We have investigated the Jahn-Teller transition accompanied by orbital order-disorder transition in ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x\mathrm{Mn}{\mathrm{O}}_3$ 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 $T_{\mathit{JT}}\approx 750\mathrm{K}$ and then undergoes a volume collapse at $T_{\mathit{JT}}$. The transition is first order. Similar behavior is also obtained in Ca-doped ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x\mathrm{Mn}{\mathrm{O}}_3$ 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 ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x\mathrm{Mn}{\mathrm{O}}_3$ 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.

• Received 14 June 2005

DOI:https://doi.org/10.1103/PhysRevB.73.094444