Thermal behaviour of alum-(K) KAl(SO4)2·12H2O from in situ laboratory high-temperature powder X-ray diffraction data: thermal expansion and modelling of the sulfate orientational disorder. (2nd January 2018)

• Record Type: Journal Article
• Authors: Ballirano, Paolo
• Is Part Of: Mineralogical magazine. Volume 79:Number 1(2015:Feb.)
• Page Start: 157
• Page End: 170
• Publication Date: 2018-01-02
• Subjects: alum-(K), -- powder X-ray diffraction, -- Rietveld method, -- thermal behaviour, -- sulfate disorder; Mineralogy -- Periodicals; Mineralogy -- Great Britain -- Periodicals; 549.05
• DOI: 10.1180/minmag.2015.079.1.13
• Languages: English
• ISSNs: 0026-461X

Abstract

Abstract: The present work analyses the thermal behaviour of alum-(K), KAl(SO4 )2 ·12H2 O, by in situ laboratory high-temperature powder X-ray 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) × 10 –6 K –1 within the investigated thermal range. The k disorder parameter, describing the extension of the orientational disorder of the sulfate group, has been found to decrease from ∼0.70 to ∼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 a reasonable bond-valence sum of 0.81 valence units (vu) to the 'average' K + ion a instead of 0.66 vu, which is obtained in the case of six-fold coordination alone. We can describe the temperature dependence of k from 93–355 K by means of the empirical equation k = 0.798(12) + 2.5(11) × 10–4 T – 1.9(2) × 10 –6 T 2, which includes reference low-temperature data. Bond-valence analysis has shown that, on cooling, an increase of the k disorder parameter and shortening of the K–O2 bond distance act together to maintain constancy in the bond-valence sum at the K site, stabilizing the structure. Therefore, the need for keeping the 'average' K + ion at a reasonable bond-valence sum appears to be the driving force for the ordering process involving the sulfate group.