The absence of isomerization or disproportionation at the orthoborate stoichiometry implies the driving mechanism for glass formation in these glasses can be viewed as having a physical rather than chemical origin. This is to say, vitrification is dependent on the freezing-in of highly distorted, isolated trigonal borate structures - which are favored at high temperatures and in the melt compared to their tetrahedral counterpart. Several of the lithium yttrium/rare-earth orthoborate glasses were doped with Tb^3+ and the measured lifetime of the 543 nm emission using 375 nm excitation was ~2.2 ms. Measured lifetime decays, in conjunction with the relevant literature, suggest that even if trivalent rare-earth cations are present in small quantities, in a partially or fully depolymerized borate glass, the Tb^3+ ions will seek out local sites whose short range order corresponds to that of the high temperature crystalline phase. Indeed, this observation is in excellent agreement with the qualitative Raman interpretation of bulk strontium yttrium borate glasses where the introduction of the small quantities of Y2O3 is seen to induce the formation of trigonal orthoborate units below the pyroborate stoichiometry. Overall, the results obtained from the Tb^3+ doped orthoborate glasses here permit interpretation of previously unexplained published results. This is framed in the context of experimental and computational results pertaining to alkali and alkaline-earth borates that show that metal cations with high field strength will determine their local environment to a greater degree than metal cations present whose field strength is lower.
Thesis completed in partial fulfillment of the requirements for the degree of Master of Science in Materials Science and Engineering at the Inamori School of Engineering, New York State College of Ceramics at Alfred University
Glass, Chemistry, Physical and theoretical