The metal+metal tri-iodide systems for lanthanum, cerium and praseodymium are presented in the form of phase diagrams. In contrast to the behaviour of the respective chlorides, all three of these systems show the formation of a stable di-iodide, higher melting than the tri-iodide, plus a stoichiometric intermediate phase with an iodine to metal ratio between 2-4 and 2.5. The stability on melting and the melting points decrease from La12 to PrI2.The high conductivities ( >1 x 103 ohm-1 cm-1) of the black to bronze di-iodides and the small molar susceptibility (2 x 10-4 e.m.u.) of La12 indicate the presence of tripositive metal cations plus " metallic " lattice electrons, M3+e(I-)2. This is in contradistinction to the salt-like neodymium di-iodide but similar to the monosulphides and dicarbides of these and neighbouring elements. The nature of the solids and their possible conduction behaviour in the liquid state are briefly considered.The lighter rare-earth metals show a considerable variation in the degree of their solution in the respective molten trihalides. A simple solution of lanthanum in LaC13, 9 mole % metal at the eutectic (826") and 11 % at 914", takes place without the formation of a new solid phase,l and a similar behaviour is found with cerium in CeC13, the reported solubilities being 9-3 % at the monotectic (777") and 9.7 % at 950O.2 Progressively greater reaction, with the formation of reduced, solid compounds, is found between the next two elements and their molten trichlorides. * Contribution no. 1040 ; work was performed in the Ames Laboratory of the US.
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