Manifestation of a series of phase transitions in IR spectra of a multiferroic TbMnO3

We have studied IR transmittance spectra of a multiferroic TbMnO₃. Three phase transitions (T _N = 43 K, T _FE = 28 K, and T _Tb = 7 K), which manifest themselves in a shift of the absorption band edge of manganese and in changes in the spectral parameters of lines of f-f transitions of terbium, have been detected. An incommensurate character of the magnetic structure leads to nonequivalence of terbium positions and to an additional inhomogeneous broadening. The spontaneous electric polarization, which arises at T < T _FE, significantly affects the crystal field that acts on terbium and noticeably shifts its energy levels. The phase transition with ordering of terbium (7 K) is accompanied by a lowering of the energy of the ground state of the Tb³⁺ ion.