Synthesis of pyrochlore Sr ₂ Ta ₂ O ₇ via continuous‐flow supercritical hydrothermal approach

Abstract This study presents the successful synthesis of Sr 2 Ta 2 O 7 with a pyrochlore crystal structure, which was previously unreported. Traditionally, Sr 2 Ta 2 O 7 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 Sr 2 Ta 2 O 7 stands out as a rare exception to the traditional tolerance factor approach among 278 known A 2 B 2 O 7 compositions. Systematic evaluation of formation energies through first‐principles calculations revealed that the pyrochlore Sr 2 Ta 2 O 7 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.