  Polarized infrared reflectivity spectra of a (110)-oriented TbScO3 single
crystal plate were measured down to 10 K. The number of observed polar phonons
active along the crystallographic c axis at low temperatures is much higher
than predicted by factor-group analysis for the orthorhombic Pbnm space group.
Moreover, the lowest frequency phonons active in E||c as well as in E||[1-10]
polarized spectra exhibit dramatic softening tending to a lattice instability
at low temperatures. The dielectric permittivity at microwave frequencies does
not show any ferroelectric-like anomaly, but the dielectric loss exhibits a
maximum at 100 K. The origin of the discrepancy between the number of predicted
and observed polar phonons as well as the tendency toward lattice instability
are discussed. Magnetic measurements reveal an antiferromagnetic phase
transition near 3 K.
