Synthesis and structural investigations of infrared transmitting materials in rare earth chalcogenide systems.

Abstract

γ La₂S₃ (lanthanum sulfide) shows potential as a candidate window material for far IR transmission in the 8-14 μm spectral region. Novel low temperature routes using metalorganic solutions have been developed for the synthesis of γ and β La₂S₃. Two different amorphous precursors (20 nm size and SSA of 74 m²/gm) were synthesized at room temperature which undergo transformation in a sulfur atmosphere to generate sulfide particles (1-3 μm size). A sulfur content of 8.50 wt% in the precursor is critical for the transformation to the cubic (γ) form of La₂S₃. The transformation controlled by the extent of hydrolysis of the alkoxide as determined by thermal stability and chemical analysis of the precursor. Detailed microstructural characterization of the precursors and phase evolution mechanisms were studied. Hot pressed disks (9mm thick) of the cubic sulfide ceramic show a far IR cut-off at 13 μm. Structural analysis by convergent beam electron diffraction (CBED) confirmed the BCC cell structure and the space group of I43d for the powders and ceramic. Amorphous IR materials in the La₂S₃-GeS₂ system were also studied with an emphasis on bulk glass formation and structural characteristics. Glasses containing 92.5 mol% GeS₂ show evidence of primary (60-800 Å) and secondary (30-130 Å) phase separation at the molecular level. The effect of composition on the microstructure and thermal stability of these glasses were also investigated. The Molecular Dynamics simulation technique was applied to study the structure of IR transmitting materials in liquid and glassy forms. An empirical model was developed which successfully describes the liquid and glass structures of a representative IR material (ZnCl₂) and studies were performed to assess the effect of the mass of the borders (W) of the simulation cell on the glass transition temperature. An extension of the model to simulate crystalline La₂S₃ was also attempted.