Lattice effects in La1−xCaxMnO3 (x=0→1): Relationships between distortions, charge distribution, and magnetism

Abstract

X-ray-absorption fine-structure measurements from the Mn and La $K$ edges of samples of ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ $(x=\overrightarrow{0}1)$ are presented as a function of temperature. In the insulating state, distortions of the Mn-O environment are found to be linear with calcium concentration $x.\mathrm{}$ These distortions are consistent with a model where individual ${\mathrm{Mn}}^{3+}$ sites have strong Jahn-Teller (JT) distortions while ${\mathrm{Mn}}^{4+}$ sites do not, although intermediate distortions are also possible. Comparisons to simulations of various possible bond length distributions show that these distortions are best modeled as a JT distortion, as opposed to charge disproportionation or other models. In the metallic state, at least $\sim 70\%$ of the distortion is removed. A similar effect is seen in the Mn-Mn bond length distribution. The La-O bond length distribution does not change significantly through the ferromagnetic transition, constraining possible changes in the Mn-O-Mn bond angle to less than 0.5°. The changes in the ${\mathrm{MnO}}_6$ distortion that occur near $T_c$ are also present (to a lesser degree) in an insulating sample with $x=\mathrm{0.12.}\mathrm{}$ The functional relation between the ${\mathrm{MnO}}_6$ distortions and the magnetization is determined, and compared to transport measurements.