Orientation Controlled Anisotropy in Single Crystals of Quasi-1D BaTiS3

TL;DR

This study investigates the anisotropic optical, thermal, and electronic properties of BaTiS3 crystals with different orientations, revealing significant in-plane optical and thermal anisotropies linked to phonon scattering differences.

Contribution

It provides a comprehensive characterization of BaTiS3's anisotropic properties using XAS, FTIR, and thermal measurements, highlighting orientation-dependent optical and thermal behaviors.

Findings

Large in-plane optical anisotropy in a-oriented crystals

Thermal conductivity anisotropy of approximately 4.5 between axes

Similar longitudinal sound speeds suggest phonon scattering causes thermal anisotropy

Abstract

Low-dimensional materials with chain-like (one-dimensional) or layered (twodimensional) structures are of significant interest due to their anisotropic electrical, optical, thermal properties. One material with chain-like structure, BaTiS3 (BTS), was recently shown to possess giant in-plane optical anisotropy and glass-like thermal conductivity. To understand the origin of these effects, it is necessary to fully characterize the optical, thermal, and electronic anisotropy of BTS. To this end, BTS crystals with different orientations (aand c-axis orientations) were grown by chemical vapor transport. X-ray absorption spectroscopy (XAS) was used to characterize the local structure and electronic anisotropy of BTS. Fourier transform infrared (FTIR) reflection/transmission spectra show a large inplane optical anisotropy in the a-oriented crystals, while the c-axis oriented crystals were nearly isotropic in-plane. BTS platelet crystals are promising uniaxial materials for IR optics with their optic axis parallel to the c-axis. The thermal conductivity measurements revealed a thermal anisotropy of ~4.5 between the c- and a-axis. Time-domain Brillouin scattering showed that the longitudinal sound speed along the two axes is nearly the same suggesting that the thermal anisotropy is a result of different phonon scattering rates.