Pollucite Structure Research Articles

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

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.

Crystal structure of pollucite

Abstract The crystal structure of pollucite (ANA) (Cs,Na)AlSi2O6 is characterized by single-crystal X-ray diffractometry. The space group C2/c, monoclinic, is found to be the most probable space group for this mineral based on the systematic absences of reflections. Cesium ions or water molecules are disordered at the center of a channel and are slightly apart from the special position. This is the origin of symmetry breaking and the space group becomes lower from Ia-3d (analcime) to C2/c. Sodium ions or vacancies are located at the contact point of the channel. No ordering of aluminum is observed in the pollucite structure.

Phase Transition and Lattice Thermal Expansion of Cs‐Deficient Pollucite, Cs1−xAl1−xSi2+xO6 (x≤0.25), Compounds

Cs‐deficient pollucite compounds with the general composition Cs1−xAl1−xSi2+xO6 and a cubic pollucite structure at room temperature (RT) were prepared by a multistep solid‐state reaction. The single phase of the products was confirmed by powder X‐ray diffraction (XRD) analysis at RT. Structural phase transition was influenced by the Cs content of compounds of the Cs1−xAl1−xSi2+xO6 series. Cs0.8Al0.8Si2.2O6 and Cs0.75Al0.75Si2.25O6 were cubic at 123 K from the XRD patterns and did not undergo structural phase transition. The lattice thermal expansion behavior of the Cs‐deficient pollucite compounds was investigated by low‐temperature and high‐temperature XRD analyses in the temperature range of 123–1173 K and was found to show a gradual decrease with decreasing Cs content of Cs1−xAl1−xSi2+xO6 series. The variation in unit cell volume with temperature for each composition showed that Cs0.8Al0.8Si2.2O6 and Cs0.75Al0.75Si2.25O6 had significantly smaller degrees of lattice thermal expansion than CsAlSi2O6 and Cs0.9Al0.9Si2.1O6. CsAlSi2O6, Cs0.9Al0.9Si2.1O6, Cs0.8Al0.8Si2.2O6, and Cs0.75Al0.75Si2.25O6 showed linear coefficients of thermal expansion of 2.7 × 10−6, 2.2 × 10−6, 1.5 × 10−6, and 1.1 × 10−6 K−1, respectively, which are attributed to not only an increase in space ratio but also a decrease in Al/Si ratio in the framework in the unit cell.

Effects of PMMA on porous structure of pollucite

The effects of PMMA as a pore-forming reagent and the powder for Cs-deficient pollucite, Cs 9Al 0.9Si 2.1O 6, calcined at 1073 K, on the microstructure of the porous body of Cs 0.9Al 0.9Si 2.1O 6 were investigated. The Cs 0.9Al 0.9Si 2.1O 6 porous bodies were fabricated by sintering the green compacts of the calcined powder and PMMA adding 35 mass% to the calcined powder. When the green compact was heated at 873 K in air for 20 h, pores <1 μm were observed in the porous body, suggesting that the PMMA previously dissolved in acetone was uniformly distributed in the calcined powder by the ball milling. The pore size of the obtained porous structure increased with increasing the size of the aggregated particles and the pore size distribution was significantly related to the size of Al 2O 3 balls and the time for the ball milling for mixing the calcined powders and PMMA.

High Temperature Synthesis of a Noncentrosymmetric Site‐Ordered Cobalt Aluminophosphate Related to the Pollucite Structure.

AbstractFor Abstract see ChemInform Abstract in Full Text.

High temperature synthesis of a noncentrosymmetric site-ordered cobalt aluminophosphate related to the pollucite structure.

A straightforward synthesis of a transition metal-loaded derivative of the pollucite structure is presented, with non-centric cation ordering over the tetrahedral sites.

Beta radiation effects in 137Cs-substituted pollucite

The effect of high-energy beta radiation on the long-range and local structure of 137Cs-substituted CsAlSi 2O 6 (pollucite) was studied by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) techniques at the Cs K-edge. Analysis of the XRD pattern of pollucite with an absorbed dose of 10 18 beta decays/g using Rietveld analysis indicates a 0.5–1% volume expansion of the tetrahedral structure as measured at 50 K and a minor displacement of the Cs cation toward the face of one of the six-membered rings. Analysis of the real-space pair-distribution function obtained from Fourier transformation of the diffraction pattern indicates significant correlated movement of the (Si,Al)–O pairs and large static disorder between Cs–O pairs. Analysis of the Cs K-edge XAS revealed substantial contributions from the Cs atomic X-ray absorption. This likely results from the exceedingly long Cs–O bond distances in the pollucite structure, which diminish the fine structure of the XAS oscillations, and the destructive interference of the backscattered photoelectron from static disorder between Cs–O pairs. In addition, a significant lengthening of the Cs–Cs interatomic distance was observed.

