X-ray-absorption fine-structure measurements at the Mn $K$ edge as a function of temperature were performed on ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x\mathrm{Mn}{\mathrm{O}}_3$ and ${\mathrm{La}}_{0.67}$${\mathrm{Pb}}_{0.33}$Mn${\mathrm{O}}_3$. All samples have a metal-insulator (MI) transition near the ferromagnetic transition, except the ${\mathrm{Ca}}_{0.5}$ sample, which does not have a MI transition. Near $T_c$ for the samples exhibiting a MI transition, the Debye-Waller ${\sigma }^2$ for the Mn-O and Mn-Mn atom pairs increases rapidly with temperature. Since the change in ${\sigma }^2$ for the Ca/La atoms is much smaller, this nonthermal disorder must involve mainly the Mn and O atoms. These results strongly suggest that small polarons delocalize as the magnetization increases in materials which exhibit a MI transition.

• Received 29 July 1996

DOI:https://doi.org/10.1103/PhysRevB.54.R15606