Local Coordination about La3+ in Molten LaCl3 and Its Mixtures with Alkali Chlorides

The local structure around the La3+ ions in molten LaCl3 and its mixtures with alkali and alkaline earth chlorides has been investigated by using extended X-ray absorption fine structure (XAFS) and molecular dynamics (MD) techniques. Such mixtures, which are of current technological interest, are known to be thermodynamically nonideal, and there has been a good deal of work to understand the structural effects factors that contribute to the nonideality. New experimental methods allow observations at the La K-absorption edge at the high temperatures of interest, and the ability of the technique to obtain reliable information even at very low La3+ concentrations in multicomponent mixtures is demonstrated. Both the mean La3+-Cl- interionic separation and the mean La3+ coordination number are found to decrease as the concentration of La3+ in the mixture decreases. The rate of decrease depends on the identity of the alkali and alkaline earth cations present in the mixtures in a way that parallels the degree of nonideality of the different systems; it is greatest for those alkali cations that coordinate CL weakly. In dilute mixtures with such cations La3+ is able to adopt a very stable octahedral coordination geometry but this is inhibited by the presence of more strongly coordinating cations like Li+ and Mg2+.