Crystal Growth And Structure Characterization Of Tunnel, Layered And Spinel Manganese Oxides

In recent years, much attention has been paid to synthesizing and designing manganese oxides of a diversity of structures and morphologies owing to their structural flexibility and manganese valence variety. They have found applications in many fields such as batteries, catalysts, ion sieves and magnetic materials.For years, porous manganese oxides with layered or tunnel structures are very attractive, among which Na0.44MnO2 is well-known as an electrode material for its unique double tunnel structure that can greatly facilitate the Na+ mobility. It also has great potential to be used as sodium ions sensors.Na0.44MnO2 was early synthesized via high temperature solid state reaction of stoichiometric Na2CO3 and Mn2O3. Recently, Na0.44MnO2 nanowires with high aspect ratio have been synthesized using hydrothermal method. The size of the crystals obtained by the above methods is small with diameters below 500nm. In contrast, flux method is particularly suitable for improving the crystal size. We have discovered that large Na0.44MnO2 whiskers (0.01×0.01×0.1mm3) can be obtained by reacting Mn2O3 with NaCl. Single-crystal X-ray structure shows that the composition of this compound approaches Nao.5Mn02 through incorporating a new Na site in the S-shaped tunnel. NaCl acted as not only the flux but also the reactant. For the slow reaction rate, the reaction of Mn2O3 and NaCl favors larger whiskers.Eftekhari et. al. have studied the formation of Na0.5MnO2 nanowire bundles by solid state reaction at 800℃and proposed the nanowires are formed by longitudinal diffusion of Na+ across the layered MnO2. Here, NaCl flux is introduced in the reaction system to improve the crystal size of Na0.5MnO2 whiskers (needle crystal) and further examine the growth mechanism. We found that NaCl played two roles here, i.e., as flux and reactant, respectively. As a flux, NaCl can not only accelerate the reaction but also provide a suitable liquid environment for crystal growth. Meanwhile, as a reactant, NaCl can consume the residue Mn2O3 that has not reacted with Na2CO3. Therefore, pure products can be easily obtained due to the activity difference between Na2CO3 and NaCl. On the other hand, by quenching the high temperature flux, the crystals of layered structure as an intermediate product has been identified, and the crystal growth of Na0.5MnO2 was achieved via a layer-to-tunnel structure transformation that was coupled with anisotropic crystal growth and nucleus oriented aggregation.Birnessite is a layered mixed-valent manganese oxide built from edge-sharing MnO6 octahedra with Na+, K+, or other cations and H2O molecules filling the interlayer space. Its interlayer spacing is about 7 A and can be tuned by incorporation of different species, such as metallic cations, oxides and organic molecules, etc.. It is usually observed as an intermediate or used as precursor in the synthesis of tunnel structures. However, the intrinsic mechanism of layer-to-tunnel transformation has not been clear yet. Silvester et al. have studied the conversion of synthetic Na-rich buserite to hexagonal H+-exchanged birnessite at low pH and found that the process starts with a disproportionation reaction of neighboring Mn3+ ions in the manganese oxide layers to Mn4+ and Mn2+ ions. The Mn2+ ions migrate into the interlayer space, undergo further oxidation to Mn3+ by oxygen and assist the formation of corner-sharing MnO6 octahedra. However, the transformations in alkaline conditions or solid state reactions are obviously different because there is nearly no soluble Mn2+ Unexpected fascinating phases may appear when nanoporous materials are subjected to high pressure (HP), because HP not only affects the structure of the flexible open framework but also the fate of extraframework cations and the guest molecules in the nanopores, resulting in volume contraction or expansion. The layered structures, such as graphite oxide (GO), have attracted considerable interest because of their ability to accommodate different solvents under HP. GO layers are buckled, deviating from the ideal planar shape at the positions of functional group bonding. Birnessite-related materials present similar layered structure and the MnO6 octahedral layers may be also buckled due to the mixed-valent MnO6 octahedral layer of Mn3+/Mn4+ with distinct steric environment, which is expected to be magnified under HP. Because it is usually difficult to grow large crystals enough for single-crystal XRD study, to date, however, structure models of birnessite determined from powder XRD show us only simple planar MnO6 layer.In this study, HP can not only enhance the crystallinity and crystal size of birnessite, but also force Mn3+ and Mn4+ in ordered state by elongation and orientation of the Mn3+O6 octahedra. A 2×2 buckled MnO6 octahedral layers of K-BT-1 were stabilized at HP (50MPa) and transformed to a 2×1 buckled MnO6 octahedral layers of K-BT-2 when annealed at room temperature in air for one year. Both structures were determined by single-crystal XRD. The transformation of K-BT-1 to K-BT-2 involves Mn3+/Mn4+ order-disorder-order transition. Based on these buckled layers, which is a result of ordering of Mn3+/Mn4+ in separate rows and cooperative Jahn-Teller distortion of Mn3+O6 octahedra, a mechanism of structure transformation from birnessite to tunnel structures was proposed. In addition, temperature and pressure are both important factors for the crystal growth of K-BT-1. Higher temperature and pressure favor larger K-birnessite crystals. Abrupt pressure decrease is responsible for the formation and crystal growth of K-BT-1 crystals. Mn3O4 is a well-known candidate as an active catalyst for the decomposition of waste gases and also a raw material for the production of manganese zinc ferrite for magnetic cores in transformers for power supplies. Bulk Mn3O4 undergoes a ferromagnetic transition at 42 K, under which it has magnetodielectric properties. It is well-known that the properties of crystalline materials rely on their structures. As a result, the exploration of new crystal structure is very important fundamentally and practically. To our knowledge, Mn3O4 has three crystalline phases:tetragonal, cubic and orthorhombic. At ambient condition, Mn3O4 has the spinel structure. It is tetragonal distorted because of the Jahn-Teller distortion of Mn3+. Above 1443 K, it transforms to cubic spinel for the degenerate d-orbitals. At high pressure (above 10 GPa), Mn3O4 can transform to an orthorhombic marokite type structure even at room temperature.We successfully synthesized large Mn3O4 single crystals at 450℃and 100MPa in KOH flux. The crystals are shaped in octahedron, parallelepiped and triangle. Single-crystal X-ray study reveals that the main exposed surface is(101) planes. Its average crystal structure can be described as the tetragonal Mn3O4. Because of the incorporated strain and the distortion of the structure, the structure becomes incommensurate. XRD and HRTEM both unambiguously conform the contraction of the (101) plane.In short, we successfully synthesized the single crystals of tunnel Na0.5MnO2, layered K-birnessite and spinel Mn3O4. Their crystal structures were resolved by single-crystal X-ray diffraction. In addition, the crystal growth mechanism and pressure effect on the crystal growth and structure distortion are discussed.