Dehydration process and transient channel deformations of slightly hydrated boron leucite: An “ in situ” time-resolved synchrotron powder diffraction study

Abstract

The step by step thermal dehydration process of slightly hydrated boron leucite K 16[B 16Si 32O 96], was studied in situ by synchrotron radiation powder diffraction. A time-resolved experiment was performed using a translating imaging plate system. Rietveld refinements were carried out on 41 consecutive powder patterns in the 25–940 °C temperature range. Results show that temperature-induced transformations can be schematized into two main steps. In the 25–565 °C temperature range, the symmetry remained cubic I-43 d and the unit cell parameter increased with an increase in temperature. The migration of H 2O molecules through the [1 1 1] channels during dehydration determined an opening of the six-member ring apertures which was as wide as possible and, at the same time, a narrowing in the eight-ring along [1 1 1]. This process was accomplished by a twisting in the tetragonal prism, constituting a leucite framework, which led to an opposed tilting in the tetrahedra connecting the prisms. Above 565 °C, a continuous structural transformation led to a displacive polymorphic transition. Rietveld structure refinement of the unit cell parameters showed a remarkable change, thus indicating an I-43 d → Ia-3 d change in symmetry. This was associated with a relaxation in the continuous structural distortions of the leucite framework and the T–O–T and O–T–O angles indicated the formation of more regular apertures.