Structural evolution and electronic properties of La 1+x Sr 2−x Mn 2 O 7

A detailed study of the layered manganite La 1+x Sr 2−x Mn 2 O 7 has been performed, establishing that within the composition range 0.1 ≤ x ≤ 0.45 the phases crystallize in the I4/mmm space group. The evolution of structural parameters with x in this composition range has been followed using a novel application of an existing program for the Rietveld analysis of powder diffraction data. The structure, a familiar intergrowth of rock-salt (La,Sr)O slabs and double perovskite (La,Sr) 2 Mn 2 O 6 units, is characterized by a reluctance to deform the latter. This manifests as a pumping of the larger Sr II ion into the 12-coordinate site of the structure as x is increased. We report these features of the structure as well as electrical transport and magnetic properties, in light of recent observations of giant, negative magnetoresistance in these systems.