Selective Synthesis of Defect-Rich LaMnO3 by Low-Temperature Anion Cometathesis

The synthesis of complex oxides at low temperatures brings forward aspects of chemistry not typically considered. This study focuses on perovskite LaMnO3, which is of interest for its correlated electronic behavior tied to the oxidation state and thus the spin configuration of manganese. Traditional equilibrium synthesis of these materials typically requires synthesis reaction temperatures in excess of 1000 °C, followed by subsequent annealing steps at lower temperatures and different p(O2) conditions to manipulate the oxygen content postsynthesis (e.g., LaMnO3+x). Double-ion exchange (metathesis) reactions have recently been shown to react at much lower temperatures (500-800 °C), highlighting a fundamental knowledge gap for how solids react at lower temperatures. Here, we revisit the metathesis reaction, LiMnO2 + LaOX, where X is a halide or mixture of halides, using in situ synchrotron X-ray diffraction. These experiments reveal low reaction onset temperatures (ca. 450-480 °C). The lowest reaction temperatures are achieved by a mixture of lanthanum oxyhalide precursors: 2 LiMnO2 + LaOCl + LaOBr. In all cases, the resulting products are the expected alkali halide salt and defective La1-ϵMn1-ϵO3, where ϵ = x/(3 + x). We observe a systematic variation in defect concentration, consistent with a rapid stoichiometric local equilibration of the precursors and the subsequent global thermodynamic equilibration with O2 (g), as revealed by computational thermodynamics. Together, these results reveal how the inclusion of additional elements (e.g., Li and a halide) leads to the local equilibrium, particularly at low reaction temperatures for solid-state chemistry.

Synthesis, Structure, Electrical and Magnetic Properties of Li Substituted La/BaMnO3–δ

The characterization of the polycrystalline La0.7Ba0.3MnO3-δ (LBM) and La0.7Ba0.3Mn0.9Li0.1O3-δ (LBMLi) samples synthesized via the polymerized complex, sol-gel technique indicates the formation of cubic perovskite crystal structure of both samples by sintering at 900ºC for 2h. Substitution of Mn by Li having smaller ionic radius, leads to decrease the lattice constant and the cell volume which gives rise to a small shift and small intensity difference of the XRD patterns. A nanocrystalline structure of LBMLi with particles in the size range of 30–100nm was obtained. The temperature dependence of electrical conductivity and magnetic susceptibility of LBM and LBMLi were studied. The smaller electrical conductivity values and the calculated magnetic properties of Li substituted the Mn in LBM sample were investigated.