Direct Observation of Lattice Polaron Formation in the Local Structure ofLa1−xCaxMnO3

The local atomic structure of ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ ( $x=0.12,$ 0.21, and 0.25) has been studied using pair-distribution-function analysis of neutron powder-diffraction data. A change is seen in the local structure which can be correlated with the metal-insulator transition in the $x=0.21\mathrm{}$ and 0.25 samples. This local structural change is modeled as an isotropic collapse of oxygen towards Mn of magnitude $\delta =0.12Å$ occurring on one in four Mn sites. We argue that this is the direct observation of lattice polaron formation, associated with the metal-insulator transition in these materials.