Hydrothermal synthesis and formation mechanisms of lanthanum tin pyrochlore oxide

Well-defined La2Sn2O7 with a phase-pure pyrochlore structure was produced b y hydrothermal synthesis at temperatures as low as 200 degreesC. Production of phase-pure La2Sn2O7 requires a pH above 10, and higher pH accelerates t he crystallization process. The synthesis produced spherical particles of a verage particle size similar to0.59 mum (+/-0.12) and surface area similar to 14.1 m(2)/g. SEM and TEM observation for morphologic evolution and kinet ic analysis during crystallization indicated that La2Sn2O7 formation probab ly proceeds via a two-step reaction. First a transient dissolution-precipit ation occurs. Then the primary crystallites aggregate because of their coll oidal instability, and heterocoagulation with the lanthanum hydrous oxide p recursor particles also occurs. The sluggish reaction rate at the later sta ge of reaction is characterized by an in situ transformation, where the sol uble tin species is diffused through the porous La2Sn2O7 aggregates to reac t with entrapped lanthanum precursors.