Volume contraction and resistivity drop at the Jahn-Teller transition inLa1−xCaxMnO3

Abstract

We have investigated the Jahn-Teller transition accompanied by orbital order-disorder transition in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ by high temperature x-ray powder diffraction with synchrotron radiation and resistivity measurements. The unit cell volume of $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$ decreases with increasing temperature in a narrow temperature range below ${T}_{\mathit{JT}}\ensuremath{\approx}750\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and then undergoes a volume collapse at ${T}_{\mathit{JT}}$. The transition is first order. Similar behavior is also obtained in Ca-doped ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ for $x=0.025$ and 0.075. The amount of volume collapse, however, decreases with the doping and also the first order discontinuous transition crosses over to a quasi-continuous transition with doping. We interpret the volume contraction at the transition is due to a more efficient packing of the $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra in the orbitally liquid state and the crossover from the discontinuous to the quasi-continuous transition is due to the change in the anharmonic coupling parameter with the hole doping. The resistivity of $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$ decreases as a function of temperature and then shows abrupt drop at ${T}_{\mathit{JT}}$ becoming almost temperature independent at higher temperature. The resistivity of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ also decreases at ${T}_{\mathit{JT}}$ but the abrupt drop becomes smeared out at higher doping. The similar behavior of the unit cell volume and the resistivity at the Jahn-Teller transition suggests that the volume contraction at ${T}_{\mathit{JT}}$ causes delocalization of ${e}_{g}$ electrons.