Two displacive ferroelectric phase transitions in multiferroic quadruple perovskite $\mathrm{Bi}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$

We report on the microwave, terahertz (THz), infrared, and Raman spectroscopic studies of $\mathrm{Bi}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ ceramics, shedding more light onto the nature of two structural phase transitions and their possible relation with ferroelectricity in this compound. We observed a softening of one polar phonon in the THz range on cooling towards 460 and 300 K, i.e., temperatures at which $\mathrm{Bi}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ undergoes subsequent structural phase transitions from monoclinic $I2/m$ to polar monoclinic $\mathit{Im}$ and triclinic $P1$ phases. The soft phonon causes dielectric anomalies typical for displacive ferroelectric phase transitions. Microwave measurements performed at 5.8 GHz up to 400 K qualitatively confirmed not only the dielectric anomaly at 300 K, but also revealed two other weak dielectric anomalies near the magnetic phase transitions at 60 and 28 K. This evidences the multiferroic nature of the low-temperature phases, although the relatively high conductivity in the kHz and Hz spectral range prevented us from directly measuring the permittivity and ferroelectric polarization. Some Raman modes sense the magnetic phase transitions occurring near 60 and 25 K, showing that spin-phonon coupling is relevant in this compound and in this temperature range. The deviation of the Mn-O stretching mode frequency from the anharmonic temperature behavior was successfully explained by the spin correlation function calculated from the magnetic contribution to the specific heat.

Figure 1

Crystal structure of $\mathrm{Bi}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ at room temperature and schematic representation of its structural (purple) and magnetic (orange) phase sequence. The illustration of the crystal structure was obtained using vesta software [15].

Figure 2

(a) $\varepsilon {}^{\prime }\left(\omega \right)$ and (b) ${\varepsilon }^{\prime \prime }\left(\omega \right)$ spectra of $\mathrm{Bi}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ obtained by THz time-domain spectroscopy at several temperatures.

Figure 3

(a) Temperature dependence of the frequency of two components of the soft mode in $\mathrm{Bi}{\mathrm{Mn}}_7{\mathrm{O}}_{12}, {\omega }_{02}={\omega }_{SM}$ and ${\omega }_{01}$, where the latter is discernable only below room temperature. (b) Temperature dependence of dielectric strength of the soft mode and the sum of the contributions of all phonons to the static permittivity. The dashed lines are a guide for the eye. Inset: Contribution (in %) of the SM to the total static permittivity.

Figure 4

Relative changes of the (a) real and (b) imaginary parts of the dielectric permittivity measured at 5.8 GHz.

Figure 5

Unpolarized IR reflectivity spectra of $\mathrm{Bi}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ ceramics recorded at 20, 300, and 500 K. The corresponding fits are shown as dotted lines. The reflection band below $50\mathrm{c}{\mathrm{m}}^{-1}$ corresponds to the soft mode.

Figure 6

(a) ${\varepsilon }^{\prime }$ and (b) ${\varepsilon }^{\prime \prime }$ (in logarithmic scale) spectra of $\mathrm{Bi}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ obtained from fitting the IR reflectivity at several temperatures.

Figure 7

Temperature dependence of the specific heat divided by temperature of $\mathrm{Bi}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$. Solid red line was determined by the best fit of Debye equation to the experimental data recorded above 120 K. Temperature dependence of the magnetic contribution to the specific heat, calculated from the difference of the experimental data and the extrapolated lattice contribution described by Debye behavior, extrapolated to 2 K.

Figure 8

Unpolarized Raman spectra of $\mathrm{Bi}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ ceramics recorded at several fixed temperatures. The spectra are vertically offset from each other for better resolution.

Figure 9

(a),(b) Temperature dependence of the wave number of selected Raman modes. The best fit with Eq. (3) in the paraelectric phase is represented by a solid line and is extrapolated down to 0 K. (c) The deviation, $\mathrm{\Delta }\omega$, of the highest phonon frequency seen near $650\mathrm{c}{\mathrm{m}}^{-1}$ from the anharmonic temperature behavior [Eq. (3)] as a function of the spin-spin correlation function, $6J\langle {\mathit{S}}_{\mathit{i}}·{\mathit{S}}_{\mathit{j}}\rangle$, obtained from integrating the magnetic contribution to the specific heat, shown in Fig. 7. Inset: temperature dependencies of $\mathrm{\Delta }\omega$ and $6J\langle {\mathit{S}}_{\mathit{i}}·{\mathit{S}}_{\mathit{j}}\rangle$.