Optical study of the vibrational and dielectric properties of ${\text{BiMnO}}_3$

${\mathrm{BiMnO}}_3$ (BMO), ferromagnetic (FM) below $T_c\simeq 100\mathrm{K}$, was believed to also be ferroelectric (FE) due to a noncentrosymmetric $C2$ structure, until diffraction data indicated that its space group is the centrosymmetric $C2/c$. Here we present infrared phonon spectra of BMO, taken on a mosaic of single crystals, which are consistent with $C2/c$ at any $T>10\mathrm{K}$, as well as room-temperature Raman data which strongly support this conclusion. We also find that the infrared intensity of several phonons increases steadily for $T\to 0$, causing the relative permittivity of BMO to vary from 18.5 at 300 K to 45 at 10 K. At variance with FE materials of the displacive type, no appreciable softening has been found in the infrared phonons. Both their frequencies and intensities, moreover, appear insensitive to the FM transition at $T_c$.

Figure 1

Magnetization vs temperature (in electromagnetic units, emu) of a single crystal extracted from the mosaic used for the infrared reflectivity measurements. The sample was cooled either in zero field (ZFC) or in a field of 100 oersted (FC). A sharp and single FM transition occurs at $T_c=100$ K.

Figure 2

Reflectivity spectra of bulk ${\mathrm{BiMnO}}_3$ in the far-infrared range of frequencies at different temperatures.

Figure 3

Optical conductivity of bulk ${\mathrm{BiMnO}}_3$ in the far-infrared range, at different temperatures. Inset: The fit (dotted line) to the optical conductivity data (solid line) used to obtain the parameters at 10 K listed in Table 1.

Figure 4

Raman spectrum of a single crystal of ${\mathrm{BiMnO}}_3$ at room temperature and infrared spectrum of the mosaic at 10 K, extracted from Fig. 3. Given the weak $T$ dependence observed in both spectra (see Table 1 and Ref. [22]), the best resolved IR spectrum has been chosen for a more meaningful comparison.

Figure 5

Systematic increase of the oscillator strength of ${\mathrm{BiMnO}}_3$ phonons for $T\to 0$. The modes (M) are numbered as in Table 1 and the lines are guides to the eye. Within errors, no reproducible features are observed around the FM transition at $T_c\simeq 100$ K.

Figure 6

Behavior with frequency and temperature of the real part of the dielectric function ${\epsilon }_1\left(\omega \right)$ in ${\mathrm{BiMnO}}_3$. Inset: The temperature dependence of its extrapolation to zero frequency, the (relative) dielectric constant ${\epsilon }_1\left(0\right)$.