Lattice displacements above T-C 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 magnetoresis tance manganite La1.2Sr1.8Mn2Mn2O7. The anomalous temperature behavior of t hermal diffuse scattering in (La1.2Sr1.8MnO7)-O-2 strongly suggests that it arises from lattice displacements and correlates directly with anomalies i n the displacement parameters of the O and Mn atoms and the Mn-O bond lengt hs. From our measurements, the insulator-metal transition can be described as a transition from a high-temperature state with disordered Mn-O bond len gths to a low-temperature stare with a more uniform distribution on Mn-O bo nds. These observations are in agreement with polaronic charge transport ab ove T-C in the perovskite manganites; as electron hopping is responsible fo r bond disorder above T-C, below the transition where e(g) carriers are del ocalized, any lattice displacements are uniformly averaged. [S0163-1829(99) 04429-X].