Rietveld structure refinement of Sr-exchanged phillipsites

The structures of three Sr-exchanged phillipsites with different Si/(Si+Al) ratio were refined by the Rietveld method on data collected with a conventional Bragg-Brentano geometry. The distribution of Sr in the extra-framework sites depends on its amount in the sample and hence on the value of Si/(Si+Al). The typical PZ,lin cell with Sr distributed over sites I and II is displayed in the two samples with Si/(Si+Al) = 0.72 and 0.76 respectively. The sample with Si/(Si+Al) = 0.63 instead undergoes a cell modification with Sr distributed in an ordered fashion in sites I, I', II, and II' of four adjacent cavities along the a axis. This ordered site distribution has the effect of changing the typical unit cell with a approximate to 9.8 Angstrom, b approximate to 14.0 Angstrom, c approximate to 8.6 Angstrom, and beta approximate to 124 degrees to a pseudo B-centered monoclinic cell with a approximate to 16.5 Angstrom, b approximate to 14.0 Angstrom, c approximate to 8.6 Angstrom and beta approximate to 99.4 degrees. Nevertheless, the B-centering is broken by the ordered extra-framework site distribution. This study may help in the understanding of the interaction mechanism of Sr-cation exchange in phillipsite for the potential use as a countermeasure amendment in radiostrontium-contaminated soils, and as sieve for Sr-90 in natural waters of nuclear facilities. (C) 1999 Elsevier Science B.V. All rights reserved.