On the substitution of Fe and B for Al in the pollucite (CsAlSi 2O 6) structure

The partial or total substitution of Fe and B for Al in the pollucite (CsAlSi 2O 6) structure was examined or re-examined by means of XRD and FTIR. The samples, prepared through a sol-gel technique, showed a complete solid solution series along the join CsAlSi 2O 6-CsFeSi 2O 6 and along the join CsAlSi 2O 6-CsBSi 2O 6, whilst along the join CsFe-Si 2O 6-CsBSi 2O 6 the mutual substitution of boron for iron is no longer complete; indeed a miscibility gap exists, probably due to the large difference between the ionic radii of Fe and B. Along the latter join, XRD intensity measurements on selected samples (CsFeSi 2O 6, Cs(Fe 0.5B 0.5)Si 2O 6 and CsBSi 2O 6) allowed to carry out their structural analysis in the Space Group Ia3d.

Glass Waste Form Performance for Disposal of the Cesium and Strontium Concentrate Resulting From the Partitioning of HLW

ABSTRACTThe recent advances in HLW partitioning indicate that cesium–137 and strontium–90 can be separated in the form of a pure concentrate. Based on this concentrate the glass RT2–1 containing 4.7 wt.% Cs2O and 2.7 wt.% SrO was selected for more intensive and wider studies to predict long-term performance of this waste form under repository conditions.Leach tests were made in distilled and mineralized water before and after crystallization at 20, 90 and 130°C.Fully devitrified glass contained only one crystalline phase corresponding to the pollucite structure.

Hydrothermal Synthesis of Hydroxyapatite and Pollucite from Calcium Phosphate

AbstractHydroxyapatite (HAP : Ca10(PO4 ) 6 (OH) 2 ) and aluminosili-cate were easily synthesized using a hydrothermal process with mixing ratios of Ca/P=1.67 (Ca(PO3)2 and Ca(OH)2) and Si/Al=4 (SiO2 and Al(OH)3). The temperature was kept constant at 573 K for 60 minutes. .Cesium (Cs) and Cerium (Ce) contained in Ca(PO3)2 , were found to take on a stable form in the pollucite (CsAlSi2O6) structure and the monazite (CePO4) structure.

Thermal Expansion Characteristics of Li-Replaced Type Pollucite (Cs&lt;sub&gt;1-&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Li&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;AlSi&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;6&lt;/sub&gt;) Powder

Pollucite (Cs1-xLixAlSi2O6) powders of single phase have been synthesized by isomorphous replacement of part of the Cs+ ion by Li+ ion in the pollucite structure using sol-gel processing. Thermal expansion of the synthesized powders has also been investigated with a high-temperature XRD. The pollucite powders of single phase were synthesized in the case of the composition (Cs2O:Li2O: SiO2:Al2O3 (molar ratio)=0.95-0.90: 0.05-0.10: 4.5:1). Thermal expansion was less than 0.2% (room temperature to 1200°C). In particular, the powders showed almost zero thermal expansion in the range of room temperature to 500°C.

Novel CsAl 2PO 6 of pollucite structure: Synthesis and characterization

A novel cesium aluminophosphate, CsAl 2PO 6 of pollucite structure was crystallized under mild hydrothermal conditions using reactive Cs, Al and P components. The CsAl 2PO 6 has a cubic unit cell with the lattice constant a o = 13.54 A ̊ and with cell volume of 2480Å 3. The new CsAl 2PO 6 phase is potentially useful as a cesium host in phosphatic waste forms of high-level nuclear wastes.

Immobilization of cesium into pollucite structure by hydrothermal hot-pressing.

