Abstract
The metal-rich tantalum-arsenic system has been reinvestigated. X-ray powder diffraction techniques were used in conjunction with controlled thermal decomposition experiments in an effusion cell connected to a vacuum electrobalance. Monophase polycrystalline samples of Ta 3As, Ta 2As, Ta 5As 4 and TaAs have been prepared. Single crystals of each of these phases have been obtained by halogen vapour transport. Single-crystal precession camera studies indicated that Ta 3As was of a new structure type with a monoclinic lattice and space group B2 b , a = 14.683(2), b = 14.558(1), c = 5.098(1) A ̊ , γ = 90.562° (3). The structure of TaAs was refined by using single-crystal diffractometer data. The results confirm the structure assigned earlier on the basis of powder data. In addition, a modification of the TaAs phase has been identified but not, as yet, fully characterized. Powder and single-crystal data indicate a primitive tetragonal cell with structure and lattice parameters very similar to the body-centered form. Refined lattice parameters have been determined for all the phases. The difficulties of constructing a phase diagram for the system are discussed.
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