Thermal dehydration kinetics of a rare earth hydroxide, Gd(OH)3

This paper reports the synthesis, characterization, and dehydration kinetics of a rare earth hydroxide, Gd(OH)(3). Uniform rod-like Gd(OH)(3) powder was prepared by a colloidal hydrothermal method. The powder thus obtained dehydrated into its oxide form in a two-step process, where crystalline GdOOH was obtained as the intermediate phase. Crystal structure study revealed a monoclinic structure for GdOOH, with space group P-2/1M and lattice parameters a = 6.0633, b = 3.7107, c = 4.3266, and beta = 108.669. The first-step dehydration follows the F2 mechanism, while the second step follows the F1 model, indicating that both the steps are controlled by nucleation/growth mechanism, The activation energy Ea and frequency factor A are 231 +/- 12 kJ/mol and 2.08 x 1018 s(-1) for the first step and 496 +/- 32 kJ/mol and 7.88 x 10(33) s(-1) for the second step, respectively. Such high activation energy calculated from the experimental data can be ascribed to the high bonding energy of Gd-O bond, and the difference in activation energy for the two steps is due to the change in the bond length of hexagonal Gd(OH)3 and monoclinic GdOOH. (c) 2006 Wiley Periodicals, Inc.