Growth of ternary oxides in the Gd2O3-Fe2O3 system. A diffusion couple study

The simultaneous, diffusion-controlled growth of GdFeO3 (perovskite) and Gd3Fe5O12 (garnet) was studied at 1200-1400 C in Gd2O3-Fe2O3 diffusion couples. Both compounds were found to grow as parallel layers according to the parabolic rate law. The parabolic rate constants of the second kind for the exclusive growth of each compound (k1 II for GdFeO3, k2 II for Gd3Fe5O12) were calculated from the experimentally determined rate constants of the first kind assuming coupling between diffusion fluxes and chemical reactions at phase boundaries. In the case of GdFeO3, the calculated values of k1 II are in good agreement with the experimental values measured on Gd3Fe5O12-Fe2O3 couples, where exclusive growth of GdFeO3 is observed. Growth of the perovskite phase, probably related to formation of gaseous Fe(OH)2, is also observed on Gd2O3-gas- Fe2O3 couples. The values of k2 II for of Gd3Fe5O12 are very close to those found for the growth of the isostructural compound Y3Fe5O12. The most likely reaction mechanism is the coupled diffusion of Gd3 + and O2 for garnet growth and the coupled diffusion of Fe3 + and O2 for perovskite growth. The activation energy is 550 kJ mol -1 for Gd3Fe5O12 and 400 kJ mol -1 for GdFeO3.