Topotactic reduction as a synthetic route for the preparation of low-dimensional Mn(II) oxide phases: The structure and magnetism of LaAMnO4-x (A = Sr, Ba)

Abstract

Reaction of LaSrMnO₄ with CaH₂ at 420 °C yields LaSrMnO_3.67(3). Raising the temperature to 480 °C yields the Mn(II) phase LaSrMnO_3.50(2). Neutron powder diffraction data show both phases adopt body-centred orthorhombic crystal structures (LaSrMnO_3.67(3), Immm: a = 3.7256(1) Å, b = 3.8227(1) Å, c = 13.3617(4) Å; LaSrMnO_3.50(2), Immm: a = 3.7810(1) Å, b = 3.7936(1) Å, c = 13.3974(3) Å) with anion vacancies located within the equatorial MnO_2-x planes of the materials. Analogous reactivity is observed between LaBaMnO₄ and CaH₂ to yield body-centred tetragonal reduced phases (LaBaMnO_3.53(3), I4/mmm: a = 3.8872(1)Å, c = 13.6438(2) Å). Low-temperature neutron diffraction and magnetisation data show that LaSrMnO_3.5 and LaBaMnO_3.5 exhibit three-dimensional antiferromagnetic order below 155 K and 135 K respectively. Above these temperatures, they exhibit two-dimensional antiferromagnetic order with paramagnetic behaviour observed above 480 K in both phases. The origin of the low dimensional magnetic order and ordering of the anion vacancies in the reduced phases is discussed.