Pressure-induced structural phase transition and suppression of Jahn-Teller distortion in the quadruple perovskite structure

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 Im3 over bar symmetry characterized by a pronounced contraction of the unit cell and by a significant modification in the Raman phonon modes. The phase transition is also marked by the suppression of a 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 a paramagnetic insulator at ambient conditions, and that the Jahn-Teller distortion is frozen in the high-pressure Im3 over bar 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.