X-ray absorption study of molten yttrium trihalides.

Area of Science:

• Materials Science
• Solid-State Chemistry
• High-Temperature Chemistry

Background:

• Understanding the local structure and coordination behavior of metal halides in molten salts is crucial for applications in metallurgy and energy storage.
• Molten YCl3-LiCl-KCl and YBr3-LiBr systems are relevant for electrochemical processes, necessitating detailed structural characterization.

Purpose of the Study:

• To investigate the local structure and structural changes in molten YCl3-LiCl-KCl and YBr3-LiBr systems.
• To elucidate the coordination behavior of halide ions (Cl-, Br-) around Y(3+) ions using high-temperature EXAFS.
• To determine the effect of alkali halide addition on the stability and structure of Y(III) coordination polyhedra.

Main Methods:

• High-temperature extended X-ray absorption fine structure (EXAFS) technique.
• EXAFS analysis of the Y K-absorption edge to study Y(3+)-halide interactions.
• EXAFS analysis of the Br K-absorption edge to investigate bridging structures in YBr3-LiBr melts.

Main Results:

• Nearest Y(3+)-Cl(-) and Y(3+)-Br(-) distances and coordination numbers remain unchanged upon mixing with alkali halides.
• Addition of alkali halides stabilizes the (YCl6)(3-) and (YBr6)(3-) octahedral coordination, indicated by reduced Debye-Waller factors and anharmonicity.
• In molten YBr3-LiBr, the coordination number of Y(3+) around Br(-) decreases from 2 to approximately 1 in LiBr-rich melts, suggesting broken bridging structures.

Conclusions:

• Alkali halides stabilize the Y(III) coordination octahedra in molten YCl3 and YBr3 systems.
• The bridging structure of (YBr6)(3-) octahedra is significantly diminished in LiBr-rich melts, leading to the formation of stable, free octahedra.
• EXAFS is a powerful tool for characterizing local structures and coordination changes in high-temperature molten salt systems.