Using CPS120 (curved position-sensitive detector covering 120°) powder diffraction data in Rietveld analysis. The dehydration process in the zeolite thomsonite

Abstract

A powder diffractometer (CPS120 by INEL) equipped with a position-sensitive detector covering 120° in 2θ has been tested and the data have been successfully used with the Rietveld method. The detector was found to have a characteristic variation in linearity of ±1.5%. A calibration compound with well known 2θ values was used to determine a cubic spline calibration function to describe the 2θ versus channel-number function, which was then incorporated in the Rietveld analysis program. A procedure is also described to correct for air and capillary scattering, taking sample absorption into account. The methods have been applied to a structure refinement of quartz and to a study of the dehydration process of the zeolite thomsonite. Quartz: Rp = 11.0%, Rw = 14.0% and RB = 7.9%, based on 10 < 2θ < 116.3° in 3470 steps, λ = 1.540598 Å. Space group P312, Z = 3, a = 4.91201 (7), c = 5.40301 (5) Å, x(Si) = 0.4702 (3), B ave(Si) = 0.77 (6) Å2, x(O) = 0.4112 (6), y(O) = 0.2712 (5), z(O) = −0.2811 (4) and B ave(O) = 1.0 (1) Å2. Thomsonite: Rp = 8.18%, Rw = 11.04% and RB = 3.33%, based on 10 < 2θ < 110° in 3270 steps. Space group Pncn, Z = 4, a = 13.0778 (3), b = 13.0398 (3) and c = 13.1970 (3) Å. The unit cell changes continuously with increasing dehydration temperatures up to 523 K, at which a = 12.9725 (3), b = 12.9537 (3) and c = 13.2288 (3) Å. In this first dehydration step, the W3 and W4 molecules coordinated to the Ca/Na site are lost and the Ca/Na and W1 sites are shifted approximately 0.3 Å from their original positions towards the framework O atoms. After dehydration at 573 K, the mode of unit-cell variations is changed: a = 12.908 (3), b = 13.039 (3) and c = 13.034 (3) Å. In this dehydration step, W1 is lost. The W2 site appears unaffected by the dehydration below 573 K. On dehydration above 573 K, the diffraction intensities decrease as the compound becomes amorphous.