Crystal Chemistry of the Microporous Zirconosilicate Na6Zr3[Si9O27], a Product of High-Temperature Transformation of Catapleiite, and Its Ag-Exchanged Form

Abstract

The crystal structure of the Ag-exchanged form of the zirconosilicate with the simplified formula (Na6-2xCax▯x)Zr3[Si9O27] with x < 1 (the idealized formula Na6Zr3[Si9O27]), a product of thermal transformation of catapleiite, ideally Na2Zr[Si3O9]·2H2O, was studied using single crystal X-ray diffraction data. The crystal structure of Na6Zr3[Si9O27] is based on a heteropolyhedral framework built by nine-membered tetrahedral rings [Si9O27] and isolated [ZrO6] octahedra. This zirconosilicate demonstrates high exchange capacity to Ag (experiment with 1 M AgNO3 aqueous solution, 250 °C, 30 days). Its Ag-exchanged form with the simplified formula (Ag5Ca0.5)Zr3[Si9O27] is characterized by a significant distortion of the heteropolyhedral framework and strongly disordered arrangement of extra-framework cations (Ag) which results in the doubling of a parameter of the hexagonal unit cell [a = 23.3462(3), c = 10.10640(10) Å, V = 4770.45(13) Å3] and space group P63cm. Ag+ cations preferably occupy the sites that are close to the Na sites in Na6Zr3[Si9O27].