The synthesis and complex anion-vacancy ordered structure of La 0.33Sr 0.67MnO 2.42

Abstract

The low-temperature topotactic reduction of La 0.33Sr 0.67MnO 3 with NaH results in the formation of La 0.33Sr 0.67MnO 2.42. A combination of neutron powder and electron diffraction data show that La 0.33Sr 0.67MnO 2.42 adopts a novel anion-vacancy ordered structure with a 6-layer OOTOOT' stacking sequence of the ‘octahedral’ and tetrahedral layers ( Pcmb, a=5.5804(1) Å, b=23.4104(7) Å, c=11.2441(3) Å). A significant concentration of anion vacancies at the anion site, which links neighbouring ‘octahedral’ layers means that only 25% of the ‘octahedral’ manganese coordination sites actually have 6-fold MnO 6 coordination, the remainder being MnO 5 square-based pyramidal sites. The chains of cooperatively twisted apex-linked MnO 4 tetrahedra adopt an ordered -L–R–L–R- arrangement within each tetrahedral layer. This is the first published example of a fully refined structure of this type which exhibits such intralayer ordering of the twisted tetrahedral chains. The rationale behind the contrasting structures of La 0.33Sr 0.67MnO 2.42 and other previously reported reduced La 1− x Sr x MnO 3− y phases is discussed.