Crystal Chemistry and Phase Transitions in Substituted Pollucites along the CsAlSi₂O₆‐CsTiSi₂O_6.5 Join: A Powder Synchrotron X‐ray Diffractometry Study

Abstract

Crystal Chemistry and Phase Transitions in Substituted PollucitesAlong the CsAlSi 2 O 6 -CsTiSi 2 O 6.5 Join: A Powder Synchrotron XRay Diffractometry Study.-As revealed by powder XRD, CsAlSi 2 O 6 crystallizes in the tetragonal space group I4 1 /a. The substituted pollucites CsTixAl 1−x Si 2 O 6+0.5x (0 ¡ x ≤ 1) crystallize in the cubic space group Ia3d, presumably because the excess O 2− anions effectively hold open the expanded framework. On cooling, however, these cubic phases transform to the tetragonal modifications.These low-temperature transformations are close to being tricritical in nature, as is consistent with the tetragonal-to-cubic transition in pollucite on heating.

Heat capacities, standard entropies and Gibbs energies of Sr-, Rb- and Cs-substituted barium aluminotitanate hollandites

“…15 of the three hollandite crystal lattices are similar, which is consistent with their similarities in structures -having the same crystal symmetry (I4/m) and small differences in unit cell parameters [11].…”

“…An alternative strategy is to immobilize the radionuclides as components in the crystal structures of dense minerals, such as silicate, phosphate, and titanate [9][10][11][12][13][14][15][16]. Compared to glass waste forms, these dense minerals have robust thermal stability and aqueous durability rendering them viable candidates for hosting radionuclides.…”

Thermochemistry of glass forming Y-substituted Sr-analogues of titanite (SrTiSiO5)

“…10 Most studies related to the Cs/Sr-loaded titanosilicate waste forms have focused on their structure and selectivity. [11][12][13][14][15] What is still lacking in this area is a fundamental understanding of the thermodynamics of the titanosilicate waste forms. Moreover, the relations among crystalline and amorphous or glassy states for waste forms are important due to the possibility of producing material instability through amorphization from radiation damage and during vitrification.…”

The Crystallization of Ba‐Substituted CsTiSi2O6.5 Pollucite Using CsTiSi2O6.5 Seed Crystals

“…1 The substitution of Ti 41 for Al 31 in the lattice is balanced by the additional oxygen that results in half of the Ti 41 ions in fivefold coordination and half in fourfold coordination. 2,3 CsTiSi 2 O 6.5 is the cesium (Cs)-containing phase formed by the heat treatment of the highly selective and commercially available Cs getter silicotitanate material (UOP IE-910 and IE-911). 4,5 As a result, CsTiSi 2 O 6.5 has been highly recommended as a waste form for immobilizing radioactive Cs.…”