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Matrix-isolated Al2OF6(2-) ion in molten and solid LiF/NaF/KF
M H Brooker1, R W Berg, J H von Barner
1Department of Chemistry, Technical University of Denmark, Building 207, Kemitorvet, DK-2800 Lyngby, Denmark. artceps@morgan.ucs.mun.ca
Inorganic Chemistry
|February 24, 2001
Summary
Researchers identified the Al2OF6(2-) ion in molten FLINAK salt by adding sodium oxide to aluminum fluoride. This oxyfluoroaluminate ion was characterized in both molten and solid states, offering insights into oxide-fluoride melt chemistry.
Area of Science:
- Inorganic Chemistry
- Materials Science
- Spectroscopy
Background:
- Molten salts, particularly fluoride systems like LiF/NaF/KF (FLINAK), are crucial in various industrial processes.
- Understanding the speciation of aluminum compounds in these melts is essential for process control and optimization.
- Previous studies on alumina solubility in molten cryolite showed limitations due to high temperatures and spectral overlap.
Purpose of the Study:
- To identify and characterize the aluminum oxyfluoroaluminate ion formed in FLINAK melts.
- To investigate the reaction between dissolved aluminum fluoride and added sodium oxide in FLINAK.
- To compare spectral data of the Al2OF6(2-) ion in molten and solid states.
Main Methods:
- Raman spectroscopy was used to analyze the FLINAK melt at 500°C.
- Infrared (IR) spectroscopy was employed to study the isolated solid compound at 25°C.
- Matrix isolation technique was used to trap and stabilize the Al2OF6(2-) ion.
Main Results:
- A Raman spectrum consistent with the Al2OF6(2-) ion was observed upon dissolving Na2O in FLINAK containing aluminum fluoride.
- Characteristic Raman bands for Al2OF6(2-) were identified at 494 cm⁻¹ (polarized) in the melt and 509 cm⁻¹ in the solid state.
- The AlF6(3-) ion, in the absence of oxide, showed characteristic Raman bands at 542 and 325 cm⁻¹.
- The Al2OF6(2-) ion was successfully trapped in frozen FLINAK and its IR spectrum recorded (approx. 780-900 cm⁻¹).
- Alumina (Al2O3) was found to be largely insoluble, but Na2O addition converted AlF6(3-) to Al2OF6(2-), forming metastable solutions.
Conclusions:
- The formation and spectral characteristics of the Al2OF6(2-) ion in FLINAK melts have been established.
- The study provides a method for dissolving and characterizing oxide species in fluoride melts.
- The results offer valuable data for understanding oxide-fluoride interactions in high-temperature systems.