  We performed factor-group analysis of all phonons in possible monoclinic C2/c
and C2 structures of BiMnO3 and compared it with our experimental infrared and
Raman spectra. We conclude that the crystal structure is centrosymmetric C2/c
in the whole investigated temperature range from 10 to 550 K, therefore BiMnO3
cannot be ferroelectric. We revealed a dielectric relaxation in THz spectra
above the structural phase transition taking place at T_C1=475 K giving
evidence in strong lattice anharmonicity and a large dynamical disorder of Bi
cations above T_C1. Step-like dielectric anomaly observed at T_C1 in THz
permittivity reminds antiferroelectric phase transition. Nevertheless, the
low-temperature dielectric studies did not reveal any antiferroelectric or
ferroelectric hysteresis loop. Our experimental results support theoretical
paper of P. Baettig et al. (J. Am. Chem. Soc. 129, 9854 (2007)) claiming that
BiMnO3 is not multiferroic, but only antipolar ferromagnet.
