Synthesis of pyrochlore Sr2Ta2O7 via continuous-flow supercritical hydrothermal approach

This study presents the successful synthesis of Sr₂Ta₂O₇ with a pyrochlore crystal structure, which was previously unreported. Traditionally, Sr₂Ta₂O₇ is synthesized in an orthorhombic layered perovskite-type structure using traditional solid-state reaction and batch-type subcritical hydrothermal methods. Here, we utilize a continuous-flow supercritical hydrothermal method, a technique primarily applied for nanoparticle synthesis and minimally explored for the synthesis of metastable crystals. Through detailed evaluations employing X-ray diffraction and scanning electron microscopy, we confirmed the synthesis of the pyrochlore phase, characterized by particles with a diameter of less than 100 nm, synthesized at 633 K. Notably, the synthesized pyrochlore structure demonstrated remarkable stability, even after thermal treatment at 773 K for 1 h. Pyrochlore Sr₂Ta₂O₇ stands out as a rare exception to the traditional tolerance factor approach among 278 known A₂B₂O₇ compositions. Systematic evaluation of formation energies through first-principles calculations revealed that the pyrochlore Sr₂Ta₂O₇ synthesized in this study is located 0.06 eV/atom above the convex hull. This achievement underscores the potential of the continuous-flow supercritical hydrothermal method in automating the exploration and discovery of novel crystal structures, suggesting a systematic pathway for advancing the field of material synthesis.