Computer study of structure, thermodynamic, and electrical transport properties of Na3AlF6-Al2O3 and CaF2-Al2O3 melts

Structure, thermodynamic, and electrical transport properties of Na3AlF6-Al2O3 and CaF2-Al2O3 melts were examined by molecular dynamics. Ionic models were constructed for Na3AlF6-Al2O3 and CaF2-Al2O3 melts at 1283 and 2000 K, respectively. It was found that in the Na3AlF6-Al2O3 melts, stable aluminum-fluorine-oxygen groups are formed. Although bonds between F− and Al3+ ions in the first coordination shell are weaker than between O2− and Al3+ ions, very stable negatively charged AlF63− groups are formed at low oxygen concentrations in the Na3AlF6-Al2O3. This results in migration of aluminum to the anode in an external electric field. In the CaF2-Al2O3 melts, positively charged aluminum-oxygen groups dominate. This results in migration of aluminum to the cathode at almost all Al2O3 concentrations. Therefore, in Na3AlF6-Al2O3 melts, the Al3+ ion as a component of the complex anion has a negative partial conductivity and the O2− ion has positive partial conductivity; in CaF2-Al2O3 melts, Al3+ has a positive transport number while O2− has a negative transport number.