Subsolidus phase diagram of the Bi2O3-Gd2O3 system and the morphology of phase separation

The low-temperature, subsolidus phase diagram for the binary system Bi2O3-Gd2O3 was determined using X-ray diffraction (XRD), electron microprobe analysis (EPMA), and conductivity measurements. The high-temperature phase in Bi2O3-rich compositions is cubic (CaF2 structure). A simple eutectoid reaction from cubic --> monoclinic + rhombohedral phases was observed, leading to the formation of a cellular structure. The eutectoid temperature and composition are similar to 683 degrees C and 8.8% Gd2O3,dagger respectively, The cubic phase containing similar to 27.5% Gd2O3 congruently transforms into the rhombohedral phase at similar to 810 degrees C, The solubility of Gd2O3 in the monoclinic phase was determined to be negligible, The concentration of Gd2O3 in the rhombohedral phase in equilibrium with the monoclinic phase is similar to 18% Gd2O3. Samples with Gd2O3 concentration greater than or equal to 10% undergo a composition-invariant, thermally activated (massive) transformation to a metastable rhombohedral phase below a composition-dependent critical temperature, T-c(2), followed by its decomposition into a rhombohedral (of equilibrium composition) + monoclinic two-phase mixture upon further annealing, The resulting monoclinic phase appears to form by nucleation and growth. Samples containing less than or equal to 7% Gd2O3 undergo cubic --> tetragonal displacive transformation upon cooling below another composition-dependent critical temperature, T-c(1). A long-term annealing leads to decomposition of the tetragonal phase into a mixture of rhombohedral and monoclinic phases. The resulting microstructure was extremely fine and typical of a cellular reaction.