Structural role of molybdenum in nuclear glasses: an EXAFS study

Calas, Georges; Grand, Mathilde; Galoisy, Laurence; Ghaleb, D.

doi:10.1016/s0022-3115(03)00277-0

Cited by 109 publications

(137 citation statements)

References 17 publications

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“…The MRN model is able to describe the existence of large cation-rich clusters in glass, e.g., clusters of Ca in CaSiO 3 glasses [80] and Na 2 MoO 4 in French HLW glasses. [81] (Figure 3) MRO in glasses and melts has been measured for many single component mineral melts and glasses, e.g. SiO 2 glass [73], diopside glass [73], and nepheline glass [82], as well as in complex natural silicate melts [73].…”

Section: Structural Control Of Glass Dissolution (1) Role Of Medmentioning

confidence: 99%

Durable Glass for Thousands of Years

Jantzen

Brown

Pickett

2010

Int J of Appl Glass Sci

180

156

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The durability of natural glasses on geological time scales and ancient glasses for thousands of years is well documented. The necessity to predict the durability of high level nuclear waste (HLW) glasses on extended time scales has led to various thermodynamic and kinetic approaches. Advances in the measurement of medium range order (MRO) in glasses has led to the understanding that the molecular structure of a glass, and thus the glass composition, controls the glass durability by establishing the distribution of ion exchange sites, hydrolysis sites, and the access of water to those sites. During the early stages of glass dissolution, a "gel" layer resembling a membrane forms through which ions exchange between the glass and the leachant. The hydrated gel layer exhibits acid/base properties which are manifested as the pH dependence of the thickness and nature of the gel layer. The gel layer ages into clay or zeolite minerals by Ostwald ripening. Zeolite mineral assemblages (higher pH and Al 3+ rich glasses) may cause the dissolution rate to increase which is undesirable for long-term performance of glass in the environment. Thermodynamic and structural approaches to the prediction of glass durability are compared versus Ostwald ripening.

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Section: Structural Control Of Glass Dissolution (1) Role Of Medmentioning

confidence: 99%

Durable Glass for Thousands of Years

Jantzen

Brown

Pickett

2010

Int J of Appl Glass Sci

180

156

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“…Moreover, they can be synthesized at reasonable conditions and show good chemical stability when subjected to aqueous environments [1,3]. Though they have proved beneficial in many regards, there are material limitations introduced by insoluble species that can result in unexpected phase separation [4][5][6] and thus degradation of physical properties [7,8].…”

Section: Introductionmentioning

confidence: 99%

“…Compositionally, the preferential charge balancing of (BO 4 ) − and (MoO 4 ) 2 − anionic entities by Na + and Ca 2 + cations can determine molybdate speciation and network connectivity [22]. Due to charge, mobility, size and sterics (BO 4 ) − prefers Na + ions as charge balancers.…”

Section: Introductionmentioning

confidence: 99%

Impacts of composition and beta irradiation on phase separation in multiphase amorphous calcium borosilicates

Patel

Boizot

Facq

et al. 2017

Journal of Non-Crystalline Solids

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A B S T R A C TBorosilicate glasses for nuclear waste applications are limited in waste loading by the precipitation of watersoluble molybdates. In order to increase storage efficiency, new compositions are sought out that trap molybdenum in a water-durable CaMoO 4 crystalline phase. Factors affecting CaMoO 4 combination and glass-inglass phase separation in calcium borosilicate systems as a function of changing [MoO 3 ] and [B 2 O 3 ] are examined in this study in order to understand how competition for charge balancers affects phase separation. It further examines the influence of radiation damage on structural modifications using 0.77 to 1.34 GGy of 2.5 MeV electron radiation that replicates inelastic collisions predicted to occur over long-term storage. The resulting microstructure of separated phases and the defect structure were analyzed using electron microscopy, XRD, Raman and EPR spectroscopy prior to and post irradiation. Synthesized calcium borosilicates are observed to form an unusual heterogeneous microstructure composed of three embedded amorphous phases with a solubility limit~2.5 mol% MoO 3. Increasing [B 2 O 3 ] increased the areas of immiscibility and order of (MoO 4 ) 2 − anions, while increasing [MoO 3 ] increased both the phase separation and crystallization temperature resulting in phases closer to metastable equilibrium, and initiated clustered crystallization for [MoO 3 ] > 2.5 mol%. β-irradiation was found to have favorable properties in amorphous systems by creating structural disorder and defect assisted ion migration that thus prevented crystallization. It also increased reticulation in the borosilicate network through 6-membered boroxyl ring and Si ring cleavage to form smaller rings and isolated units. This occurred alongside an increased reduction of Mo 6 + with dose that can be correlated to molybdenum solubility.In compositions with existing CaMoO 4 crystallites, radiation caused a scattering effect, though the crystal content remained unchanged. Therefore β-irradiation can preferentially prevent crystallization in calcium borosilicates for [MoO 3 ] < 2.5 mol%, but has a smaller impact on systems with existing CaMoO 4 crystallites.

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“…Mo predominantly occurs as a hexavalent state Mo 6+ in glasses prepared under oxidising and neutral atmospheres, regardless of glass composition [9][10] with each Mo 6+ cation being coordinated with four oxygens to form a MoO4 2-tetrahedron. The average Mo-O distance range is 1.76-1.78 Å, indicating that Mo 6+ has a high field strength range of 1.89-1.94 Å -2 in glass [6,[10][11][12][13] and thus Mo 6+ cations have a strong ordering effect on surrounding oxygens [14]. Consequently, the MoO4 2-tetrahedra can be easily separated from the silicate glass network.…”

Section: Introductionmentioning

confidence: 99%

“…Consequently, the MoO4 2-tetrahedra can be easily separated from the silicate glass network. Meanwhile, it has been observed that MoO4 2-ions are preferentially associated with the network modifying cations [12] and are thus located in alkalis and alkaline earth enriched domains in glass network [11]. MoO4 2-tends to form alkali or alkaline earth molybdate crystals on separating out from the glass network and therefore the molybdate solubility can be related to the cations with which the MoO4 2-tetrahedra are associated.…”

Section: Introductionmentioning

confidence: 99%