  We report on the microwave, terahertz (THz), infrared and Raman spectroscopic
studies of $BiMn_{7}O_{12}$ ceramics, shedding more light into 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
$BiMn_{7}O_{12}$ undergoes subsequent structural phase transitions from
monoclinic $\textit{I2/m}$ to polar monoclinic $\textit{Im}$ and triclinic
$\textit{P1}$ 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 K 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.
