Structures and Magnetism of La_1–<i>x</i>Sr_<i>x</i>MnO_{3–(0.5+<i>x</i>)/2} (0.67 ≤ <i>x</i> ≤ 1) Phases

Topotactic reduction of La1–xSrxMnO3 (0.67 ≤ x ≤ 1) phases with sodium hydride yields a series of isoelectronic materials of composition La1–xSrxMnO3–(0.5+x)/2. Lanthanum rich members of the series (0.67 ≤ x ≤ 0.83) adopt anion deficient perovskite structures with a 6-layer -OTOOT′O- stacking sequence of sheets of octahedra/square-based pyramids (O) and sheets of tetrahedra (T). The strontium rich members of the series (0.83 ≤ x ≤ 1) incorporate “step defects” into this 6-layer structure in which the OTOOT′O stacking sequence is converted into either OOTOOT′ or TOOT′OO at a defect plane which runs perpendicular to the [201] lattice plane. The step defects appear to provide a mechanism to relieve lattice strain and accommodate additional anion deficiency in phases with x > 0.83. Magnetization and neutron diffraction data indicate La1–xSrxMnO3–(0.5+x)/2 phases adopt antiferromagnetically ordered states at low-temperature in which the ordered arrangement of magnetic spins is incommensurate with the crystallographic lattice

Topotactic Oxidative and Reductive Control of the Structures and Properties of Layered Manganese Oxychalcogenides

Topotactic modification, by both oxidation and reduction, of the composition, structures, and magnetic properties of the layered oxychalcogenides Sr4Mn3O7.5Cu2Ch2 (Ch = S, Se) is described. These Mn3+ compounds are composed of alternating perovskite-type strontium manganese oxide slabs separated by anti-fluorite-type copper chalcogenide layers and are intrinsically oxide deficient in the central layer of the perovskite slabs. The systems are unusual examples of perovskite-related compounds that may topotactically be both oxidized by fluorination and reduced by deintercalation of oxygen from the oxide-deficient part of the structure. The compounds exhibit antiferromagnetic ordering of the manganese magnetic moments in the outer layers of the perovskite slabs, while the other moments, in the central layers, exhibit spin-glass-like behavior. Fluorination has the effect of increasing the antiferromagnetic ordering temperature and the size of the ordered moment, whereas reduction destroys magnetic long-range order by introducing chemical disorder which leads to both further disorder and frustration of the magnetic interactions in the manganese oxide slab

Topotactic Oxidative and Reductive Control of the Structures and Properties of Layered Manganese Oxychalcogenides

Topotactic modification, by both oxidation and reduction, of the composition, structures, and magnetic properties of the layered oxychalcogenides Sr4Mn3O7.5Cu2Ch2 (Ch = S, Se) is described. These Mn3+ compounds are composed of alternating perovskite-type strontium manganese oxide slabs separated by anti-fluorite-type copper chalcogenide layers and are intrinsically oxide deficient in the central layer of the perovskite slabs. The systems are unusual examples of perovskite-related compounds that may topotactically be both oxidized by fluorination and reduced by deintercalation of oxygen from the oxide-deficient part of the structure. The compounds exhibit antiferromagnetic ordering of the manganese magnetic moments in the outer layers of the perovskite slabs, while the other moments, in the central layers, exhibit spin-glass-like behavior. Fluorination has the effect of increasing the antiferromagnetic ordering temperature and the size of the ordered moment, whereas reduction destroys magnetic long-range order by introducing chemical disorder which leads to both further disorder and frustration of the magnetic interactions in the manganese oxide slab

Synthesis and Structural Characterization of La_1−<i>x</i>A_<i>x</i>MnO_2.5 (A = Ba, Sr, Ca) Phases: Mapping the Variants of the Brownmillerite Structure

Analysis of the structural parameters of phases that adopt brownmillerite-type structures suggests the distribution of the different complex ordering schemes adopted within this structure type can be rationalized by considering both the size of the separation between the tetrahedral layers and the tetrahedral chain distortion angle. A systematic study using structural data obtained from La1−xAxMnO2,5 (A = Ba, Sr, Ca,) phases, prepared by the topotactic reduction of the analogous La1−xAxMnO3 perovskite phases, was performed to investigate this relationship. By manipulating the A-cation composition, both the tetrahedral layer separation and tetrahedral chain distortion angle in the La1−xAxMnO2,5 phases were controlled and from the data obtained a “structure map” of the different brownmillerite variants was plotted as a function of these structural parameters. This map has been extended to include a wide range of reported brownmillerite phases showing the structural ideas presented are widely applicable. The complete structural characterization of La1−xAxMnO2,5 0.1 ≤ x ≤ 0.33, A = Ba; 0.15 ≤ x ≤ 0.5 A = Sr, and 0.22 ≤ x ≤ 0.5 A = Ca is described and includes compositions which exhibit complex intralayer ordered structures and Mn2+/Mn3+ charge ordering

Fragmentation of an Infinite ZnO₂ Square Plane into Discrete [ZnO₂]²⁻ Linear Units in the Oxyselenide Ba₂ZnO₂Ag₂Se₂

