Topotactic Reduction As a Route to New Close-Packed Anion Deficient Perovskites: Structure and Magnetism of 4H-BaMnO_(2+x)

摘要

The anion-deficient perovskite 4H-BaMnO_(2+x) has been obtained by a topotactic reduction, with LiH, of the hexagonal perovskite 4H-BaMnO_(3-x). The crystal structure of 4H-BaMnO_(2+x) was solved using electron diffraction and X-ray powder diffraction and further refined using neutron powder diffraction (S.G. Pnma, a = 10.375(2) A, b = 9.466(2) A, c = 11.276(3) A, at 373 K). The orthorhombic superstructure arises from the ordering of oxygen vacancies within a 4H (chch) stacking of close packed otype BaO_(2.5) and h-type BaO_(1.5) layers. The ordering of the oxygen vacancies transforms the Mn_2O_9 units of face-sharing MnO_6 octahedra into Mn_2O_7 (two comer-sharing tetrahedra) and Mn_2O_6 (two edge-sharing tetrahedra) groups. The Mn_2O_7 and Mn_2O_6 groups are linked by corner-sharing into a three-dimensional framework. The structures of the BaO_(2.5) and BaO_(1.5) layers are different from those observed previously in anion-deficient perovskites providing a new type of order pattern of oxygen atoms and vacancies in close packed structures. Magnetization measurements and neutron diffraction data reveal 4H-BaMnO_(2+x) adopts an antiferromagnetically ordered state below T_n ≈ 350 K.