Charge ordering and phase competition in the layered perovskite LaSr2Mn2O7

Charge-lattice fluctuations are observed in the layered perovskite manganite LaSr2Mn2O7 by Raman spectroscopy at temperatures as high as 340 K, and with decreasing temperature they become static, forming a charge-ordered ~CO! phase below TCO5210 K. In the static regime, superlattice reflections are observed by neutron and x-ray diffraction with a propagation vector ~1/4, 21/4, 0!. Crystallographic analysis of the CO state demonstrates that the degree of charge and orbital ordering in this manganite is weaker than that in the three-dimensional perovskite manganites. Below TN5170 K, type-A antiferromagnetism ~AF! develops and competes with the charge ordering, causing it to eventually melt below T*5100 K. High-resolution diffraction measurements suggest that the CO and AF states do not coincide within the same region of material, but rather coexist as separate phases. The transition to type-A antiferromagnetism at lower temperatures is characterized by the competition between these two phases.