Solid-State NMR Investigation Of Mixed-Alkali Distribution In Phosphate Glasses
- PDF / 401,517 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 90 Downloads / 189 Views
Solid-State NMR Investigation Of Mixed-Alkali Distribution In Phosphate Glasses S. Prabakar and K.T. Mueller Department of Chemistry, The Pennsylvania State University 152 Davey Laboratory, University Park, PA 16802 ABSTRACT The correlation between 31P and 23Na (or 133Cs) nuclei in a mixed-alkali phosphate glass has been investigated by TRAnsfer of Populations by DOuble Resonance (TRAPDOR) NMR spectroscopy. The variation in spatial proximity between 31P nuclei in phosphate tetrahedra and sodium (or cesium) modifier ions has been demonstrated in a phosphate glass with molar composition 25 Cs2O: 31 Na2O: 44P2O5. Interactions between 31P and 23Na (or 133Cs) nuclei are shown to exist even at low dephasing times indicating the short-range nature of these couplings. The ratios of resonance intensities from Q1 and Q2 phosphate units show different trends depending on whether couplings to 23Na or 133Cs are investigated, demonstrating the differing structural roles of the modifier alkali cations within the glass. INTRODUCTION The “mixed-alkali effect” is a term coined to reflect the non-linear variation in glass properties (e.g. density and glass transition temperature) when one alkali in a multicomponent glass is gradually replaced by another alkali species [1]. Controversy remains over the atomic-level details (both structural and dynamical) that can provide such an effect. For example, the role of two dissimilar alkali ions in the same glass structure has been interpreted and modeled using both a random [2] or a clustered distribution [3] of the two alkali ions, depending on interpretations of the experiments used in different studies. Several experimental, theoretical, and computational approaches have been employed to understand the origins of the mixed-alkali effect, but the mixedalkali distribution is not completely understood at this time [4-6]. We have proposed the use of multiple sets of dipolar “connectivity” experiments over a range of glass compositions in order to study the distributions of alkali atoms, and mixed-alkali phosphate glasses present a useful, and easily studied, model system. Phosphate glasses are gaining increased attention [7] due to potential applications as glassto-metal seals and laser glasses. Cesium-containing phosphate glasses may find applications in electron photomultiplier tubes and within containment systems for nuclear waste. Solid-state nuclear magnetic resonance (NMR) spectroscopy has been widely used as a tool to investigate complex microstructures in glasses [8], and 31P magic-angle-spinning (MAS) NMR is known to be sensitive to the local environments of phosphorus atoms. For example, the number of bridging and non-bridging oxygens around phosphorus, the bond angles, and the degree of hybridization of phosphorus are all reflected in measurable NMR parameters [9]. Recently the dipolar interactions between 31P and modifying cations have been exploited to probe structural correlations that exist between these nuclei [10]. In this article we investigate the alkali ion interaction with
Data Loading...
