Origin of the Non-Linear Pressure Effects in Perovskite Manganites*

High-pressure resistivity and x-ray diffraction measurements were conducted on La$_{0.85}$MnO$_{3-\delta }$ to $\sim $6 GPa and $\sim $7 GPa, respectively. At low pressures the metal-insulator transition temperature (T$_{MI})$ increases linearly up to a critical pressure, P* $\sim $ 3.4 GPa, followed by reduction of T$_{MI}$ at higher pressure. Analysis of the bond distances and bond angles reveal that a bandwidth increase drives the increase of T$_{MI}$ below P*. The reduction of T$_{MI}$ at higher pressures is found to result from Jahn-Teller distortions of the MnO$_{6 }$octahedra. The role of anharmonic interatomic potentials is discussed.