Microstructural insight into the thermal decomposition of MgCl2·6H2O examined by in-situ high-temperature X-ray powder diffraction

Abstract

A sequential thermal conversion of MgCl2·6H2O, including dehydration, hydrolysis and decomposition, was studied in terms of microstructure using in-situ high-temperature X-ray diffraction. The dehydration is occurred serially from room temperature to 208 ​°C that resulted in ∼48% weight loss. A decrease in unit cell volume is more prominent during dehydration until the monohydrate formed. The volume of monohydrate is slightly increased upon monoclinic to orthorhombic conversion. A gradual decrease in crystallite size is observed during the entire period of dehydration, resulting from lattice distortions upon ongoing lattice vibrations. The dehydration is also accounted for increased lattice strain. The hydrolysis of monohydrate produced magnesium chloride hydroxide, which is partly crystalline in nature. The magnesium chloride hydroxide increased its crystallinity upon further heating and decomposed to stable MgO at 458 ​°C. The hydrolysis and decomposition, both are occurred by the elimination of HCl that resulted in ∼28% weight loss. The MgO is formed under increased crystallite size and decreased lattice strain. The unit cells of MgO are packed tightly, minimizing the influence of ongoing lattice vibrations. At elevated temperatures, major steps that are involved in lattice stabilization are the elimination of water and HCl followed by rearrangement to minimize unfavorable interactions.