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 (ZnMn3O7·3H2O) 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 (ZnMn3O7) 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 [(Zn0.75Mn2+0.25)Mn23+O4]. 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 Mn4+ to Mn3+ and Mn2+.