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Enthalpies of drop solution for a suite of substituted pollucites with the compositions CsTi x Al 1؊x Si 2 O 6؉0.5x , 0 < x < 1, which are synthesized using the sol-gel method, have been measured in molten lead borate (2PbO⅐B 2 O 3 ) at 701°C. As Ti 4؉ substitutes for Al 3؉ , the enthalpies of drop solution become less endothermic and show exothermic heats of mixing within the composition range from x ‫؍‬ 0.3 to 1. This nonideal mixing behavior is consistent with the trend seen in variation of lattice parameters, and we interpret it to be a result of the shortrange order associated with the framework cations Al 3؉ , Si 4؉ , and Ti 4؉ in the structure. Using enthalpies of drop solution of SiO 2 , Al 2 O 3 , TiO 2 , and Cs 2 O, standard molar enthalpies of formation of these phases from their constituent oxides and from the elements are derived for the first time.

The structure of Cs 2 ZrSi 3 O 9 synthesized using a sol-gel method was determined from the Rietveld refinement of experimental powder X-ray diffraction data. The refinement confirmed that this compound was isostructural with wadeite: its structure was hexagonal (space group P6 3 /m), and its lattice parameters were a ‫؍‬ 7.2303(2) Å and c ‫؍‬ 10.2682(4) Å. The aqueous durability of Cs 2 ZrSi 3 O 9 varied, depending on the solution conditions. In modified leach tests with buffered (pH 7) and unbuffered solutions, the 7-d cesium release rates were <1.2 ؋ 10 ؊4 g⅐(m 2 ⅐day) ؊1 , which indicated high aqueous durability. However, in unsaturated, unbuffered solutions with a pH of 9 -10, the durability was much lower, with 7-d cesium release rates of 2.2 ؋ 10 ؊3 g⅐(m 2 ⅐day) ؊1 . The ability of this material to retain cesium in aqueous environments can be explained by its condensed ring structure, in which the size of the channel openings is smaller than the diameter of a Cs ؉ ion. However, dissolution of the network silicate occurred at high pH, which resulted in the release of cesium.