The equilibrium phase diagrams for the tantalum-tantalum bromide and tantalum-tantalum iodide systems

Article abstract of DOI:10.1021/ic50032a029

The tantalum-tantalum bromide and the essential features of the tantalum-tantalum iodide system have been elucidated. In the bromide system four phases of composition lower than TaBr₅ were established, viz., TaBr₄, TaBr_2.5, TaBr_2.5, and TaBr_2.33. By comparison the iodide system is very simple as evidenced by only the two lower phases TaI₄ and TaI_2.33. In the bromide system a eutectic occurs at 267° and the composition TaBr_4.87. The TaBr₄ melts incongruently at 392° to form liquid of composition near TaBr_4.85 and the next lower solid TaBr_2.83. Equilibrations of TaBr₄ at a series of temperatures above 400° established that TaBr_2.83 melts incongruently within the interval 447-453° to form bromine-rich liquid and the phase TaBr_2.5. Similarly it was determined that TaBr_2.5 melts incongruently to form TaBr_2.33 in the interval 671-680°. Exact location of the melting temperatures for the bromides with Br/Ta < 4 was hindered by the very sluggish reactions. That TaBr_2.33 was the lowest phase in the system was established by equilibrations of the higher halides with tantalum at temperatures up to 750°; a melting point for TaBr_2.33 was not observed. In the iodide system the melting point of TaI₅ occurred at 382°, with a eutectic at 365° and the composition TaI_4.87. Decomposition of TaI₄ into iodine-rich liquid and TaI_2.33 occurred in the interval 395-402° at a very low rate. Equilibrations of TaI_2.33 and Ta up to 815° provided no evidence for any new phases. Both TaBr_2.33 and TaI_2.33 provide aqueous solutions of the ions Ta₆Br₁₂²⁺ and Ta₆I₁₂²⁺. Hence these compounds are formulated correctly as (Ta₆Br₁₂)Br₂ and (Ta₆I₁₂)I₂. The chemical properties and stoichiometry of the phases TaBr_2.5 and TaBr_2.83 suggest that they should be formulated as (Ta₆Br₁₂)Br₃ and (Ta₆Br₁₂)Br₅, respectively.