Influence of NaX (X=Cl, Br, I) additive on phase composition, thermostability and high infrared performances of γ-La₂S₃ transparent ceramics

A simple method to synthesize Na⁺-doped La₂S₃ powder with only γ phase was developed where NaX (X = Cl, Br, I) served as sodium source. In initial molar ratio Na/La ≥ 0.8 of the mixed raw materials, γ-La₂S₃:Na⁺ tended to reach a constant ratio Na/La = 0.2 and could be represented as Na_0.5La_2.5S₄, which exhibited enough high-temperature stability available for sintering. A modified process was designed to prepare γ-La₂S₃ ceramics with NaCl additive. The microstructure, typical grain boundary composition, photoluminescence and IR performance of as-sintered γ-La₂S₃ ceramics were investigated in details. The optical transparency with mean transmittance 61.3% in 8–14 μm and maximum 64.03% at 13.1 μm was achieved for pellet with Na:La = 0.2 and 1.0 wt% NaCl nanocrystals. NaCl was proved to play multiple roles in governing phase composition, enhancing stability, optimizing microstructure, improving IR properties and affecting luminescence of γ-La₂S₃ ceramics. The mechanisms of action of NaCl additive are proposed by combining the defects reaction, ionic radius and the diffusion behavior of related ions with the cation-deficient structure of γ-La₂S₃.