Thermal behaviour of K-alum KAl(SO4)2 · 12H2O from in-situ laboratory high-temperature X-ray powder diffraction data. Thermal expansion and modelling of the sulfate orientational disorder

The present work analyses the thermal behaviour of alum-(K), KAl(SO4)2 · 12H2O, by in-situ laboratory high-temperature X-ray powder diffraction data from 303 K to melting, which starts at 355 K and is completed, due to kinetics, at 359 K. The calculated a0 linear thermal expansion coefficient is of 14.68(11) x 10-6 within the investigated thermal range. The k disorder parameter, describing the extension of the orientational disorder of the sulphate group, has been found to decrease from ca. 0.70 to ca. 0.65 just before melting. It has been demonstrated that the occurrence of the disorder implies the coexistence of K+ ions in both six- and seven-fold coordination. This is necessary for assigning to the “average” K+ ion a reasonable bond valence sum of 0.81 valence units (v.u.) instead of 0.66 v.u., which is obtained in the case of a six-fold coordination alone. We can describe the temperature dependence of k from 93-355 K by the empirical equation k = 0.798(12) + 2.5(11) x 10-4 T – 1.9(2) x 10-6 T2, which includes reference low-temperature data. Bond valence analysis has shown that, on cooling, increase of the k disorder parameter and shortening of the K-O2 bond distance together act to maintain constancy in the bond valence sum at the K site, stabilizing the structure. Therefore, the need for keeping at a reasonable bond valence sum the “average” K+ ion seems to apparently be the driving force for the ordering process involving the sulphate group.