Synthesis and Crystallization of La_1-xSr_xMnO_3+δ from Mechanochemically Prepared Precursors

Abstract

Mechanochemical process with the peculiarity of forming La(OH)₃ and Sr(OH)₂ as the reactive grinding aids in grinding liquids has been applied to the synthesis of solid solution La_1-xSr_xMnO_3+δ. The other starting material γ-MnO₂ is kneaded into the grinding aids to form mixed precursors of ground and hydrated MnO₂ and the hydroxides. The precursor consisting of La (OH)₃ and MnO₂ · yH₂O (LM) can be converted to orthorhombic-LaMn^IIIO₃ in argon or rhombohedral-LaMnO_3.15 in air above 700°C. Hexagonal-SrMn^IVO₃ is obtained by the heat-treatment of the precursor consisting of Sr(OH)₂ and MnO₂ · yH₂O (SM) at 600°C in air. The reactions of the mixed precursors are promoted from LM and SM, forming amorphous products at 400-600°C and crystallized to the intermediate pseudo-cubic phase at 600-800°C. The pseudo-cubic phase is transformed into orthorhombic (x≤0.1 in Ar), rhombohedral (x≤0.4 in air, x=0.2 in Ar), tetragonal (x=0.5 in air) and cubic phase (x=0.6 in air, 0.3≤x≤0.5 in Ar) at 1000°C depending on the composition. The substitution of La³⁺ in LaMn^IIIO₃ with Sr²⁺ leads the increased fraction of Mn⁴⁺ and related shrinkage of MnO₆-octahedra, which results in the reduction of lattice volume. The solubility limit of Sr²⁺ into LaMnO₃ is evaluated as x=0.57 from the liner relationship between the composition x and the lattice volume.