Either the three Al atoms or Al (1a) and the Ni atoms form a trigonal omega ($C6$) structure. Using the choices of internal parameters for Al$_{3}$Ni$_{2}$, this can be viewed as a five-layer close-packed unit cell with stacking ABCBCA.
Al$_{3}$Ni$_{2}$ is isostructural with La$_{2}$O$_{3}$ ($D5_{2}$) and Ce$_{2}$O$_{2}$S. We use this structure for the binary intermetallics, $D5_{2}$ for the binary oxides and related structures, and Ce$_{2}$O$_{2}$S for the ternary structures.
$\frac{1}{2}a \,\mathbf{\hat{x}}- \frac{\sqrt{3}}{6}a \,\mathbf{\hat{y}}- c z_{3} \,\mathbf{\hat{z}}$
(2d)
Ni I
References
A. J. Bradley and A. Taylor, The crystal structures of Ni$_2$Al$_3$ and NiAl$_3$, Philosophical Magazine 23, 1049–1067 (1937), doi:10.1080/14786443708561875.
Found in
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First cited in
M. J. Mehl, D. Hicks, C. Toher, O. Levy, R. M. Hanson, G. L. W. Hart, and S. Curtarolo, The AFLOW Library of Crystallographic Prototypes: Part 1, Comp. Mat. Sci. 136, S1-S828 (2017). (doi=10.1016/j.commatsci.2017.01.017)