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

Boyang Zhao, Md Shafkat Bin Hoque, Gwan Yeong Jung, Hongyan Mei, Shantanu Singh, Guodong Ren, Milena Milich, Qinai Zhao, Nan Wang, Huandong Chen, Shanyuan Niu, Sang-Jun Lee, Cheng-Tai Kuo, Jun-Sik Lee, John A. Tomko, Han Wang, Mikhail A. Kats, Rohan Mishra, Patrick E. Hopkins, and Jayakanth Ravichandran

Low-dimensional materials with chain-like (one-dimensional) or layered (two-dimensional) structures are of significant interest due to their anisotropic electrical, optical, and thermal properties. One material with a 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 (a– and c-axis orientations) were grown by chemical vapor transport. X-ray absorption spectroscopy was used to characterize the local structure and electronic anisotropy of BTS. Fourier transform infrared reflection/transmission spectra show a large in-plane 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 infrared 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.