Phase equilibria and chemical vapor transport in the system Mo–Ta–As

The phase diagram Mo–Ta–As was studied in two partial isothermal sections at 1050 °C (in the As-rich corner) and at 1400 °C (As-poor alloys) using powder X-ray diffraction and electron probe microanalysis. A complete solid solution was found to exist between isostructural Mo₅As₄ and Ta₅As₄ and the ternary solubility of Mo in Ta₃As at 1400 °C was determined. A ternary phase Mo_xTa_1−xAs with MnP-type structure was found to exist in the As-rich part of the system. Lattice parameters were investigated as a function of composition for (Mo,Ta)₅As₄ and for Mo_xTa_1−xAs. Additional experiments of chemical vapor transport (CVT) from 1000 °C to 900 °C using different ternary source compositions and I₂ and Br₂ (PtBr₂) as transport agents were performed. Only Ta compounds were found in the sink and no ternary transport was observed.