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Acta Cryst. (2014). A70, C70
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Microporous materials such as zirconium silicates have the potential to be of importance in the nuclear industry for the selective uptake of cationic radionuclides and environmental pollutants. The structural behaviour of these materials at elevated temperatures is of interest for two reasons, the first is the densification of the exchanged materials prior to long term storage and the second is the formation of new porous phases which may have increased ion exchanged affinity for certain cations. The work presented here focuses on the umbite system. Umbite is a naturally occurring microporous zirconium titanium silicate found in northern Russia and synthetic analogues, K2ZrSi3O9·H2O, can be prepared using hydrothermal methods. It has an orthorhombic cell with a = 10.2977(2)Å, b = 13.3207(3)Å and c = 7.1956(1)Å. The ion-exchange of umbite with cations such as rubidium, caesium and strontium and the structures of the resulting exchanged materials have been studied. Exchanges with certain cations were found to cause a change in crystal system to a monoclinic cell. Recently Rocha and co-workers found that synthetic umbite will undergo a topotactic transformation when heated 9100C to form a new microporous zirconium silicate (AV-15) with the formula K2ZrSi3O9·2H2O, but to date no in-situ work has been carried out on this phase transition. In this work the high temperature structural behaviour of five umbite samples with different exchanged cations (K+, Na+, Mg2+ Ca2+and Cu2+) was studied up to a temperature of 10000C. All samples behaved very differently, indicating that the nature and location of the charge balancing cation plays an important part in determining which high temperature phases are formed. Certain general trends were observed, with group 1 cations the samples remain crystalline to high temperatures. With group 2 cations dense phases are formed at high temperatures and with transition metal cations there is a loss of crystallinity at low temperature.

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Acta Cryst. (2014). A70, C1526
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Microporous materials have a wide range of commercial uses from ion-exchangers to catalysts and have been used in the treatment of nuclear waste. The acidity associated with legacy waste pools often limits the effectiveness of these zeolites due to a loss of crystallinity. Microporous titanium silicates display different structural characteristics compared to conventional zeolites. Sitinakite, KNa2Ti4Si2O13(OH)·4H2O and the synthetic niobium doped analogue are being used as ion-exchange materials for the removal of Cs+ and Sr2+ from nuclear waste . Natisite is a layered titanium silicate with titanium in an unusual 5 coordinate square pyramidal environment. Natisite, Na2TiSiO5, crystallises in the tetragonal space group P4/nmm. [1, 2] A series of samples have been prepared with varying levels of zirconium doping ranging from 10% to 50%. Powder x-ray diffraction (PXRD) showed no obvious impurities attributed to zirconium containing phase. X-Ray Fluorescence (XRF) was carried out and showed the presence of zirconium indicating that doping had been successful. Ion-exchange experiments were carried out on the doped and undoped natiste samples using Cs and Co containing solutions. It was found that increasing the levels of zirconium increased the affinity towards Cs with the undoped materials taking up very little Cs. The rate of exchange with Co seemed to increase as the zirconium level was increased within the sample. This suggests that the presence of zirconium in the framework has a considerable effect on the ion-exchange properties of natisite. EXAFS has been useful in determining the coordination environment of titanium and zirconium in order to help fully understand the chemistry of this material. Also it has helped with determining if the exchanged Cs and Co have a preference for the sites close to Zr rather than Ti. It is therefore believed that the inclusion of zirconium in the natisite framework has potential use as an ion-exchanger in the nuclear industry.
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