Lattice anharmonicity and polar soft mode in ferrimagnetic M-type hexaferrite BaFe

The polar phonon modes in BaFe12O19 single crystal are studied in the temperature range from 6 to 300 K by polarized infrared spectroscopy. The phonon spectrum of the crystal is strongly anharmonic and unstable with respect to long-wavelength fluctuations of the dielectric permittivity along the hexagonal axis. Our results suggests that in BaFe12O19 hexaferrite symmetry lowering to the polar phase with the space group P63mc can be expected