Optical transmittance range of High-Temperature stable phase germanium dichalcogenide (GeS2) single crystal grown by the Bridgman method

A high-temperature (HT) stable phase germanium dichalcogenide (GeS₂) was grown in an evacuated quartz ampoule to investigate its transparency range. First, as a starting material for the crystal growth of HT-GeS₂, GeS₂ glass containing HT-GeS₂ crystals was synthesized using our previously improved process for synthesizing polycrystalline sulfide compounds in a conventional horizontal furnace. The HT-GeS₂ crystal was grown using the Bridgman method under a temperature gradient of 20–30 °C/cm. The grown HT-GeS₂ single crystal was easily cleaved along the growth direction, and this cleaved surface was identified as the (001) face using X-ray diffraction (XRD). Single-crystal XRD analysis confirmed the growth of an HT-GeS₂ single crystal with a monoclinic structure (space group P2₁/c) and lattice parameters of a = 6.7061(4) Å, b = 16.0877(13) Å, and c = 11.4242(11) Å, and β = 91.069 (8)°. The optical transmittance spectra of the HT-GeS₂ single crystal were measured in the visible and infrared (IR) regions, revealing a transparency range of 0.36–22.5 μm. Differential thermal analysis revealed the melting point of HT-GeS₂ as 841 ± 1 °C. The HT-GeS₂ single crystal is expected to be a new candidate for IR optical crystal.