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 T-CO = 210 K. In the static regime, superlattice reflections are observed by neutron and x-ray diffraction with a propagation vector (1/4, - 1/4, 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 T-N= 170 K, type-ii antiferromagnetism (AF) develops and competes with the charge ordering, causing it to eventually melt below T* = 100 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 typed antiferromagnetism at lower temperatures is characterized by the competition between these two phases.