Persistence of Jahn-Teller Distortion up to the Insulator to Metal Transition in ${\mathrm{LaMnO}}_3$

High pressure, low temperature Raman measurements performed on ${\mathrm{LaMnO}}_3$ up to 34 GPa provide the first experimental evidence for the persistence of the Jahn-Teller distortion over the entire stability range of the insulating phase. This result resolves the ongoing debate about the nature of the pressure driven insulator to metal transition (IMT), demonstrating that ${\mathrm{LaMnO}}_3$ is not a classical Mott insulator. The formation of domains of distorted and regular octahedra, observed from 3 to 34 GPa, sheds new light on the mechanism behind the IMT suggesting that ${\mathrm{LaMnO}}_3$ becomes metallic when the fraction of undistorted octahedra domains increases beyond a critical threshold.

Figure 1

(a) High $P$ Raman spectra collected at ambient $T$. (b) Pressure dependence of the main phonon peak frequencies. (c) Pressure dependence of peak widths. Dashed lines are guides for the eye.

Figure 2

Raman spectra for ${\mathrm{LaMnO}}_3$ collected as a function of $P$ and $T$. The gray spectra were collected when ${\mathrm{LaMnO}}_3$ enters metallic state. The black arrow indicates the ${\nu }_2$ peak that is still observed at 32 GPa.

Figure 3

(a) Raman spectra collected at 10 K at two selected pressures. Inset: Pressure dependence of the inverse of the asymmetry Fano parameter $q$. (b) Pressure dependence of phonon peak frequencies. (c) Pressure dependence of peak widths.

Figure 4

$P\mathrm{\text{−}}T$ phase diagram. Paramagnetic phase (PM), antiferromagnetic phase (AFM). Gray dashed area and white circles frame the $P\mathrm{\text{−}}T$ region with coexistence of domains. $P\mathrm{\text{−}}T$ evolution of $T_N$ (white squares) [31]. The star and the black square indicate the JT transition and the IMT temperatures, respectively [3, 4]. Dark gray area indicates the insulating $P\mathrm{\text{−}}T$ region. The light gray color is used for the $P\mathrm{\text{−}}T$ region of the phase diagram which has been poorly explored.