[en] HfNi₃ occurs in two modifications: a high-temperature α form, stable from its melting point to 1200⁰C, and a low-temperature β form, stable below 1200⁰C. The crystal structures of the two modifications have been determined from single-crystal X-ray diffraction data. α-HfNi₃ crystallizes with the BaPb₃-type structure, space group R3^-m (No. 166). The parameters of the triply-primitive hexagonal unit cell are a = 5.2787 (1), c = 19.2324 (10) A at room temperature. The structure can be described as a stacking of nine triangularly ordered, close-packed AB₃ layers in the sequence ABABCBCAC. β-HfNi₃ has the γ-Ta(Pd, Rh)₃-type structure, with a stacking of ten AB₃ layers in the sequence ABCBCACBCB. The space group is P6₃/mmc (No. 194) and the unit-cell dimensions are a = 5.2822 (2), c = 21.3916 (18) A at room temperature. (Auth.)