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 $\mathrm{La}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$. At 34 GPa, the data unveil a first-order structural phase transition from monoclinic $I2/m$ to cubic $Im\overline{3}$ 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 $\mathrm{La}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ is a paramagnetic insulator at ambient conditions, and that the Jahn-Teller distortion is frozen in the high-pressure $Im\overline{3}$ 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.

Figure 1

(a) Refinement of the SXRD patterns of LMO at ambient (monoclinic $I2/m$ phase) and at 35.5 GPa pressure (cubic $Im\overline{3}$ phase). The peaks originating from unidentified secondary phase(s) are denoted with asterisk (*). Insets show the Miller indices of characteristic regions. (b) Crystal structure of LMO in the monoclinic $I2/m$ (left) and cubic $Im\overline{3}$ (right) phases. Elongated Mn-O bonds due to the JT distortion in $\mathrm{Mn}{\mathrm{O}}_6$ octahedra are marked with black lines.

Figure 2

Pressure dependence of (a) unit cell volume, (b) lattice parameters of LMO in monoclinic $I2/m$, and cubic $Im\overline{3}$ phases. Error bars in pressure, volume, and lattice parameters are smaller than the symbol size. Solid lines are EOS fit to the experimental data (see text).

Figure 3

(a) Unpolarized Raman spectra of LMO at different pressures. Deconvolution of the background corrected spectrum in $I2/m$ phase (at ambient) and $Im\overline{3}$ phase (35.2 GPa) is shown. (b) Pressure dependence of Raman phonon frequencies. Error bars are smaller than the symbol.

Figure 4

Pressure dependence of Raman mode linewidths in the $I2/m$ phase. Error bars are smaller than the size of the symbol. The dash-dotted line is guide to the eye.