Jahn-Teller-induced crossover of the paramagnetic response in the singly valent $e_g$ system ${\text{LaMn}}_7{\text{O}}_{12}$

We investigate the high-temperature magnetic and transport properties of ${\text{LaMn}}_7{\text{O}}_{12}$, which displays a similar perovskitelike structure and the same single-valent ${\text{Mn}}^{3+}$ properties of ${\text{LaMnO}}_3$ but a much simpler Jahn-Teller (JT) distortion at $T_{\text{JT}}=650\text{K}$. We find that the magnetic response of ${\text{LaMn}}_7{\text{O}}_{12}$ is similar to that of ${\text{LaMnO}}_3$ below $T_{\text{JT}}$, but strikingly different in the undistorted phase above $T_{\text{JT}}$, where the Curie-Weiss susceptibility is strongly suppressed. Electrical resistivity and thermopower measurements unveil a concomitant crossover from nonadiabatic to adiabatic small polaron regime. This suggests that the above suppression is due to low-spin electron-hole dimers formed by the $e_g$ charge transfer between Mn sites and stabilized by the slow JT dynamics above $T_{\text{JT}}$.

Figure 1

(Color online) Crystal structure of the $Im\overline{3}$ cubic phase of ${\text{LaMn}}_7{\text{O}}_{12}$.

Figure 2

(Color online) Temperature dependence of the magnetic susceptibility in the 300–800 K range of a ${\text{LaMn}}_7{\text{O}}_{12}$ sample measured at 100 Oe upon field cooling and field warming. Inset: inverse susceptibility data. The drop at 80 K is due to the antiferromagnetic transition of ${\text{LaMn}}_7{\text{O}}_{12}$ discussed in Ref. 7. The drop at 200 K is due to ${\text{LaMnO}}_{3+x}$ impurity (see text). Red and blue data points indicate two distinct data sets.

Figure 3

(Color online) Solid line: electrical resistivity $\varrho \left(T\right)$ of ${\text{LaMn}}_7{\text{O}}_{12}$. Open circles: selected $\varrho \left(T\right)$ data of ${\text{LaMnO}}_{3.02}$ from Ref. 24 after rescaling of the temperature to the $T_{\text{JT}}$ value of ${\text{LaMn}}_7{\text{O}}_{12}$. The rescaling puts into evidence the similar behavior of the two compounds. Upper inset: semilogarithmic $\varrho \left(T\right)$ vs $1/T$ plot showing the thermally activated behavior predicted by the small polaron scenario in both adiabatic $\left(s=1\right)$ and nonadiabatic $\left(s=1.5\right)$ regimes [see Eq. (1)]. Solid lines are linear fits of the data, dashed lines indicate the $T$ ranges for the fit. Lower inset: detail of the thermal hysteresis in the vicinity of $T_{\text{JT}}$.

Figure 4

Behavior of thermopower $S\left(T\right)$ in ${\text{LaMn}}_7{\text{O}}_{12}$. Inset: The same data plotted as a function of $1/T$, which yields $E_S\simeq 91\text{meV}\left(67\text{meV}\right)$ above (below) $T_{\text{JT}}$.