Fe-W phase-diagram and literature note: is tetragonal Fe17W3 known?

Question. We predict a tetragonal Fe17W3 phase (validated CIF, candidate row, space group 115, P-4m2, 20 atoms/cell, ~15 at% W, independently replicated e_above_hull = 0.0087 eV/atom at include_user_materials=false). Is anything like it known in the experimental or database literature, and what does the assessed Fe-W phase diagram say should exist at that composition? This note is the phase-diagram evidence for the quest checkpoint that decides whether Fe17W3 earns a large-cell anisotropy calculation.

Short answer. Every intermetallic compound in the assessed Fe-W system is W-richer than Fe17W3, and no experimental, thin-film, or crystallographic-database record of a Fe17W3 (or W3Fe17) phase surfaced in any search performed for this note. At ~15 at% W the equilibrium constitution is a supersaturated bcc alpha-Fe(W) solid solution plus the C14 Laves phase lambda-Fe2W, which means our predicted phase is not contradicted by the phase diagram (it does not sit inside a known compound's homogeneity range) but it is also entirely unevidenced: it is an unknown, not a rediscovery.

The assessed equilibrium phase diagram

The Fe-W system contains two well-established intermetallics. lambda-Fe2W is the hexagonal C14 Laves phase (MgZn2 type, P6_3/mmc, Pearson hP12); Materials Project carries it as a hexagonal Laves structure in P6_3/mmc [8,9]. mu-Fe7W6 is the rhombohedral D8_5 mu phase (R-3m, Pearson hR13) [8,9]. Early galvanic-cell thermodynamics also treated a Fe3W2 compound [10], but Fe3W2 does not appear as a stable phase in the modern assessments discussed below and is best regarded as a historical report. The existence of a binary FeW sigma phase has been a recurring question: sigma (P4_2/mnm, 30 atoms/cell) is stable in Cr-W and Cr-Fe-W, and Chvatalova et al. explicitly used first-principles thermodynamic modelling of sigma for "systems where it is not stable at all", including Fe-W [11]. The binary phase-diagram literature is summarised in Kubaschewski von Goldbeck's Fe-W compilation [1] and was partially redetermined experimentally by Antoni-Zdziobek, Commeau and Joubert [2]; a Calphad evaluation with a magnetic multi-sublattice model followed the same framework [3], and the most recent full modelling effort (Jacob et al., 2015) combines first-principles energies and phonons with Calphad [4]. Waki et al., quoting the accepted diagram, note that Fe7W6 is stable only between 1215 and 1637 °C at near-equiatomic Fe:W, and that no liquid exists below 1500 °C in this system, so all low-temperature Fe-W compounds form by solid-state (peritectoid) reactions, not by primary solidification [7]. Kirchner, Harvig and Uhrenius provide the classic experimental/thermodynamic study of ferrite and austenite equilibria with the intermediate phases [5], and Takayama, Wey and Nishizawa showed that the magnetic transition of bcc Fe measurably shifts the W solubility in ferrite [6].

Crucially for our composition: both Fe2W (33.3 at% W) and Fe7W6 (46.2 at% W) are far W-richer than Fe17W3 (15.0 at% W). Equilibrium at 15 at% W is the alpha-Fe(W) solid solution coexisting with lambda-Fe2W, with a very narrow W solubility in ferrite at low temperature.

Has Fe17W3 ever been reported?

Not in anything the search could find. Searches across the open web, Google Scholar-style indexing, the Crystallography Open Database, and Materials Project returned no entry for Fe17W3, W3Fe17, or any non-centrosymmetric tetragonal binary Fe-W compound; the Materials Project Fe-W records surfaced were the binaries Fe2W and Fe7W6, with the other "Fe-W" hits (Fe3W3C, Fe3W3N, Fe6W6N) being ternary eta-carbide-type phases stabilised by interstitials [7,9]. Caveat: this is a literature search, not an exhaustive ICSD/AFLOW database sweep, so absence of evidence here carries the usual weight. The closest crystallographic neighbour found is a metastable W-enriched body-centred-tetragonal nanophase (a ≈ 2.96 Å, c ≈ 3.17 Å) precipitating in Fe-Co-Cr-W-Ga magnets, paramagnetic at room temperature and of unresolved structure [12]; it is not Fe17W3 but it shows that Fe-rich Fe-W-containing systems do accommodate unknown metastable W-enriched tetragonal phases under the right processing.

