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.

中文翻译：

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准一维 BaTiS3 单晶中的取向控制各向异性

具有链状（一维）或层状（二维）结构的低维材料因其各向异性的电学、光学和热学特性而备受关注。一种具有链状结构的材料，BaTiS ₃ (BTS)，最近被证明具有巨大的面内光学各向异性和玻璃般的热导率。要了解这些效应的起源，有必要充分表征 BTS 的光学、热和电子各向异性。为此，具有不同取向的 BTS 晶体（a - 和c轴方向）通过化学气相传输生长。X射线吸收光谱用于表征BTS的局部结构和电子各向异性。傅里叶变换红外反射/透射光谱显示a取向晶体中的面内光学各向异性很大，而c轴取向晶体在面内几乎是各向同性的。BTS 片晶是一种很有前途的红外光学单轴材料，其光轴平行于c轴。热导率测量显示c - 和a之间的热各向异性约为 4.5-轴。时域布里渊散射表明沿两个轴的纵向声速几乎相同，表明热各向异性是不同声子散射率的结果。