Thermal behaviour of alum-(Rb) RbAl(SO4)2 · 12H2O from in-situ laboratory high-temperature X-ray powder diffraction data

Abstract

The present work investigates in-situ the thermal behaviour of alum-(Rb), RbAl(SO₄)₂ · 12H₂O, by high-temperature X-ray powder diffraction from 303 K to melting, which starts at 359 K and is completed, due to kinetics, at 363 K. The calculated a₀ linear thermal expansion coefficient is of 10.33(12) x 10^-6 within the investigated thermal range. The k order parameter, which is the measure of the extension of the orientational disorder of the sulphate group, decreases from ca. 0.80 at 303 K to ca. 0.75 just before melting. This behaviour has been proved to depend on the need to keep the bond valence sum of the Rb⁺ cation, which coexists in both six- and seven-fold coordination, at a reasonable value.