Polar phonon anomalies in single-crystalline TbScO₃

Abstract

Polarized infrared reflectivity spectra of a (1 1 0)-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 the factor group analysis for the orthorhombic Pbnm- space group. Moreover, the lowest frequency phonons active in E| c as well as in E| 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.

Keywords:

• phonons
• infrared and microwave spectroscopy
• antiferromagnetic phase transition
• rare-earth scandates

Acknowledgments

This work was supported by the Czech Science Foundation (Project no. P204/12/1163) and MŠMT (COST MP0904 project LD12026). We thank J. Drahokoupil for his X-ray diffraction measurements, Xianglin Ke for helpful discussions, and J. Petzelt for his careful reading of the manuscript.