Tetragonal lattice instability and monoclinic distortion in A-type antiferromagnetic La_0.4Sr_0.6MnO₃ manganites

The microstructural properties of A-type antiferromagnet La _0.4Sr_0.6MnO₃ were investigated through Rietveld analysis of neutron power diffraction data. Above 240 K, the temperature dependence of lattice parameters of the tetragonal structure exhibited a negative slope for c/2 and a positive slope for a (=b). An increase of tetragonality ((c/2)/a) on cooling probably causes a lattice instability, which may trigger a structural distortion to a monoclinic structure. At ∼240 K, the A-type spin/orbital ordering was presumably related to the two dimensional-like anisotropic structural distortion (ΔaΔc+0.9% and Δb/2=-2.0%) and a significant volume discontinuity ΔV/V∼0.002. These microscopic structural changes were attributed to a distortion of the MnO₆ octahedron, which results in a change in the Mn-O bond length and Mn-O-Mn bond angles.