title: Effects of hydration during strontium exchange into nanoporous hydrogen niobium titanium silicate.
abstract: A Nb-substituted titanium silicate with the sitinakite (NbTS) topology was exchanged with Sr(2+) to determine the mechanisms and pathways of ion diffusion through this mixed polyhedral nanoporous framework. The refined structural models yield unit cell parameters and atomic positions of Sr(2+) and suggest that there was a two-step process during cation diffusion. The starting material of the exchange experiment was the H(+)-exchanged material, H(1.4)Nb(0.6)Ti(1.4)SiO(7)·1.9H(2)O, with space group P4(2)/mcm. In the beginning of the exchange process, Sr filled the 8-membered-ring channel near the 4(2) axis in the center. Once the Sr(2+) fractional occupancy reached approximate 0.11, Sr positions and extra-framework H(2)O molecules shifted away from the central 8-membered-ring toward the framework, and an increase in Sr hydration and framework bonding was observed. The new H(2)O positions resulted in a lowering of symmetry to the P ̅42m space group, and it is thought that the Sr migration served to enhance Sr(2+) ion diffusion capacity into the channels of NbTS since the exchange rate briefly accelerated after the 0.11 fractional occupancy level was passed. Exchange of Sr(2+) into the nanoporous material reached maximum fractional site occupancy of approximately 0.20 using a 10.0 mM SrCl(2) solution.
authors.affiliation: Geography and Geology, Advanced Materials Institute, Western Kentucky University, 1906 College Heights Boulevard, Bowling Green, Kentucky 42101, USA.
authors.forename: Samantha
authors.initials: S
authors.lastname: Kramer
authors.forename: Aaron J
authors.initials: AJ
authors.lastname: Celestian
mesh_terms: D004058:Diffusion; D006859:Hydrogen / Q000737:chemistry*; D008958:Models, Molecular; D009556:Niobium / Q000737:chemistry*; D016062:Porosity; D033362:Powder Diffraction; D017640:Silicates / Q000737:chemistry*; D013324:Strontium / Q000737:chemistry*; D014025:Titanium / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D017640:Silicates; D009556:Niobium; C103828:titanium silicide; D006859:Hydrogen; D014025:Titanium; D013324:Strontium