On Some Ternary Phases in the Systems Nb-Zn-Al and Ta-Zn-Al

Aluminium, Niobium, Tantalum, Zinc, Alloy In the systems NbZi^-aALr and TaZn2_^Alz a great number of samples was prepared and investigated by means of X-ray powder diffraction. In both systems at the equiatomic composition two new ternary phases were found. Their crystal structures were determined and found to belong to the Friauf-Laves type (MgZm prototype, S. G. P 63/mmc). The unit cell parameters of NbZnAl are a = 5Ö6.4 ± 0.2 pm, c = 829.9 ± 0.8 pm, c/a = 1.639, and for TaZnAl a = 503.8 ± 0.2 pm, c = 827.9 ± 0.3 pm, c/a= 1.643. In the system NbZno-a-Al̂ , at the composition NbZn1.25Alo.75, a ternary phase of the A11C113 structure (a= 394.4 i 0.3 pm, S. G. Pm3m) was also observed, while in the system NbZns-zAl^ a single phase region, having the A11C113 structure, was found to exist up to the composition NbZmAl. The stability of these Friauf-Laves phases and those found previously in the systems ZrZn2_J-Alj; and HfZm-zAlz is discussed in terms of atomic sizes and valence electron concentration.