Tentative Phase Relationships in the System CaHfTi 2 O 7 ‐Gd 2 Ti 2 O 7 with up to 15 mol% Additions of Al 2 TiO 5 and MgTi 2 O 5

The phase relationships in the CaHfTi 2 O 7 ‐Gd 2 Ti 2 O 7 (zirconolite‐pyrochlore) pseudobinary system were investigated, after heating at 1500°C, because of their importance in the design of pyrochlore‐rich titanate ceramics for immobilization of impure surplus plutonium. Up to 15 mol% of MgTi 2 O 5 and Al 2 TiO 5 were added to CaHfTi 2 O 7 ‐Gd 2 Ti 2 O 7 compositions to elucidate the effects of divalent and trivalent impurities on the phase stability within these systems. From X‐ray diffractometry analysis, scanning electron microscopy, and energy dispersive X‐ray spectrometry, phase formation and compositional stability limits were evaluated. The main phases observed in these systems were pyrochlore, perovskite, and polytypes of zirconolite. The formation of the 2 M ‐, 4 M ‐, and 3 O ‐zirconolite polytypes was dependent on the amount of aluminum or magnesium present. In the magnesium system, a large area of pyrochlore‐only was observed, which indicated that divalent impurities of appropriate ionic size could be readily incorporated in the eightfold site of the pyrochlore. The locations of the tentative phase boundaries are discussed with respect to the chemical composition.