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

• Published in: Journal of solid state chemistry 2023-06, Vol.322, p.123965, Article 123965
• Main Authors: Karunadasa, Kohobhange S.P., Rajapakse, R.M.G., Pitawala, H.M.T.G.A., Manoratne, C.H.
• Format: Article
• Language: English
• Subjects: Crystallite size; Lattice strain; MgCl2·6H2O; Microstructure; Thermal decomposition
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2023.123965

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. MgCl2·6H2O undergoes dehydration, hydrolysis and decomposition sequentially upon heating. During dehydration, the lattice strain increased with temperature while crystallite size decreased. Monohydrate undergoes hydrolysis at 259 ​°C forming MgOHCl, which is partially crystalline in nature. The MgOHCl is thermally decomposed to MgO, which consists of small unit cells, ensuring tight packing in space. Thermal stability of MgO is achieved by decreasing lattice strain and increasing crystallite size. [Display omitted] •MgCl2·6H2O undergoes dehydration, hydrolysis and decomposition sequentially upon heating.•During dehydration lattice strain increased with temperature while crystallite size decreased.•Monohydrate undergoes hydrolysis at 259 ​°C forming MgOHCl, which is partially crystalline in nature.•MgOHCl is thermally decomposed to MgO, which consists of small unit cells, ensuring tight packing in space.•Thermal stability of MgO is achieved by decreasing lattice strain and increasing crystallite size.