  By means of in situ synchrotron X-ray diffraction and Raman spectroscopy
under hydrostatic pressure, we investigate the structural stability of the
quadruple perovskite LaMn7O12. At 34 GPa, the data unveil a first-order
structural phase transition from monoclinic I2/m to cubic Im-3 symmetry
characterized by a pronounced contraction of the unit cell and by a significant
modifications in the Raman phonon modes. The phase transition is also marked by
the suppression of Jahn-Teller distortion which is present in the ambient
monoclinic phase. In addition, above 20 GPa pressure, a sudden and simultaneous
broadening is observed in several Raman modes which suggests the onset of a
sizable electron-phonon interaction and incipient charge mobility. Considering
that LaMn7O12 is paramagnetic insulator at ambient, and Jahn-Teller distortion
is frozen in the high-pressure Im-3 phase, we argue that this phase could be a
potential candidate to host a purely electronic insulator-metal transition with
no participation of the lattice
