Impact of rare earth ion size on the phase evolution of MoO3-containing aluminoborosilicate glass-ceramics

Abstract

Transition metal and rare earth cations are important fission products present in used nuclear fuel, which in high concentrations tend to precipitate crystalline phases in vitreous nuclear waste forms. Two phases of particular interest are powellite (CaMoO 4) and oxyapatite (Ca 2 RE 8 (SiO 4) 6 O 2). The glass compositional dependencies controlling crystallization of these phases on cooling from the melt are poorly understood. In the present study, the effect of rare earth identity and modifier cation field strength on powellite and apatite crystallization were studied in a model MoO 3-containing alkali/alkaline-earth aluminoborosilicate glass with focus on (1) influence of rare earth cation size (for RE 3+ : Ce, La, Nd, Sm, Er, Yb) and (2) influence of non-framework cations (RE 3+ , Mo 6+ , Na + , Ca 2+). Quenched glasses and glass-ceramics (obtained by slow cooling) were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray absorption (XAS), and electron probe microanalysis (EPMA). All samples were X-ray amorphous upon quenching, except the Ce-containing composition which crystallized ceria (CeO 2), and the sample devoid of any rare earth cations which crystallized powellite (CaMoO 4). On heat treatment, powellite and oxyapatite crystallized in the majority of the samples with the former crystallizing in the volume, while the latter on the surface. The EPMA results confirmed a small concentration of boron in the oxyapatite crystal structure. RE cations were incorporated in the glass, as well as in powellite, oxyapatite, and in the case of Yb 3+ , keiviite (Yb 2 Si 2 O 7). Raman spectroscopy showed that the primary vibration band for molybdate MoO 4 2in the glasses was strongly affected by the ionic field strength of the modifying cations (alkali, alkaline earth, and RE), suggesting their proximity to the MoO 4 2ions in the glass, though the MoO bond length and coordination according to XAS suggested little local change.

Crystallization study of rare earth and molybdenum containing nuclear waste glass ceramics

“…There is a trade‐off between simplifying a complex glass in order to better understand it and over‐simplifying it, such that behavior becomes substantially different than the analogous complex glass. For instance, it is known that some of the alkali behave differently in these glass‐ceramic systems, in that Cs tends to partition to a soluble molybdate or stay in the glassy phase (depending on details of the glass composition), Li goes normally to the glass, and Na can go to the glass, powellite, or oxyapatite phase . Cesium (and even more so rubidium) is expected to be a small fraction of the alkali on a molar basis .…”

“…For alkaline earths, the presence of Ba and Sr cause a separate partitioning of two powellite crystalline phases with different amounts of Ca, which would not be reproduced when including only Ca; unlike the case of Cs, the molar concentrations of Ba plus Sr in the waste are expected to be about half that of added Ca . Finally, it has been shown that the specific rare earth chosen does somewhat influence the RE phases formed, but given that Nd is by far the most prevalent rare earth in the waste (see Table ), it was chosen as representative. The 8‐oxide composition studied in detail here still provides the basic chemistry of charge compensation (Na), powellite inclusion (mainly Ca), and oxyapatite formation (Nd, with contributions from Ca and sometimes Na), such that useful physical insight can be gained without excessive compositional complexity.…”

“…The importance of RE for the stabilization of Mo in the glass matrix has been well-described, along with the effect of removing RE from the glass by crystallization of oxyapatite. [13][14][15][16][17] At the same time, when Mo does precipitate in a crystalline phase, it is desirable to avoid the formation of water-soluble alkali molybdates above the Mo solubility limit, by controlling the distribution of Na and Cs, to force the more chemically durable calcium molybdate (powellite) phase to form preferentially. [18][19][20][21][22] A key barrier to maturation and exploitation of glass-ceramic technologies for nuclear waste forms is the gap in fundamental understanding of the molecular-scale mechanisms of phase separation and crystallization leading to the development of the desired phase assemblage and microstructure, ultimately determining long-term product performance.…”

“…For instance, it is known that some of the alkali behave differently in these glass-ceramic systems, in that Cs tends to partition to a soluble molybdate 18 or stay in the glassy phase 1 (depending on details of the glass composition), Li goes normally to the glass, 1 and Na can go to the glass, powellite, or oxyapatite phase. 17 Cesium (and even more so rubidium) is expected to be a small fraction of the alkali on a molar basis. 1 For alkaline earths, the presence of Ba and Sr cause a separate partitioning of two powellite crystalline phases with different amounts of Ca, 1,24 which would not be reproduced when including only Ca; unlike the case of Cs, the molar concentrations of Ba plus Sr in the waste are expected to be about half that of added Ca.…”

Partitioning of rare earths in multiphase nuclear waste glass‐ceramics

“…Oxyapatite, Ca 2 RE 8 (SiO 4 ) 6 O 2 , was detected in samples doped with RE of larger ionic radius (Pr, Nd, Sm), while glass with RE of smaller ionic radius (Eu, Gd) did not crystallize apatite. Later, Patil et al 29 reported on single RE containing peralkaline aluminoborosilicate compositions with varying RE, showing that compositions with large RE ionic radii (La, Nd, Sm, Er) form oxyapatite, while compositions with small RE ionic radii (Yb) form keiviite (Yb 2 Si 2 O 7 ). By contrast, in alkaline earth (CaO‐MgO‐SrO) alumino‐borosilicate GC systems containing RE, it has been reported that the Yb‐containing system, as well as the analogous Gd‐containing one, precipitate an oxyapatite phase 30 …”