In order to immobilize Cs into pollucite structure, hydrothermal reactions of Cs with amorphous aluminosilicate (siliceous sinter) and with mixtures of low-quartz (silicastone) and Al(OH)3, were performed. In both cases, pollucite was formed in 5-N NaOH solution above 200°C for 10 min. Synthesized pollucite was solid solution between pollucite and analcite. Waste forms in which Cs was immobilized in pollucite structure were successfully produced by the hydrothermal hot-pressing method using the silica matrix (mixture of low-quartz and amorphous aluminosilicate) containing Al(OH)3 and NaAlO2. Effect of hot-pressing conditions on leachability of the waste forms was investigated. The waste form containing 10w/o CsOH produced at 300°C and 49 MPa for 24 h, had Cs leach rate of 3.15 g/m2·d by Soxhlet leach tests for 7 d. The waste form was porous, but had high compressive strength and thermal stability.

Immobilization of Cesium into Pollucite Structure by Hydrothermal Hot-Pressing

In order to immobilize Cs into pollucite structure, hydrothermal reactions of Cs with amorphous aluminosilicate (siliceous sinter) and with mixtures of low-quartz (silicastone) and Al(OH)3, were performed. In both cases, pollucite was formed in 5-N NaOH solution above 200°C for 10 min. Synthesized pollucite was solid solution between pollucite and analcite. Waste forms in which Cs was immobilized in pollucite structure were successfully produced by the hydrothermal hot-pressing method using the silica matrix (mixture of low-quartz and amorphous aluminosilicate) containing Al(OH)3 and NaAlO2. Effect of hot-pressing conditions on leachability of the waste forms was investigated. The waste form containing 10w/o CsOH produced at 300°C and 49 MPa for 24 h, had Cs leach rate of 3.15 g/m2·d by Soxhlet leach tests for 7 d. The waste form was porous, but had high compressive strength and thermal stability.

New family of silicate phases with the pollucite structure

New family of silicate phases with the pollucite structure

New family of silicate phases with the pollucite structure

Abstract A new family of silicate phases, Cs2MSi5O12: M = Be, Mg, Fe, Co, Ni, Zn, Cd; Rb2MSi5O12: M = Mg, Fe, (Co, Zn) has been prepared with the cubic pollucite structure, a in the range 13.3 to 13.8 Å, space group Ia3d, Z = 8. The structure of one, Rb2MgSi5O12, has been refined to R = 10.8% using X-ray powder data. The structures are built of a 3D, [MSi5O12]2− framework containing large, 12-coordinated cavities for the alkali cations. These structures are unusual in that a wide range of divalent cations have partially replaced Si4+ in the silicate anion framework.

Synthesis of Cs 2BeSi 5O 12 with a pollucite structure

The new compound, Cs 2BeSi 5O 12, has been synthesized by solid state reaction at ∼1200°C. It is cubic with a = 13.406 ± 0.001 Å, space group Ia3 d, and Z = 8. The structure was assumed to be essentially similar to that of pollucite, CsAlSi 2O 6, and was refined on this basis using X-ray powder intensity data to an R value of 6.2% using 36 reflections. The structure has a three-dimensional beryllosilicate framework in which rings containing 4, 6, and 12 (Be,Si)O 4 tetrahedra, sharing corners, are present. No evidence for ordering of Be and Si was found. The Cs + ions occupy large cavities within the framework and have a coordination number of 12. Cs 2BeSi 5O 12 is thermodynamically stable and melts congruently at 1420 ± 20°C to give a glass-forming melt.

The crystal structure and chemical composition of pollucite

The crystal structure and chemical composition of pollucite

The crystal structure and chemical composition of pollucite

The structure of pollucite from Rumford, Maine has been determined and refined. The space group is Ia 3 d with cell edge a = 13.69 Å. The structure was investigated using 213 independent reflection intensities measured on a single crystal diffractometer. Pollucite has the analcime framework with the cesium atoms occupying the large voids in the framework, as initially suggested by N áray -S zabó . The water molecules occupy the large voids of this same set which are not occupied by the cesium atoms. The sodium atoms are located in equipoint 24 c , at ¼ ⅛ 0, in the positions between the water molecules. The water molecules and the sodium atoms occupy the same positions as they do in analcime, but they occur only in randomly distributed clusters of atoms whose outer members are restricted to water molecules. The final structure was refined by least squares to an R value of 0.050 for all reflections. The chemical composition of pollucite can be expressed by a general formula similar to: Cs x Na y Al x+y Si 48− x − y · O 96 (16− x ) H 2 O, in which 2 y ⩾ 16 − x ⩾ y .