Infrared Optical Anisotropy in Quasi-1D Hexagonal Chalcogenide BaTiSe3

Polarimetric infrared (IR) detection bolsters IR thermography by leveraging the polarization of light. Optical anisotropy, i.e., birefringence and dichroism, can be leveraged to achieve polarimetric detection. Recently, giant optical anisotropy is discovered in quasi-1D narrow-bandgap hexagonal perovskite sulfides, A(1+)(x)TiS(3), specifically BaTiS3 and Sr9/8TiS3. In these materials, the critical role of atomic-scale structure modulations in the unconventional electrical, optical, and thermal properties raises the broader question of the nature of other materials that belong to this family. To address this issue, for the first time, high-quality single crystals of a largely unexplored member of the A(1+)(x)TiX(3) (X = S, Se) family, BaTiSe3 are synthesized. Single-crystal X-ray diffraction determined the room-temperature structure with the P31c space group, which is a superstructure of the earlier reported P6(3)/mmc structure. The crystal structure of BaTiSe3 features antiparallel c-axis displacements similar to but of lower symmetry than BaTiS3, verified by the polarization dependent Raman spectroscopy. Fourier transform infrared (FTIR) spectroscopy is used to characterize the optical anisotropy of BaTiSe3, whose refractive index along the ordinary (E perpendicular to c) and extraordinary (E & Vert; c) optical axes is quantitatively determined by combining ellipsometry studies with FTIR. With a giant birefringence Delta n similar to 0.9, BaTiSe3 emerges as a new candidate for miniaturized birefringent optics for mid-wave infrared to long-wave infrared imaging.