Charge ordering and phase competition in the layered perovskite LaSr2Mn2O7

Charge-lattice fluctuations are observed in the layered perovskite manganit e LaSr2Mn2O7 by Raman spectroscopy at temperatures as high as 340 K, and wi th decreasing temperature they become static, forming a charge-ordered (CO) phase below T-CO = 210 K. In the static regime, superlattice reflections a re observed by neutron and x-ray diffraction with a propagation vector (1/4 , - 1/4, 0). Crystallographic analysis of the CO state demonstrates that th e degree of charge and orbital ordering in this manganite is weaker than th at in the three-dimensional perovskite manganites. Below T-N= 170 K, type-i i antiferromagnetism (AF) develops and competes with the charge ordering, c ausing it to eventually melt below T* = 100 K. High-resolution diffraction measurements suggest that the CO and AF states do not coincide within the s ame region of material, but rather coexist as separate phases. The transiti on to typed antiferromagnetism at lower temperatures is characterized by th e competition between these two phases.