A systematic study of lanthanide titanates (A₂Ti₂O₇) chemical durability: corrosion mechanisms and control parameters

Dense lanthanide titanates ranging from La₂Ti₂O₇ to Yb₂Ti₂O₇ were synthesized by spark plasma sintering with similar micro-sized microstructure, and their chemical durability and degradation mechanisms were investigated by semi-dynamic leaching tests. Different corrosion mechanisms from incongruent to congruent dissolution were observed with preferential elemental releases of lanthanides in A₂Ti₂O₇ (A = La to Gd) and Ti in Er₂Ti₂O₇ and Yb₂Ti₂O₇. Initial leaching was controlled by surface dissolution and long-term dissolution shifted to a diffusion-controlled mechanism for titanate pyrochlores from Sm₂Ti₂O₇ to Yb₂Ti₂O₇. Both short-term and long-term leaching rates decreased from La₂Ti₂O₇ to Yb₂Ti₂O₇, and materials became more corrosion resistant with decreasing ionic radius ratios of the rare-earth elements and titanium. Amorphous passivation films of several nm-thick enriched with Ti formed on the surfaces of the corroded materials, and the formation mechanisms were governed either by surface reorganization with preferential release of lanthanides or dissolution-precipitation from saturated bulk solutions with preferential release of titanium. These results demonstrate that the ionic size plays a dominant role in determining chemical durability of lanthanide titanates and a strong correlation is established among chemical composition/structural variation, passivation film formation and corrosion mechanisms, and materials chemical durability.