Low-temperature chemical routes to formation and IR properties of lanthanum sesquisulfide (La₂S₃) ceramics

Lanthanum sulfide (La₂S₃) exists in three different allotropic forms in the crystalline state. The low-temperature orthorhombic phase (α) transforms to the intermediate tetragonal phase (β) which further undergoes transformation to the high-temperature cubic phase (γ). The intermediate tetragonal phase (β) is an oxysulfide with possible use as a phosphor, and the cubic phase (γ) of La₂S₃ shows potential for application as a far IR (8–14 μm) window material. We report metal-organic synthesis routes for the preparation of the β- and γ-phases of La₂S₃ by processing modifications and choice of precursor chemistry. The alkoxide-derived precursors and the transformed sulfide powders were characterized for morphology, microstructure, and thermal stability. Phase evolution studies were also conducted, and the results are discussed in the context of formation of the β- and γ-phases. Results of densification, microstructure, and infrared transmission characteristics are also presented.