Mild Hydrothermal Syntheses and Thermal Behaviors of Hydrogarnets Sr3M2(OH)12 (M = Cr, Fe, and Al)

Hydrogarnets Sr3M2(OH)12 (M = Cr, Fe, and Al) were synthesized from mild hydrothermal systems at 240 °C with Sr(OH)2·8H2O, Cr(NO3)3·9H2O, Fe(NO3)3·9H2O, and Al(OH)3 as the starting materials. High solubility of the input species and basic concentration in the reaction systems considerably lower crystallization temperatures and enhance the crystallinity of the resulting hydrogarnets. The structural stability and thermal properties of Sr based hydrogarnets were investigated by X-ray diffraction, DTA-TG, infrared, and X-ray photoelectron spectroscopies. With increasing temperature, Sr based hydrogarnets underwent fast dehydration due to the loss of the nearest OH- species of Sr−OH dodecahedra. During the course of decomposition, valence variations of the hydrogarnet framework ions, e.g., Fe and Cr directly determined the decomposition process and phase compositions in final decomposition products. The decomposition product of Sr3Cr2(OH)12 in air was a mixture of SrCrO4 and SrO, but in N2, a mixture of a rhom...