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

Time-resolved synchrotron Xâray 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 R3Ì... 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_0.75Mn²⁺_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⁺.