Phase relations and high temperature-XRD studies of Gd2−xNdxTi2O7 solid solutions

A series of compositions with the general formula Gd_2−xNd_xTi₂O₇ (0.0 ≤ x ≤ 2.0) was prepared by ceramic sintering and characterized by powder XRD. Nd³⁺ has been used as a surrogate for Am³⁺, an actinide found in spent nuclear fuel. One end member, Gd₂Ti₂O₇, had the pyrochlore structure and formed solid solutions with Nd₂Ti₂O₇ up to the nominal composition Gd_1.2Nd_0.8Ti₂O₇. An increase in lattice parameter was observed as a function of x in the series Gd_2−xNd_xTi₂O₇ in the composition range 0.0 ≤ x ≤ 0.8. Compositions Gd_2−xNd_xTi₂O₇ in the range 0.8 ≤ x ≤ 1.4 were biphasic. From x ≥ 1.6, the solid solutions are monoclinic, isotypic with Nd₂Ti₂O_7. These results were explained based on the radius (r_A/r_B) ratio of the cations. High temperature-XRD studies on cubic pyrochlores showing thermal expansion are reported.