Structural Evolution and Electronic Properties of La_1+<i>x</i>Sr_2-<i>x</i>Mn₂O₇

A detailed study of the layered manganite La1+xSr2-xMn2O7 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)2Mn2O6 units, is characterized by a reluctance to deform the latter. This manifests as a "pumping" of the larger SrII 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.