SrMn3O6:: an incommensurate modulated tunnel structure

The crystallographic structure of a mixed valent manganite SrMn3O6 with a 1D modulated structure is reported. The SrMn3O6 structure can be described with the basic subcell space group Pnma (a = 9.1334(5)Angstrom; b = 2.8219(2)Angstrom; c = 12.0959(7)Angstrom), but transmission electron microscopy revealed that the study of the real structure requires a 4D-formalism approach with superspace group P2(1)/a(alphabeta0)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 NaxFexTi2-xO4 and Pb1.5BaMn6Al2O16, comprising of unusual figure-of-eight-shaped tunnels, made up of strings of edge- and corner-sharing (Fe/Ti)O-6 or (Mn/Al)O-6 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 similar to 46 K. (C) 2004 Elsevier Inc. All rights reserved.