Structural changes during dehydration and subsequent decomposition in cavansite, Ca(VO)(Si4O10)•4H2O were studied by in situ synchrotron powder diffraction between 298 and 900 K. Evolution of the crystal structure was observed through 20 structure refinements, by full profile Rietveld analysis performed in the Pnma space group, between 298 and 810 K, whereupon the cavansite structure was observed to breakdown. Full-profile Rietveld structure refinements provide complete information on the structural and volume modifications, and the thermal expansion of cavansite undergoing thermal dehydration, with particular attention to dynamic and transient effects. Within 298-810 K temperature range, the cell parameters of cavansite showed a step-like decrease as a function of temperature corresponding to semi-discrete water removal steps The variation of the shortest O3-O3 and O5-O5 ‘‘free diameters” across the elliptical eight-membered rings of tetrahedra dynamically record the effects of water removal and provide insights on the framework flexibility.In fact, while the water loss centred at about 470 K is associated to a minor flexible distortion of the 8-ring the further water loss after 540 K triggers a dramatic change in the pore shape leading to a strong increase in the ring ellipticity, reduction of the aperture crystallographic free area (C.F.A.), and volume contraction of the Ca polyhedron . Above 798K, the refined occupancy of water molecules drops below a critical level leading to instability in the cavansite structure which turns amorphous
DEHYDRATION DYNAMICS OF CAVANSITE PROBED BY IN SITU SYNCHROTRON POWDER DIFFRACTION
MARTUCCI, Annalisa;CRUCIANI, Giuseppe
2012
Abstract
Structural changes during dehydration and subsequent decomposition in cavansite, Ca(VO)(Si4O10)•4H2O were studied by in situ synchrotron powder diffraction between 298 and 900 K. Evolution of the crystal structure was observed through 20 structure refinements, by full profile Rietveld analysis performed in the Pnma space group, between 298 and 810 K, whereupon the cavansite structure was observed to breakdown. Full-profile Rietveld structure refinements provide complete information on the structural and volume modifications, and the thermal expansion of cavansite undergoing thermal dehydration, with particular attention to dynamic and transient effects. Within 298-810 K temperature range, the cell parameters of cavansite showed a step-like decrease as a function of temperature corresponding to semi-discrete water removal steps The variation of the shortest O3-O3 and O5-O5 ‘‘free diameters” across the elliptical eight-membered rings of tetrahedra dynamically record the effects of water removal and provide insights on the framework flexibility.In fact, while the water loss centred at about 470 K is associated to a minor flexible distortion of the 8-ring the further water loss after 540 K triggers a dramatic change in the pore shape leading to a strong increase in the ring ellipticity, reduction of the aperture crystallographic free area (C.F.A.), and volume contraction of the Ca polyhedron . Above 798K, the refined occupancy of water molecules drops below a critical level leading to instability in the cavansite structure which turns amorphousI documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
