Abstract
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 ${T}_{\mathrm{JT}}\ensuremath{\approx}750\mathrm{K},$ 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.
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