Thermal Behavior of M+B5O6(OH)4•2H2O(M+ = K, Rb, Cs) and polymorphic transformations of CsB5O8

Phase transformations (dehydration, amorphization, crystallization, polymorphic transitions) and thermal expansion of M+B5O6(OH)(4) (.) 2H(2)O (M+ = K, Rb, Cs) compounds and the polymorphic transformations of the CsB5O8 compound are investigated using the methods of glass crystallization, thermal X-ray diffraction, and differential thermal analysis (DTA). It is shown that the dehydration of RbB5O6(OH)(4) (.) 2H(2)O and CsB5O6(OH)(4) (.) 2H(2)O, like KB5O6(OH)(4) (.) 2H(2)O, proceeds in two stages. After the first stage of the dehydration (with a loss of three water molecules), all three compounds transform into the amorphous state. At the second stage, the fourth water molecule leaves the amorphous phase. At temperatures above the glass transition point, anhydrous pentaborates MB5O8 (M = K, Rb, Cs) crystallize and subsequently undergo polymorphic transformations.