Synthesis and Structure of Vacancy-Ordered Perovskite Ba ₆ Ta ₂ Na ₂ X ₂ O ₁₇ (X = P, V): Significance of Structural Model Selection on Discovered Compounds

Vacancy-ordered 12H-type hexagonal perovskites Ba₆Ru₂Na₂X₂O₁₇ (X = P, V) with a (<i>c</i>'<i>cchcc</i>)₂ stacking sequence of [BaO₃]_<i>c</i>, [BaO₃]_<i>h</i>, and [BaO₂]_<i>c</i>' layers, where <i>c</i> and <i>h</i> represent a cubic and hexagonal stacking sequence, were previously reported by Quarez et al. in 2003. They also synthesized Ba₆Ta₂Na₂V₂O₁₇, but structural refinement was absent. Very recently, Szymanski et al. reported 43 new compounds, including 12H-type Ba₆Ta₂Na₂V₂O₁₇, using large-scale ab initio phase-stability data from the Materials Project and Google DeepMind with the assistance of an autonomous laboratory. But their structural refinement was very poor. Here, we report the synthesis and structure of Ba₆Ta₂Na₂V₂O₁₇, which does not have 12H-type structure but has a vacancy-ordered 6C-type perovskite with a (<i>c</i>'<i>ccccc</i>) stacking sequence of [BaO₃]_<i>c</i> and [BaO₂]_<i>c</i>' layers. We also report the phosphite analogue Ba₆Ta₂Na₂P₂O₁₇ as a new compound. We claim an importance of careful structural characterization on newly discovered compounds; otherwise, the database constructed will lose credibility.