Hydrothermal synthesis and formation mechanisms of lanthanum tin pyrochlore oxide

Well-defined La2Sn2O7 with a phase-pure pyrochlore structure was produced by hydrothermal synthesis at temperatures as low as 200 degreesC. Production of phase-pure La2Sn2O7 requires a pH above 10, and higher pH accelerates the crystallization process. The synthesis produced spherical particles of average 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 kinetic analysis during crystallization indicated that La2Sn2O7 formation probably proceeds via a two-step reaction. First a transient dissolution-precipitation occurs. Then the primary crystallites aggregate because of their colloidal instability, and heterocoagulation with the lanthanum hydrous oxide precursor particles also occurs. The sluggish reaction rate at the later stage of reaction is characterized by an in situ transformation, where the soluble tin species is diffused through the porous La2Sn2O7 aggregates to react with entrapped lanthanum precursors.