SrMn₃O₆: An incommensurate modulated tunnel structure

The crystallographic structure of a mixed valent manganite SrMn ₃O₆ with a 1D modulated structure is reported. The SrMn₃O₆ structure can be described with the basic subcell space group Pnma (a=9.1334(5)Å; b=2.8219(2)Å; c=12.0959(7)Å), but transmission electron microscopy revealed that the study of the real structure requires a 4D-formalism approach with superspace group P2 ₁/a(αβ0)00 (unique axis c), with a modulation wave vector q having the approximate components (0.52a*+0.31b*). The crystallographic structure is closely related to those of Na_xFe _xTi_2-xO₄ and Pb_1.5BaMn ₆Al₂O₁₆, comprising of unusual "figure-of-eight"-shaped tunnels, made up of strings of edge- and corner-sharing (Fe/Ti)O₆ or (Mn/Al)O₆ octahedra, with the other cations situated in the tunnel cavities. Structural refinement was performed on X-ray and neutron powder diffraction data using the 4D formalism. All atoms in the crystal are affected by a displacive modulation wave, and a saw tooth function is employed to model the displacement and occupancy of the Sr sites. Magnetic susceptibility measurements reveal a sharp antiferromagnetic transition with T_N∼46K.