SYNTHESES AND X-RAY-POWDER STRUCTURES OF K-2(ZRSI3O9)CENTER-DOT-H2O AND ITS ION-EXCHANGED PHASES WITH NA AND CS

A zirconium trisilicate compound, with composition K2ZrSi3O9 . H2O (1) , was prepared under mild hydrothermal conditions and was structurally characterized by using its X-ray powder diffraction data. The compoun d crystallizes in the space group P2(1)2(1)2(1) With a 10.2977(2) Angs trom, b = 13.3207(3) Angstrom, c = 7.1956(1) Angstrom, and Z = 4. The asymmetric unit consists of a metal atom, a trisilicate group, and thr ee lattice positions corresponding to two cations and a water oxygen a tom. In the structure, the Zr atom is octahedrally coordinated by the six terminal oxygens of the trisilicate group. The trisilicate groups exist as linear chain polymers connected to each other through the Zr atoms. This arrangement leads to channels and cavities in the structur e that are occupied by the cations and water molecules. The K+ ions in compound 1 were exchanged for Cs+ ions in two steps. Ln the first cas e about 50% of the K+ ions were exchanged to give a compound with comp osition K0.9Cs1.1ZrSi3O9 . H2O (2). Compound 2 was then loaded with ad ditional Cs+ ions which resulted in a phase K0.5Cs1.5ZrSi3O9 . H2O (3) . These exchanged phases retain the crystal symmetry of compound 1, bu t their unit cell dimensions have expanded as a result of large Cs+ io ns replacing the smaller K+ ions. Structure analyses of the exchanged phases show that the cations found in the cavities of compound 1 are h ighly selective for Cs+ ions. A small amount of Cs ions also go to a s ite in the large channel that is very close to that occupied by the wa ter oxygen in compound 1. In the absence of Cs, this site is filled wi th water molecules. The second cation found in the channel of 1 is par tially occupied by water and K+ ions. The K+ ions in compound 1 were c ompletely exchanged for Naf ions, and the compound thus obtained, Na2Z rSi3O9 . H2O (4), was treated with Cs+ ions in a manner similar to tha t carried out for compound 1. The low Csf ion phase, Na0.98Cs1.02ZrSi3 O9 . H2O (5), and the high Cs+ ion phase, Na0.6Cs1.4-ZrSi3O9 . H2O (6) , show ion distributions very similar to compounds 2 and 3 except for the fact that in the Na phases a small amount Csf ion also goes to the second cation site. Compound 1 on heating releases the lattice water and transforms into a hexagonal phase, K2ZrSi3O9, corresponding to the mineral wadeite. In the high-temperature phase the silicate group exi sts as a condensed cyclic group and the K+ ions are sandwiched between trisilicate groups. A possible pathway for this conversion is also di scussed.