The synthesis and complex anion-vacancy ordered structure of La0.33Sr0.67MnO2.42

The low-temperature topotactic reduction of La0.33Sr0.67MnO3 with NaH results in the formation of La0.33Sr0.67MnO2.42. A combination of neutron powder and electron diffraction data show that La0.33Sr0.67MnO2.42 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) angstrom, b=23.4104(7)angstrom, c=1 1.2441(3)angstrom). 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 MnO6 coordination, the remainder being MnO5 square-based pyramidal sites. The chains of cooperatively twisted apex-linked MnO4 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 La0.33Sr0.67MnO2.42 and other previously reported reduced La1-xSrxMnO3-y phases is discussed. (C) 2011 Elsevier Inc. All rights reserved.