  By means of in situ synchrotron X-ray diffraction and Raman spectroscopy
under hydrostatic pressure, we investigate the stability of the quadruple
perovskite LaMn7O12. At 34 GPa, the data unveil a first-order structural phase
transition from the monoclinic I2/m symmetry stable at ambient conditions to
cubic Im-3 symmetry. Considering that the same structural transition occurs at
653 K upon heating at ambient pressure, we propose a rare scenario of
reentrant-type phase transition. In the high-pressure Im-3 phase, the
Jahn-Teller distortion of the MnO6 octahedra and the orbital order present in
the I2/m phase are suppressed, which is promising to investigate the
possibility of pressure-induced Mott insulator-metal transition in the ideal
situation of no structural distortions. The observation of a progressive line
broadening of almost all Raman modes with pressure suggests that this
transition may be incipient above 20 GPa.
