New oxygen-deficient cationic-ordered perovskites containing turquoise-coloring Mn5+O4 tetrahedral layers
Ba 6 Na 2 M 2 Mn 2 O 17 (M=Nb, Ta) oxides were synthesized by high-temperature solid-state reaction. The compounds adopt 6-layer perovskite-related structure (referred to as 6C) in P 3 ̅ m 1, analogous to Ba 6 Na 2 Nb 2 P 2 O 17 . The 6C structure consists of cubic ( c ) BaO 3 layers and pseudo-cubic ( c' ) oxygen-vacancy-ordered BaO 2 layers stacked according to a sequence of c'ccccc . Ordering of oxygen vacancies in oxygen-deficient c' -BaO 2 layers leads to two successive isolated tetrahedral layers, which stabilize an unusual +5 oxidation state for Mn cations in the tetrahedral sites. In Ba 6 Na 2 M 2 Mn 2 O 17 , these two Mn 5+ O 4 layers are sandwiched by two single octahedral NaO 6 layers that connected by two successive octahedral NbO 6 layers, forming alternative 2:1-ordered (Ba 3 NaM 2 O 9 ) - and (Ba 3 NaMn 2 O 8 ) + perovskite-like units along the stacking direction. The Mn 5+ O 4 tetrahedral units act as a turquoise chromophore in Ba 6 Na 2 M 2 Mn 2 O 17 , making these two compounds potential turquoise-coloring materials for the cool pigments .
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