Powder preparation and high infrared performance of NaLaS₂ transparent ceramics

NaLaS₂ powders with various Na/La molar ratios were prepared via a gas-solid reaction method where NaF nanocrystal was used as the sodium source, and CS₂ served as the sulfurizing agent. The effective sulfurization parameters were designed based on the thermodynamic properties of the precursor obtained via the reverse coprecipitation way. The phase compositions and morphologies of sulfide powders depended strongly on NaF amount. The powder with a molar ratio of Na/La = 1.3 was selected for sintering NaLaS₂ ceramic via hot-pressing process under vacuum due to its highest phase purity and lowest oxidesulfide, fluorosulfide impurities level. As-sintered NaLaS₂ ceramic exhibited a consolidated microstructure, a minor amount of oxysulfide impurities and high 8–14 μm mean transmissivity with the maximum transmittance up to 67.5% at 9 μm. Contrasting study proves that NaLaS₂ ceramic has the tendency to possess the better IR performance and antioxidation capacity than γ-La₂S₃ and CaLa₂S₄ ceramics.