Time-resolved synchrotron X-ray diffraction study of the dehydration behavior of chalcophanite

Abstract

Time-resolved synchrotron X‑ray powder diffraction data were used to investigate the dehydration behavior of the chalcophanite (ZnMn 3 O 7 ·3H 2 O) structure from 300 to 1060 K. Rietveld refinements revealed two obvious phase changes, at ∼450 and ∼950 K, corresponding to the dehydration of chalcophanite followed by transformation to a spinel structure (Mn-hetaerolite). Only small changes were observed in the chalcophanite unit cell from RT to ∼438 K; the volume increased by ∼0.8%, mostly caused by thermal expansion of ∼0.5% along c. Above ∼427 K, the interlayer water molecules were lost, resulting in a collapse of the interlayer spacing from ∼7 to ∼4.8 Å. The newly formed anhydrous phase (ZnMn 3 O 7 ) retained chalcophanite’s R3̅ space group and a dimension, but the c dimension decreased from ∼21 to 14.3 Å, and the Zn coordination changed from octahedral to tetrahedral. Above ∼775 K the anhydrous chalcophanite began to transform to a spinel structure, corresponding to a Mnrich hetaerolite [(Zn 0.75 Mn 2+ 0.25 )Mn 2 3+ O 4 ]. By ∼973 K the transformation was complete. The diffraction patterns did not show a significant increase in background during the transformation, indicating that the reaction did not involve transient amorphization. The phase change was likely triggered by loss of 1.25 of seven O atoms in the original anhydrous chalcophanite structure with a corresponding reduction of Mn 4+ to Mn 3+ and Mn 2+ .