The molten state of trivalent metal halides and oxides: recent progress

The structures of trivalent metal halides and oxides have been reviewed with emphasis on the stability of a six-fold coordination of metal ions, e.g. in YCl₃ and YO₃, and its competition with a four-fold coordination, e.g. in AlCl₃ and AbO₃. Upon melting in some cases the crystalline coordination is preserved, leading to the formation of either an ionic network or a molecular structure, while in other cases a transition to a different kind of structure occurs, such as that from an ionic to a molecular-type structure. Specific examples will be selected from recent experiments in which structural information at the partial level and Raman spectra have been obtained, and the X-ray average structure has been complemented by electrical transport data. Directions for future work with a potential impact on our understanding of the complex structure-property relations in the crystalline and liquid state have been proposed.