RIETVELD STRUCTURE REFINEMENT OF SR-EXCHANGED PHILLIPSITES

A.F. Gualtieri, E. Passaglia, E. Galli

Dipartimento di Scienze della Terra, Università di Modena, 41100 Modena, Italy

Keywords: Sr-exchanged phillipsite, Rietveld refinement, structure solution.

Phillipsite [PHI] is a monoclinic zeolite with a framework composed of layers of tetrahedra with eight- and four-membered rings roughly perpendicular to the a axis and vertically linked by 4-membered rings forming crankshafts with the 4-membered rings of the layer. The real space group is P2₁/m [1] with two extraframework sites: site I is on the mirror plane (010) in the type I octagonal prism and usually hosts large size cations such as K and/or Ba; site II is in a general position located near an intersection of the two sets of channels and usually hosts cations such as Ca and Na). Phillipsite crystals are always twinned (simple twins or higher rank twins) preventing their use for single crystal diffraction studies. The Rietveld structure refinement of three Sr-exchanged phillipsites with different Si/Al ratio (from 1.71 to 3.16) and different amount of Sr (from 2.63 to 1.70 a.f.u.) is presented here. The sample preparation anc characterization is described in [2]. Data were collected in the angular range 5-130°2 theta: the low 2 q region (12-20°2q) was collected using steps of 0.02° and 50 s/step counting time, divergence slits of 0.08° and receiving slits of 0.1 mm. The intermediate-2 q region (20-70°) was collected using steps of 0.02° and 20 s/step, divergence slits of 0.25° and receiving slits of 0.1mm. The high-2 theta region (70-130°) was collected using steps of 0.02° and 35 s/step, divergence slits of 1° and receiving slits of 0.1mm. X-ray powder data sets were analyzed by the Rietveld method using the GSAS software system [3].

Phillipsite with Sr=1.70 and 1.90 a.f.u. show a conventional monoclinic P2₁/m cell whereas the sample with Sr=2.63 a.f.u. shows a pseudo B-centred P2₁/m cell with a=16.40, b=14.12, 8.66, and b=99.5 which has been as well solved and refined. The cell modification is induced by strong interactions of the Sr atoms with the framework oxygens.

Passaglia, E., Gualtieri, A.F., and Galli, E. 1998. Proc. Zeolite 97 Conference. Submitted.
Rinaldi, R., Pluth, J.J., and Smith, J.V. 1974. Acta Cryst.,1974, B30-10:2426-2433.
Larson, A.C., and von Dreele R.B. 1998. Generalized Structure Analysis System, Los Alamos National Laboratory, Los Alamos, New Mexico, LAUR, 86-748.