Probing alkali coordination environments in alkali borate glasses by multinuclear magnetic resonance Vladimir K. Michaelis, Pedro M. Aguiar, Scott Kroeker * Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 Received 8 September 2006; received in revised form 5 February 2007 Available online 13 June 2007 Abstract Five series of binary alkali borate glasses were prepared to compare the alkali dependence of network and modifier short-range order. 11 B magic-angle spinning (MAS) NMR reveals that the fraction of four-coordinate boron depends strongly upon alkali type at high- alkali concentrations: heavier alkalis favour the formation of non-bridging oxygens, whereas lithium borates contain a much higher con- centration of tetrahedral boron units. The alkali modifiers were observed directly by MAS NMR to measure the change in chemical shift with composition. All alkali peaks shift to higher frequency with increasing loading, indicative of decreasing average coordination num- bers. Relative to their known chemical shift ranges, the heavier alkalis exhibit the greatest shifts, whereas the lithium shifts are subtle. This is interpreted in terms of the availability of charged and partially charged coordinating oxygens in the network. Moreover, the 133 Cs chemical shifts plateau at 40 mol%, implying that the Cs + coordination number reaches a lower limit at this composition. This work demonstrates that NMR instrumentation and methodology have reached a level where even challenging nuclei like 39 K and 87 Rb can be probed to yield structural information in glasses. Ó 2007 Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 82.56.Dj; 82.56.Hg; 67.80.Jd Keywords: Glasses; Nuclear magnetic (and quadrupole) resonance; Borates; NMR; MAS-NMR and NQR; Medium range order 1. Introduction Alkali borate glasses have been extensively studied due to their potential importance as fast ion conductors [1–4] and alkali borosilicates as model systems for highly durable nuclear wasteforms [5,6]. One of the most valuable tech- niques for probing short-range structure in glasses is NMR spectroscopy [7–10]. Boron-11 NMR has been used for decades to quantify the types of borate polyhedra in borate glasses [9,11–13] and has played an important role in developing structural models of borate-based glasses [14–18]. Less widely studied are the alkali modifier cations. This is partly because the low field strengths characteristic of network modifiers result in highly disordered and irregu- lar local environments. This leads to large distributions in chemical shifts and quadrupole parameters, resulting in NMR peaks that are generally broad and featureless, mak- ing precise assignments difficult. The heavier alkali cations (K, Rb, Cs) have also traditionally been considered ‘diffi- cult’ nuclei to probe because of their unfavourable NMR characteristics. 39 K has a very low magnetogyric ratio, c, and hence suffers from low sensitivity and practical acquisi- tion problems [19]. 87 Rb can have very large quadrupole couplings for low-symmetry sites, resulting in extremely broad peaks, if observable. 133 Cs is highly polarisable and thus gives rise to a very broad range of chemical shifts, which can be difficult to disentangle from the spinning side- bands of the satellite transitions. However, improvements in NMR instrumentation have made it much easier to observe these nuclei in amorphous materials. The use of 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.04.029 * Corresponding author. Tel.: +1 204 474 9335; fax: +1 204 474 7608. E-mail address: Scott_Kroeker@UManitoba.ca (S. Kroeker). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 353 (2007) 2582–2590