Study of a lacunary solid phase II—Morphological and kinetic characteristics of its formation

Abstract The morphological changes which take place during the formation of the lacunary phase αH 2 C 2 O 4 ·BaC 2 O 4 by isothermal dehydration of the oxalate H 2 C 2 O 4 ·BaC 2 O 4 ·2H 2 O, are characteristic for dihydrations of hydrates which, with water vapor, form a divariant system. They show that the transformation affects the entire bulk of the solid from the very first moments of the reaction. The dihydrate crystal undergo a very regular fragmentation and the pseudomorph appears as a stacking of microcrystals whose shape and dimensions are unique and independent of the size and habit of the initial crystals. The kinetic characteristics of the reaction show that the microcrystal dimensions do not depend on dehydration rate, they enable the precise role of crystalline faces in gas evacuation to be evaluated. The origin of fragmentation, the microcrystal habit produced, the anisotropy of transformation of the crystalline faces and the process of water elimination are explained by means of structural considerations.