MILD HYDROTHERMAL SYNTHESES AND THERMAL BEHAVIORS OF HYDROGARNETS SR3M2(OH)(12) (M = CR, FE, AND AL)

Hydrogarnets Sr3M2(OH)(12)(M = Or, Fe, and Al) were synthesized from m ild hydrothermal systems at 240 degrees C with Sr(OH)(2) . 8H(2)O, Cr( NO3)(3) . 9H(2)O, Fe(NO3)(3) . 9H(2)O, and Al(OH)(3) as the starting m aterials. High solubility of the input species and basic concentration in the reaction systems considerably lower crystallization temperatur es and enhance the crystallinity of the resulting hydrogarnets. The st ructural stability and thermal properties of Sr based hydrogarnets wer e investigated by X-ray diffraction, DTA-TG, infrared, and X-ray photo electron spectroscopies. With increasing temperature, Sr based hydroga rnets underwent fast dehydration due to the loss of the nearest OH- sp ecies of Sr-OH dodecahedra. During the course of decomposition, valenc e variations of the hydrogarnet framework ions, e.g., Fe and Or direct ly determined the decomposition process and phase compositions in fina l decomposition products. The decomposition product of Sr3Cr2(OH)(12) in air was a mixture of SrCrO4 and SrO, but in N-2, a mixture of a rho mbohedral phase Sr4Cr2O8 with some amorphous phase was observed; in ei ther O-2 or N-2, Sr3Fe2(OH)(12) decomposed to an unknown phase, which was not a perovskite Sr3Fe2O7-x. Sr3Al2(OH)(12) decomposed to a single phase Sr3Al2O6. In Sr-Fe hydrogarnet, two different symmetric octahed ra of Pe(OH)B were revealed by Mossbauer spectroscopy. Mild hydrotherm al crystallization and the thermal decomposition behaviors of Sr-based hydrogarnets are discussed.