The niobium iodides. Characterization of niobium(IV) iodide, niobium(III) iodide, and triniobium octaiodide

Article abstract of DOI:10.1021/ic50024a011

The condensed niobium-niobium(V) iodide system contains three intermediate phases, NbI₄, NbI₃, and Nb₃I₈, all qualitatively diamagnetic at room temperature. Niobium tetraiodide is evidently trimorphic. The weakly endothermic α → β transition observed at ca. 348° only on heating is without effect in the powder pattern data. A more substantial β → γ transition occurs at 417°, and γ-NbI₄ melts incongruently at 503° to form an iodide-rich liquid and solid Nb₃I₈. Niobium(III) iodide may be obtained by thermal decomposition of NbI₄, as a sublimate from mixtures with Nb₃I₈, or as the initial product of oxidation of excess metal. The phase is apparently not an equilibrium compound in the system since it decomposes slowly and irreversibly at about 513° into iodide-rich liquid and Nb₃I₈. The phase in equilibrium with the metal is Nb₃I₈ (NbI_2.67±0.02), which may also be obtained by thermal decomposition of higher iodides or as the phase transported from mixtures with metal. The two lower niobium iodides show only a limited correspondence with those reported in the niobium chloride and bromide and tantalum iodide systems.