Continuous dehydration of cavansite under dynamic conditions: an in situ synchrotron powder-diffraction study

The dehydration dynamics of cavansite, Ca(VO)(Si4O10)center dot 4H(2)O was studied by time-resolved in situ synchrotron powder diffraction between 298 and 900 K. The crystal-structure evolution was continuously monitored through twenty Rietveld structure refinements (Pnma space group) in the 298-810 K range whereupon cavansite turned amorphous without any precursor to a polymorphic phase transition to pentagonite. The results obtained from the series of time-resolved Rietveld refinements highlight the out-of-equilibrium effects that govern dehydration of cavansite powders under dynamic conditions. While confirming the general picture of cavansite dehydration, as previously reported by static single-crystal work, this dynamic study revealed an important transient phenomenon, namely the cell-volume expansion caused by framework relaxation resulting from breakdown of the hydrogen bonding network during the initial heating stages. We also documented that the channels formed by elliptical eight-membered tetrahedral rings of cavansite, when heated under the typical dynamic conditions of an industrial process, undergo a series of short-lived "pore-mouth breathing motions" which could be exploited for fine tuning of gas diffusion paths in possible applications of synthetic analogues of these vanadosilicates.