Lattice displacements above TC in the layered manganite La1.2Sr1.8Mn2O7

Neutron-diffraction data presented in this paper demonstrate the relevance of lattice displacement above Tc, in our understanding of the evolution of the crystal structure with temperature in the layered colossal magnetoresistance manganite La1.2Sr1.8Mn2Mn2O7. The anomalous temperature behavior of thermal diffuse scattering in (La1.2Sr1.8MnO7)-O-2 strongly suggests that it arises from lattice displacements and correlates directly with anomalies in the displacement parameters of the O and Mn atoms and the Mn-O bond lengths. From our measurements, the insulator-metal transition can be described as a transition from a high-temperature state with disordered Mn-O bond lengths to a low-temperature stare with a more uniform distribution on Mn-O bonds. These observations are in agreement with polaronic charge transport above T-C in the perovskite manganites; as electron hopping is responsible for bond disorder above T-C, below the transition where e(g) carriers are delocalized, any lattice displacements are uniformly averaged. [S0163-1829(99)04429-X].