Strained-tetrahedra statistical model for atomic distances and site occupations in ternary intermetallic M3(XX′) structures Ni3(AlFe) case

Abstract The strained-tetrahedra statistical model is applied to intermetallic M 3 (XX′) alloys aiming to determine from EXAFS data, elemental tetrahedron preferences and structural dimensions. Atoms M∈{Ni, Co, Cu} and X∈{Al, Ti, Nb, Mo, Hf, Ta, W, Fe, Cr, Mn} form face-centered-cubic crystal cell M 3 X binary intermetallic compounds with Cu 3 Au ordering. Ternary intermetallic alloys can be of the type M 3 (X 1− x X′ x ) 1 or (M 1− y M′ y ) 3 X 1 . As per the literature, atoms of each set cling to their respective ‘M face’ and ‘X corner’ sublattice sites, while X″∈{Fe, Cr, Mn} atoms beyond a threshold overall relative content c M ≤3/4 also invade the ‘corner’ sublattice, behaving like M″ atoms. With this in mind, the model is applied to Ni 3 (Al 1− x Fe x ) 1 EXAFS data of the intermetallic (with Ni>75%) to determine Fe-atom site occupation preferences (with respect to random). The resulting five values are {1, 1.10, 0.85, 1.33, 1}. The physical, stoichiometric conditions imposed by the statistical model show that only four of the five tetrahedral configurations occur in Ni 3 AlFe, analogous to what is observed in semiconducting II–VI group ternaries with Mn, a transition element.