Infrared spectroscopic study on lattice dynamics in ${\mathrm{CaFeO}}_3$

The change of the lattice dynamics upon the charge disproportionation (CD) transition has been investigated for the ${\mathrm{CaFeO}}_3$ crystal by measuring its infrared optical spectra. Across the CD transition, ${\mathrm{CaFeO}}_3$ undergoes a metal-insulator transition, and it is found that below $T_{CD}\approx 290$ K the low-frequency optical conductivity gradually decreases to a rather low value and is dominated by a series of infrared-active phonons. Intriguingly, accompanied by the CD transition, two prominent phonon modes at $\sim 243$ and $\sim 559$ ${\mathrm{cm}}^{-1}\text{associated}$ with the vibrations of Fe-O bonds show obvious redshift and asymmetric line shapes characterized by a Fano profile, suggesting a strong electron-phonon coupling. This coupling behavior reveals an intimate relationship between charge and lattice in the CD transition of ${\mathrm{CaFeO}}_3$.

Figure 1

(a) The reflectivity in the far infrared region for ${\mathrm{CaFeO}}_3$ at several representative temperatures. (b) The low frequency optical conductivity ${\sigma }_1\left(\omega \right)$ for ${\mathrm{CaFeO}}_3$ deduced from the K-K transformation of the reflectivity. The colored symbols on the vertical axis are dc values of conductivity obtained from the measurement of resistivity. The insets in (a) and (b) show the reflectivity and optical conductivity up to $10000{\mathrm{cm}}^{-1}$, respectively.

Figure 2

The fitting curves of the real part of optical conductivity in the far infrared region at (a) $T=300$ and (b) $T=25$ K. The phonon modes discussed in the text are labeled by #1 to #4. The arrows in (b) signify the deviation of the Drude-Lorentz fitting. The insets show the spectra up to $10000{\mathrm{cm}}^{-1}$.

Figure 3

(a),(b) Line shapes of the phonons #2 and #3. (c)–(f) The resonance frequencies ${\omega }_0$ and the Fano parameters $|1/q|$ of the #2 and #3 phonon modes determined from the fittings at various temperatures.

Figure 4

Vibration patterns of optical phonon modes assigned to the peaks in $\sigma \left(\omega \right)$ of ${\mathrm{CaFeO}}_3$ at 300 K. The calculated normal frequencies in ${\mathrm{cm}}^{-1}$ are compared with the observed values at 300 K (in parenthesis).