Static ionic displacements in Fe–Ni alloys from first principles

Abstract

Static local displacements of ions in disordered face-centered cubic Fe50Ni50 alloy are studied from first principles in the framework of the density functional theory. The disordered alloy is modeled using a 64 atom supercell constructed as a special quasirandom structure. Fully relaxed atomic positions inside the supercell are calculated by means of projected augmented wave method as implemented in Vienna ab initio simulation package. According to our calculation, the relative changes of mean nearest neighbor interatomic distances due to local lattice relaxations are relatively small (⩽0.6%), in agreement with experiment. At the same time, we predict that for all types of pairs, Fe–Fe, Fe–Ni, and Ni–Ni, the dispersion of the nearest neighbor interatomic distances is rather large, and the individual changes of distances between certain pairs of atoms due to local lattice relaxations can be one order of magnitude larger than the mean values for the corresponding pair of atoms.

Interatomic Interactions in F.C.C.-Ni–Fe Alloys

“…Many theoretical investigations [15][16][17][18][19][20][21][22][23][24][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81] have been devoted to studies of effects of ferro-and (or) antiferromagnetic order in alloys with atomic LRO (SRO). Conditionally, all these theories and models can be divided into two groups: (i) models, which are based on the primary contribution of itinerant electron magnetism [64,75,76] to magnetism of an alloy, and (ii) so-called local magnetic moment model [66][67][68], which implies the carriers of uncompensated magnetic moments as atoms located at the effectively-periodic lattice sites.…”

“…Conditionally, all these theories and models can be divided into two groups: (i) models, which are based on the primary contribution of itinerant electron magnetism [64,75,76] to magnetism of an alloy, and (ii) so-called local magnetic moment model [66][67][68], which implies the carriers of uncompensated magnetic moments as atoms located at the effectively-periodic lattice sites. As for magnetism of f.c.c.-Ni-Fe alloys, the basic complexity for developing such quantitative model is the simultaneous quantification of, firstly, the magnetism of both constituents of an alloy (Ni and Fe), secondly, the significant difference between Ni and Fe magnetic moments, and, thirdly, the availability of two magnetic states of Fe atoms, namely, two so-called Weiss γ-states [18][19][20][21][22][23][24]66], namely, the low-spin (LS) and high-spin (HS) states. There are some methods and approaches for definition of 'exchange' 'integrals' for magnetic interactions in alloys, in particular, MSCF (or MF) approximations [69][70][71][72][73][74]77], cluster methods in the mean-field theory [79], Monte Carlo Ising-type approximation [80], ab initio models [18-24, 75, 76], etc.…”

“…2) at c > 0.55 is apparently conditioned by both the presence of heterogeneous (atomic and/or magnetic) states [14][15][16][17][18][19][20][21][22][23][24]66] in f.c.c.-Ni-Fe alloys and the simultaneously increasing contribu-tion of itinerant electron magnetism into 'effective' 'exchange' interactions [75,76]. In general case, in the measurands such as T C (c) (25), there is a combination of all parameters such as follow:…”

“…In a concentration region of Fe atoms, 0.45 < c < 0.55, Elinvars are formed (with atomic long-rangeordered L1 0 -CuAuI-type (NiFe) layered structure with temperature factor of electrical resistance possessing high values) [2]. A special attention should be paid to Fe-Ni Invars [1-3, [18][19][20][21][22][23][24] with relative concentration of Fe atoms close to c ≅ 0.65. Near this chemical composition, such an alloy has microheterogeneous structure, which may be probably atomic long-range-ordered of L1 2 -Cu 3 Au type (Fe 3 Ni) in part.…”

“…From Equations (19), (20), specifying the condition of zeroing first derivatives of total configuration-dependent part of free energy, ∂F conf /∂η, ∂F conf /∂σ Fe , ∂F conf /∂σ Ni , for thermody-namic equilibrium state, we have a set of three transcendental equations for estimation of the equilibrium values of state parameters, η(T, c), …”