Nepheline crystallization in boron-rich alumino-silicate glasses as investigated by multi-nuclear NMR, Raman, & Mössbauer spectroscopies

Abstract

A spectroscopic study was conducted on six simulant nuclear waste glasses using multi-nuclear NMR, Raman, and Mössbauer spectroscopies exploring the role of Si, Al, B, Na, and Fe in the glass network with the goal of understanding melt structure precursors to deleterious nepheline crystal formation. NMR showed two sites each for Al, Si, and Na in the samples which crystallized significant amounts of nepheline, and B speciation changed, typically resulting in more B(IV) after crystallization. Raman spectroscopy suggested some of the glass structure is composed of metaborate chains or rings, thus significant numbers of non-bridging oxygen and a separation of the borate from the aluminosilicate network. Mössbauer, combined with Fe redox chemical measurements, showed Fe playing a minor role in these glasses, mostly as Fe 3+ , but iron oxide spinel forms with nepheline in all cases. A model of the glass network and allocation of non-bridging oxygens (NBOs) was computed using experimental B(IV) fractions which predicted a large amount of NBO consistent with Raman spectra of metaborate features. local charge balance on the oxygens of alkaline-earth boro-aluminate crystals, where results from 11 B and 27 Al magic angle spinning (MAS)-NMR stressed the importance of three-coordinated oxygen O(III), where parentheses here and elsewhere denote nearest neighbor coordination, and indicated the importance of Al(V) and Al(VI) in all composition ranges [19]. In another example, Du and Stebbins [12] presented a modified Dell and Bray-type model [20] to include aluminum (i.e., alkali-alumino-borosilicate) and to describe boron speciation. Various tetrahedra compete for sodium, the modifier generally preferring Al, then B, then Si [21]. Tetrahedra building, on the other hand, is believed to start with Si then proceed to Al(IV), Fe(IV), and B(IV) (if present), as long as charged tetrahedron nearneighbor avoidance is maintained [12,22].Both boron and aluminum coordination are affected by modifier-cation field strength. In aluminoboro-silicates containing both Na and Ca, increasing the amount of Ca increases Al(V) [13], but has been said to increase B(IV) [23] or decrease it [13]. It has been shown in aluminoborates that higher-fieldstrength cations (e.g., Mg), can shift the normally encountered "chemical ordering" (i.e., Al and B tetrahedra avoidance due to Coulomb repulsion) seen in Na and Ca aluminoborates to the "statistical random mixing" of tetrahedra seen in Mg alumino-borate [24], and to an extent in Ca alumino-borosilicates [13]. Higher coordinated alumina units, Al(V) and Al(IV), can be charge compensated by 2 + cations (Ca, Mg) or 3 + cations (B, Al, La), and at the same time tend to "stabilize" B(IV) tetrahedra, acting as weakly positive ions themselves [19,24,25].Effects of differing Al/B ratios are explained by the local charge balance model [19], as B(III) is needed to stabilize units containing the charged Al(IV), so as more Al is added, the fraction of B(III) increases relative to B(IV) [23]. It has also been shown by ...

Structural dependence of crystallization in glasses along the nepheline (NaAlSiO4) ‐ eucryptite (LiAlSiO4) join

“…Table summarizes the published chemical shift values for crystalline species of interest for this work, including nepheline, carnegieite, eucryptite, and related phases . In general, for framework cations 29 Si and 27 Al, the NMR results for both glasses and crystals indicate primarily Q 4 units, as would be expected for a stoichiometric glass with Al/alkali ~1.…”

“…Crystallization of aluminosilicate phases has previously been shown to be detrimental to the chemical durability of HLW glasses as it extracts more durable components (i.e., alumina and silica) from the glass and enriches the residual glass in less durable components (such as boron and alkali) . The most common phase observed to form upon cooling of glass melts in the compositional space of interest is nepheline (nominally NaAlSiO 4 ), a tectosilicate feldspathoid . Electron probe microanalysis (EPMA) of nepheline dendrites in slow‐cooled HLW glasses revealed that their general formula could be written as (K,Na,Ca,Mg,vac) 2 Hex (Na,Ca,Mg,vac) 6 Oval (Si,Al,Fe) 16 O 32 where “Hex” refers to the large hexagonal rings comprising Fe‐O, Al‐O, or Si‐O tetrahedra, “Oval” refers to the so‐called “squashed” six‐membered rings of the same tetrahedral building blocks, and “vac” refers to a vacancy .…”

Modelling the sulfate capacity of simulated radioactive waste borosilicate glasses

“…Evidence from a number of sources indicates that the Al 3+ will occur predominantly as [4] Al 3+ [27,[33][34][35][36][37][38]. Manganese is readily reduced from Mn 3+ to Mn 2+ in glasses prepared under oxidising [40] also determined that the average coordination of Mn 2+ in radioactive waste borosilicate glasses is approximately 5, therefore it is assumed for our models that Mn is present as 50% [4] Mn 2+ and 50% [6] Mn 2+ , in order to obtain an average of [5] Mn 2+ .…”

A comparative study on the structural aspects of sodium borosilicate glasses and barium borosilicate glasses: Effect of Al2O3 addition

“…The peaks indicate there exists short range order (and in some cases medium range order) in glasses. Generally, in borosilicate glasses containing high concentration of Al 2 O 3 , there is tendency for the formation of crystalline alumino-silicate phase (like for example the nepheline phase) [15]. However, no sharp peaks characteristic of crystalline phases could be seen in the XRD patterns confirming that no crystalline phase separation has taken place in these samples.…”