The differing responses of various techniques in measuring the phase transformations of K2ZrF6

The compound K2ZrF6 undergoes an unusual multistep solid-solid transformation on temperature variation, whose irreversibility or reversibility depends on the target temperature: heating K2ZrF6 up to only 250 degrees C results in an irreversible process, while heating over this temperature results in reversible transformation. The reversibility of the back transformation indicates that alpha-K2ZrF6 transforms to a high-temperature phase (HTFrev) on heating above 250 degrees C, and on cooling reverts to the alpha-K2ZrF6 phase. The irreversible transformation involves transformation of alpha-K2ZrF6 to a high-temperature phase (HTFirrev) on heating to 250 degrees C, and then on cooling transforming to a mixture of meta-K2ZrF6 and K3ZrF7. With time this mixture changes to alpha-K2ZrF6, metaK(2)ZrF(6) and K3ZrF7. This transformation was monitored by DTA, DSC, 19 F MAS NMR, XRD and thermal diffusivity techniques. Surprisingly, these experimental techniques showed unexpectedly differing responses. (C) 2019 Elsevier B.V. All rights reserved.