An in situ study of the edingtonite dehydration processes from X-ray synchrotron powder diffraction

The dehydration of edingtonite has been studied, using 61 consecutive powder patterns collected between 310 and 684 K, in steps of 6 K, at 5 min intervals; the structures were refined using the Rietveld method. At 310 K, the two water sites are both 3/4 occupied; between 310 and 400 K, the occupancy of the OW(1) site drops smoothly to 1/2 and, at the same time, the OW(2) occupancy starts to increase to reach a maximum of 88% at 450 K; a simple electrostatic model is suggested to explain this behaviour. Between 450 and 600 K, OW(2) shows a regular dehydration behaviour; the remaining water is then rapidly expelled as the water drops to <<<$I> 1 per Ba ion. Finally, as a consequence of an unstable Ba-O coordination, the edingtonite structure breaks down between 660 and 680 K. The framework responds to dehydration firstly by a chain rotation and later by intra-chain folding; both mechanisms can be related to the electrostatic and spatial needs of the Ba ion.