Fragmentation of an Infinite ZnO2 Square Plane into Discrete [ZnO2]2− Linear Units in the Oxyselenide Ba2ZnO2Ag2Se2</sub

Topotactic Reduction As a Route to New Close-Packed Anion Deficient Perovskites: Structure and Magnetism of 4H-BaMnO_2+<i>x</i>

The anion-deficient perovskite 4H-BaMnO2+x has been obtained by a topotactic reduction, with LiH, of the hexagonal perovskite 4H-BaMnO3−x. The crystal structure of 4H-BaMnO2+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) Å, b = 9.466(2) Å, c = 11.276(3) Å, at 373 K). The orthorhombic superstructure arises from the ordering of oxygen vacancies within a 4H (chch) stacking of close packed c-type BaO2.5 and h-type BaO1.5 layers. The ordering of the oxygen vacancies transforms the Mn2O9 units of face-sharing MnO6 octahedra into Mn2O7 (two corner-sharing tetrahedra) and Mn2O6 (two edge-sharing tetrahedra) groups. The Mn2O7 and Mn2O6 groups are linked by corner-sharing into a three-dimensional framework. The structures of the BaO2.5 and BaO1.5 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-BaMnO2+x adopts an antiferromagnetically ordered state below TN ≈ 350 K

Fragmentation of an Infinite ZnO₂ Square Plane into Discrete [ZnO₂]²⁻ Linear Units in the Oxyselenide Ba₂ZnO₂Ag₂Se₂

Fragmentation of an Infinite ZnO2 Square Plane into Discrete [ZnO2]2− Linear Units in the Oxyselenide Ba2ZnO2Ag2Se2</sub

Coupled Cation and Charge Ordering in the CaMn₃O₆ Tunnel Structure

The synthesis and crystal structure of a mixed valent manganite CaMn3O6, equivalent to Ca2/3Mn2O4, are reported, along with the magnetic properties. The structure was determined using electron diffraction and high-resolution transmission electron microscopy and refined from X-ray and neutron powder diffraction data (a = 10.6940(3) Å, b = 11.3258(3) Å, c = 8.4881(2) Å, β = 122.358(2)°, space group P21/a, RI = 0.037, RP = 0.118). The structure is based on a framework of double chains of edge-sharing MnO6 octahedra. The corner-sharing chains form a framework with six-sided tunnels, identical to that of the CaFe2O4 structure. The Ca2+ cations are located in the tunnels. Compared to CaFe2O4, one-third of the Ca positions in the tunnels remain vacant, with an ordered distribution of vacant and occupied sites. The empty sites in neighboring Ca chains are shifted relative to each other along the c-axis by one period of the CaFe2O4 subcell, which results in a symmetry decrease from orthorhombic to monoclinic. Based on the interatomic Mn−O distances, the charge ordering in CaMn3O6 is discussed. The compound exhibits a strong anti-ferromagnetic character, and differences between the zfc and fc magnetization curves at low temperature suggest ferro- or ferrimagnetic interactions

Mn(I) in an Extended Oxide: The Synthesis and Characterization of La_1–<i>x</i>Ca_<i>x</i>MnO_2+δ (0.6 ≤ <i>x</i> ≤ 1)

Reduction of La1–xCaxMnO3 (0.6 ≤ x ≤ 1) perovskite phases with sodium hydride yields materials of composition La1–xCaxMnO2+δ. The calcium-rich phases (x = 0.9, 1) adopt (La0.9Ca0.1)0.5Mn0.5O disordered rocksalt structures. However local structure analysis using reverse Monte Carlo refinement of models against pair distribution functions obtained from neutron total scattering data reveals lanthanum-rich La1–xCaxMnO2+δ (x = 0.6, 0.67, 0.7) phases adopt disordered structures consisting of an intergrowth of sheets of MnO6 octahedra and sheets of MnO4 tetrahedra. X-ray absorption data confirm the presence of Mn(I) centers in La1–xCaxMnO2+δ phases with x 1–xCaxMnO2+δ (x = 0.6, 0.67, 0.7) phases become antiferromagnetically ordered at low temperature

Microstructural Activation of a Topochemical Reduction Reaction

The progress of the topochemical reduction reaction that converts LaSrNiRuO6 into LaSrNiRuO4 depends on the synthesis conditions used to prepare the oxidized phase. Samples of LaSrNiRuO6 that have been quenched from high temperature can be readily and rapidly converted into LaSrNiRuO4. In contrast, samples that have been slow-cooled cannot be completely reduced. This reactivity difference is attributed to the differing microstructures of the quenched and slow-cooled samples, with the former having much smaller average crystalline domain sizes and larger lattice strains than the latter. A mechanism to explain this effect is presented, in which the greater “plasticity” of small crystalline domains helps lower the activation energy of the reduction reaction. In addition, we propose that the enhanced lattice strain in quenched samples also acts to destabilize the host phase, further enhancing reactivity. These observations suggest that the microstructure of a material can be used to “activate” topochemical reactions in the solid state, expanding the scope of phases that can be prepared by this type of reaction