We have investigated the Jahn-Teller transition accompanied by the orbital order-disorder transition in ${\mathrm{LaMnO}}_3$ by high temperature x-ray powder diffraction with synchrotron radiation and also neutron powder diffraction. The unit cell volume of ${\mathrm{LaMnO}}_3$ decreases with increasing temperature in a narrow temperature range below $T_{\mathrm{JT}}\approx 750\mathrm{K},$ and then undergoes a volume collapse at $T_{\mathrm{JT}}.$ We interpret this effect as due to the more efficient packing of the ${\mathrm{MnO}}_6$ octahedra in the orbitally disordered or orbital liquid state. The orbital melting phenomenon can be qualitatively compared with the melting of ice.

• Received 9 June 2003

DOI:https://doi.org/10.1103/PhysRevB.68.052406