Stability of the Jahn-Teller effect and magnetic study ofLaMnO3under pressure

We have studied the crystal structure of ${\mathrm{LaMnO}}_3$ under pressure up to 80 kbar at room temperature by TOF neutron diffraction. The analysis of the crystal structure as a function of pressure has revealed that the Jahn-Teller (JT) distortion is stable up to about 70 kbar. The contraction of the ${\mathrm{MnO}}_6$ octahedra is nearly isotropic, but the average tilt angle diminishes with increasing pressure. This behavior shows that ${\mathrm{LaMnO}}_3$ evolves towards a wider electronic band system. Above 70 kbar the compound undergoes a transition to an unknown phase. This transition is probably due to the suppression of the Jahn-Teller effect because of the closing of the JT gap: the ionic ${\mathrm{Mn}}^{3+}$ species disappear and the system evolves towards a metalliclike phase. At low temperature the effect of the applied pressure is first to increase the transition temperature and second to suppress magnetic ordering when the metalliclike phase is reached.