Thin-film and nonequilibrium evidence (distinct from bulk equilibrium)

Nonequilibrium processing gives bcc supersaturation exactly across our composition, which is the natural synthesis route to test. Sputter-deposited Fe1-xWx films on water-cooled or LN2-cooled substrates form a single bcc phase for x < 0.2 and an amorphous phase for 0.2 < x < 0.6; the bcc phase is ferromagnetic for x < 0.8, and the Fe moment in Fe-rich bcc exceeds that of pure Fe [13]. Lu and Chien reported structural and magnetic properties of Fe-W alloy films in the same era [14]. Mechanical alloying produces Fe(W) and W(Fe) solid solutions whose lattice parameters deviate from the CALPHAD-predicted equilibrium behaviour [15]. A 2025 combinatorial sputtering study mapped the amorphous-to-crystalline transition across 9.4 to 45.5 at% W in a 169-sample Fe-W library [16]. So a metastable ordered tetragonal derivative within that bcc supersaturation window would be surprising but not structurally alien to the thin-film literature.

Magnetism of the known Fe-W compounds

The known intermetallics are magnetically weak, which is the core caution for the magnet program. For Fe2W, Edstrom's full-potential LAPW study (Phys. Rev. B 96, 064422, 2017) finds a ferrimagnetic ground state: Fe moments of opposite sign on the two sites give a net 4.45 mu_B per unit cell, a computed saturation magnetisation of only 0.35 T against an experimental low-temperature value of about 0.44 T, and a computed uniaxial MAE of 0.87 MJ/m3 along the c axis against an experimental value near 28.6 kJ/m3 (measured on nanoparticles, so the comparison is loose) [17]. No quantitative experimental magnetisation numbers for mu-Fe7W6 were located in these searches; specific values for that compound remain unverified in this note.

This matters because our candidate's Ms = 1.74 T is a route prediction (CHGNet-assisted tier-1), not a measurement, and every experimentally characterised Fe-W intermetallic so far is a ferrimagnet with heavy moment cancellation. The literature does not forbid a ferromagnetic Fe17W3, but it has never produced one, and that is the single claim most in need of independent evidence in the checkpoint.

Verdict

A tetragonal SG-115 Fe17W3 is unknown: not reported in bulk, thin-film, or database literature that this search could locate, and not contradicted by the assessed Fe-W phase diagram either. At 15 at% W the equilibrium competitors are the bcc alpha-Fe(W) solid solution and the C14 Laves lambda-Fe2W, with mu-Fe7W6 appearing only at much higher W content; a tetragonal Fe17W3 would therefore be a genuinely new, presumably metastable entry, most plausibly accessed through nonequilibrium routes (sputtering, mechanical alloying, nanocrystalline synthesis) that already demonstrably freeze bcc supersaturation across this composition range. The burden is on synthesis: if it is real, it sits in a composition window whose equilibrium phases are all W-richer, and its magnetism must beat the ferrimagnetic near-cancellation that defines every known Fe-W intermetallic.

Inputs this gives the checkpoint: (1) Fe17W3 is not contradicted by established phase-diagram data, and its composition window's equilibrium phases are all W-richer; (2) it is entirely unevidenced experimentally, so any go branch is a bet on the route predictions, not on a known phase; (3) the ferrimagnetism of all known Fe-W intermetallics is the weakest link in the chain from "near hull and dynamically stable" to "magnet candidate". Prior supporting artifacts: evidence dossier

Sources

1. O. Kubaschewski von Goldbeck, "Fe-W Iron-Tungsten", in Iron - Binary Phase Diagrams, pp. 164-167 (1982). DOI: 10.1007/978-3-662-08024-5_76

2. A. Antoni-Zdziobek, T. Commeau, J.-M. Joubert, "Partial Redetermination of the Fe-W Phase Diagram", Metallurgical and Materials Transactions A 44, 2996-3003 (2013). DOI: 10.1007/s11661-013-1658-2

3. "A thermodynamic evaluation of the iron-tungsten system", Calphad 7, 317-326 (1983). DOI: 10.1016/0364-5916(83)90011-1. (Author name not verified from accessible metadata; treat citation as partially verified.) [UNVERIFIED AUTHOR]

4. A. Jacob, C. Schmetterer, L. Singheiser, A. Gray-Weale, B. Hallstedt, A. Watson, "Modeling of Fe-W phase diagram using first principles and phonons calculations", Calphad (2015). DOI: 10.1016/j.calphad.2015.04.010

5. G. Kirchner, H. Harvig, B. Uhrenius, "Experimental and thermodynamic study of the equilibria between ferrite, austenite and intermediate phases in the Fe-Mo, Fe-W, and Fe-Mo-W systems", Metallurgical Transactions 4, 1059-1067 (1973). DOI: 10.1007/bf02645609

6. T. Takayama, M. Y. Wey, T. Nishizawa, "Effect of magnetic transition on the solubility of alloying elements in bcc iron and fcc cobalt", Transactions of the Japan Institute of Metals 22, 315-325 (1981). DOI: 10.2320/matertrans1960.22.315