Influence of the ionic size on the evolution of local Jahn-Teller distortions in cobaltites

Abstract

The thermodynamic and atomic structure properties of ${\mathrm{La}}_{1\ensuremath{-}x}{A}_{x}\mathrm{Co}{\mathrm{O}}_{3}$ ($A={\mathrm{Ba}}^{2+}$ and ${\mathrm{Ca}}^{2+}$) with $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.5$, a class of compounds that exhibit magnetoresistance in the presence of an applied magnetic field, have been investigated via magnetic and transport measurements and neutron scattering. While in the parent compound, the ${\mathrm{Co}}^{3+}$ ion undergoes a spin-state transition from the low-spin ground state to a dynamic intermediate-spin configuration thermally, with hole doping, the intermediate-spin state becomes static as evidenced by the Jahn-Teller octahedral splitting of the Co-O bonds. The size of the split depends strongly on the tolerance factor, where small or no Jahn-Teller distortions are observed in samples with small tolerance factors (i.e., Ca), but as the tolerance factor approaches 1 (i.e., Ba), the bond split can be as much as $0.2\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. At the same time, ferromagnetic ordering is also influenced by the tolerance factor. As it gets closer to 1, ferromagnetic coupling is enhanced due to the straightening of the Co-O-Co bonds, where the angle becomes almost 180\ifmmode^\circ\else\textdegree\fi{} that, in turn, favors double-exchange interactions between Co ions. With the ferromagnetic transition, the system becomes metallic and shows a negative magnetoresistance with field. As the tolerance factor is reduced from 1, the ferromagnetic coupling is weak and the bond angle is about 160\ifmmode^\circ\else\textdegree\fi{}.