Origin of ultra-wide IR transmission and ultra-large birefringence of mercurous halide series with one dimensional chain-like structure: An ab initio study

Abstract Mercurous halide single crystals with one-dimensional chain-like structure exhibit great potential as polarizers or acousto-optic devices in infrared range based on the ultra-large birefringence and ultra-wide infrared transmission range. However, the origin of these two unique properties of mercurous halide families are unclear. Herein, we theoretically investigate the electronic and vibrational properties of these analogues through first principle calculation to explore the origin of these two distinct properties. The short cut-on wavelength is due to the large bandgap stemming from the ability of halogen to bind valence electrons tightly, while the long wavelength infrared cut-off originates from low phonon frequency caused by the high atomic weight of Hg and halogen atoms. Attributed to the high covalent characteristic between Hg and halogen atoms along c orientation and the very weak van der Waals force between X-Hg-Hg-X (X = Cl, Br, I) molecules perpendicular to c orientation, mercurous halide crystals exhibit large anisotropy of polarization leading to the large birefringence. Our analysis reveals that both one-dimensional chain structure and high atomic number elements contribute to the large birefringence and wide infrared transparent range, which would inspire a direction for designing new high-performance infrared polarizing materials.