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 Ba6 Ru2 Na2 X2 O17 (X = P, V) with a ( c ' cchcc )2 stacking sequence of [BaO3 ] c , [BaO3 ] h , and [BaO2 ] c ' layers, where c and h represent a cubic and hexagonal stacking sequence, were previously reported by Quarez et al. in 2003. They also synthesized Ba6 Ta2 Na2 V2 O17 , but structural refinement was absent. Very recently, Szymanski et al. reported 43 new compounds, including 12H-type Ba6 Ta2 Na2 V2 O17 , 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 Ba6 Ta2 Na2 V2 O17 , which does not have 12H-type structure but has a vacancy-ordered 6C-type perovskite with a ( c ' ccccc ) stacking sequence of [BaO3 ] c and [BaO2 ] c ' layers. We also report the phosphite analogue Ba6 Ta2 Na2 P2 O17 as a new compound. We claim an importance of careful structural characterization on newly discovered compounds; otherwise, the database constructed will lose credibility.