  PbTe crystals have a soft transverse optical phonon mode in the terahertz
frequency range, which is known to efficiently decay into heat-carrying
acoustic phonons, resulting in anomalously low thermal conductivity. Here, we
studied this phonon via polarization-dependent terahertz spectroscopy. We
observed softening of this mode with decreasing temperature, indicative of
incipient ferroelectricity, which we explain through a model including strong
anharmonicity with a quartic displacement term. In magnetic fields up to 25T,
the phonon mode split into two modes with opposite handedness, exhibiting
circular dichroism. Their frequencies displayed Zeeman splitting together with
an overall diamagnetic shift with increasing magnetic field. Using a
group-theoretical approach, we demonstrate that these observations are results
of magnetic field-induced morphic changes in the crystal symmetries through the
Lorentz force exerted on the lattice ions. This study thus reveals a novel
process of controlling phonon properties in a soft ionic lattice by a strong
magnetic field.
