Doping has generally been served as one of the most efficient strategies for the improvement of wide band gap semiconductor photocatalysts . However, atomic arrangements of doped elements in the host crystal structure have often been overlooked, particularly during the investigation and evaluation of dopant functionality. It is known that electronic structures of a semiconductor are profoundly controlled by atomic occupancies in different crystallographic positions. Knowledge about dopant accommodations in the crystal structure and their influence towards photocatalytic performance is highly desired. Here, we investigated four compositional and structural relevant compounds CrNbO 4 , CrTaO 4 , Sr 2 CrNbO 6 and Sr 2 CrTaO 6 with the aim to study the effects of cation ordering/disordering upon their photocatalytic activity . Our results showed that ordered cations, namely Cr and Nb/Ta, are detrimental to the photocatalytic performance. Theoretical calculations indicate that cation ordering would enlarge the band gap and inhibit charge transfer between Cr and Nb/Ta. Our findings imply that ordered dopants in photocatalytic materials would severely offset the benefits of doping and might be a reason for the decreased activity at high doping levels often encountered.
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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