The Crystallization of Ba‐Substituted CsTiSi₂O_6.5 Pollucite Using CsTiSi₂O_6.5 Seed Crystals

Abstract

Barium (Ba)‐substituted CsTiSi2O6.5 materials of two types, CsxBa1−xTiSi2O(7−x/2) and CsxBa(1−x)/2TiSi2O6.5 were synthesized with the pollucite structure with 1≥x≥0.6. When the Ba‐substituted precursor materials were heat treated to 850°C for 4 h, a mixture of amorphous and unidentifiable phases formed. However, with the addition of 10 wt% of crystalline CsTiSi2O6.5 to the Ba‐containing precursors, nearly single‐phase pollucite was obtained after 20 h at 750°C for x≥0.6. The added crystalline CsTiSi2O6.5 particles act as nuclei that allow the Ba‐containing materials to crystallize into the pollucite phase and to avoid the formation of unwanted phases that would otherwise nucleate and grow. These new materials can be used to study the stability of CsTiSi2O6.5 as a durable ceramic waste form, which could accommodate with time both Cs and its decay product, Ba.

The Effect of Vacancy and Barium Substitution on the Stability of the Cesium Titanium Silicate Pollucite

“…The observed patterns of (A) the CsTiSi 2 O 6.5 seed crystals, (B) Cs 1− x Ba 0.5 x TiSi 2 O 6.5 ( x =0.1, Cs 0.9 Ba 0.05 TiSi 2 O 6.5 ), a Ba‐substituted CsTiSi 2 O 6.5 pollucite with a replacement of one Ba for every two Cs to maintain charge balance, and Cs 1− x Ba x TiSi 2 O 6.5+0.5 x ((C) x =0.1, Cs 0.9 Ba 0.1 TiSi 2 O 6.55 and (D) x =0.2, Cs 0.8 Ba 0.2 TiSi 2 O 6.6 ), a stoichiometric one‐to‐one replacement of Cs by Ba, displayed the expected peaks emanating from the CsTiSi 2 O 6.5 pollucite structure (PDF #62‐5626) with traces of impurity peaks. The Ba‐substituted CsTiSi 2 O 6.5 pollucite phases prepared using finer 1.4 μm sized seed crystals showed better crystallinity than those previously prepared using crude seed crystals 12 . This further emphasizes our previous finding on the important role of the seed crystals in the crystallization kinetics of Ba‐substituted CsTiSi 2 O 6.5 pollucite because without the seeds we could only obtain a mixture of materials without a significant amount of the desired phase.…”

“…To understand the local environments in waste forms and to separate the effects of chemistry and radiation, it is appropriate to study the Ba substitution into CsTiSi 2 O 6.5 in a nonradioactive setting. Following our previous studies, 11,12 herein we focus on two decay product series: (1) with charge‐balance, and (2) with one‐to‐one replacement of Cs by Ba. To obtain fully crystalline CsTiSi 2 O 6.5 samples with Ba‐substitutions, we utilize a recently reported seed crystal approach to produce single phase Ba‐substituted CsTiSi 2 O 6.5 materials 12 …”

New Fission-Product Waste Forms: Development and Characterization

“…To obtain fully crystalline CsTiSi 2 O 6.5 samples with Ba-substitutions, we developed a seed crystal approach to produce single phase Ba-substituted CsTiSi 2 O 6.5 materials. 6 As shown in Figure 2, the XRD patterns of these materials were indeed closer to single phase pollucite, with only minor second phase material present (<10%). Microprobe analysis was performed on the Cs 0.6 Ba 0.2 TiSi 2 O 6.5 material with 10% CsTiSi 2 O 6.5 seeds crystallized at 750˚C for 20hr to verify that the barium was indeed incorporated into the pollucite phase as opposed to remaining in an amorphous phase.…”

Crystal Structure and Thermodynamic Stability of Ba/Ti‐Substituted Pollucites for Radioactive Cs/Ba Immobilization

“…Hence, from the energetic viewpoint, the Cs + → 0.5Ba 2+ substitution mechanism in the pollucite structure may be operating up to a certain extent. This is consistent with the fact the synthesized Cs x Ba (1− x )/2 TiSi 2 O 6.5 pollucites are limited to x > 0.6 and the Ba‐pollucite end‐members (Ba 0.5 AlSi 2 O 6 and Ba 0.5 TiSi 2 O 6.5 ) do not exist. For the hypothetical Ba‐rich pollucite compositions, the formation of other Ba‐titanate phases such as fresnoite (Ba 2 TiSi 2 O 8 ) and Ba‐titanate (BaTiSiO 5 ) would be thermodynamically favorable (Fig ).…”