---
title: "Stable, unique, novel Fe-rich compounds, loop 6: Fe-Co-X gives orderings, not new compounds"
description: "555 Fe-Co-X cells screened on the Orb hull: everything near the hull is a decoration, an Fe/Co ordering of a known type, or a Heusler with a vacancy."
url: "https://ouro.foundation/posts/mmoderwell/stable-unique-novel-fe-rich-compounds-loop-6-fe-co-x-gives-orderings-not-new-compounds"
id: "01a1d795-0a55-7bbc-b642-6fc12bf69f05"
type: "post"
---

# Stable, unique, novel Fe-rich compounds, loop 6: Fe-Co-X gives orderings, not new compounds

Post by @mmoderwell in #permanent-magnets (all) · 2026-11-14

- Asset id: `01a1d795-0a55-7bbc-b642-6fc12bf69f05`
- Page: https://ouro.foundation/posts/mmoderwell/stable-unique-novel-fe-rich-compounds-loop-6-fe-co-x-gives-orderings-not-new-compounds

This loop moved the search to Fe-Co and Fe-Co-X. We put Fe and Co onto the two late-metal sites of 3,296 prototype structures, kept 555 new cells and scored them all on the Orb hull. Two landed on the hull and 38 within 25 meV/atom, but none is a new kind of compound. Every one is an fcc or bcc lattice decorated with Fe, Co and X, an Fe/Co ordering of a structure that Fe-X or Co-X already has, or a Heusler with one atom missing. Fe and Co are too alike for a ternary to beat their own mixing, so no SUN compound came out of this chemistry.

## What we measured

Cells were relaxed and scored with Orb v3 on the [hull route](https://ouro.foundation/routes/mmoderwell/calculate-energy-above-the-convex-hull), against MP, Alexandria, GNoME and Ouro references. Novelty came from the [novelty route](https://ouro.foundation/routes/mmoderwell/check-whether-a-crystal-structure-is-already-known). The five cells that survived the structure checks were also scored on the hull route's published-PBE surface, where the references are published PBE energies but the candidate is still scored by Orb. No DFT was run.

| Cell | Structure | Novelty | Orb hull (meV/atom) | Published-PBE surface (meV/atom) | What it is |
| ---- | --------- | ------- | ------------------- | -------------------------------- | ---------- |
| Fe₃Co₄Ge | R-3m | not checked | [0.0](https://ouro.foundation/actions/01a1d781-f96d-79af-a797-f14b6b9c216d) | not run | fcc-type decoration |
| Nb₄Fe₇Co | P-3m1 | [new composition](https://ouro.foundation/actions/01a1d789-c706-7c55-a5bd-edf02f944e89) | [0.0](https://ouro.foundation/actions/01a1d77d-99f3-7218-88c6-2dd17c40a9bc) | [18.1](https://ouro.foundation/actions/01a1d791-245e-752a-888d-e0a73d106f38) | Fe/Co ordering of C14 NbCo₂ |
| Zr₆Fe₅Co₁₃ | P-3m1 | [new composition](https://ouro.foundation/actions/01a1d78a-1a66-78f5-baac-59245accbd63) | [4.5](https://ouro.foundation/actions/01a1d77e-b2c6-7dec-a262-fa628e4f741b) | not run | Fe/Co ordering of a ZrCo₃ Laves-type stacking (Alexandria) |
| Zr₄Fe₅Co₃ | P-6m2 | [new composition](https://ouro.foundation/actions/01a1d789-f8c1-7f9f-b0c9-07e3c6796ae2) | [9.6](https://ouro.foundation/actions/01a1d77e-1aa5-76cf-ae1a-b7c1583d608e) | not run | Fe/Co ordering of a ZrCo₂ Laves stacking |
| TiFeCo | R-3m | [new polymorph](https://ouro.foundation/actions/01a1d78a-44f0-74b0-90eb-753300d1843d) | [10.4](https://ouro.foundation/actions/01a1d782-1610-7817-9694-15d36a1dddb0) | [20.2](https://ouro.foundation/actions/01a1d791-1aaf-7cba-84ef-52afae5ca79b) | a lower TiFeCo is already known |
| [Fe₃Co₈Ge₄](https://ouro.foundation/files/mmoderwell/fe3co8ge4-orb-relaxed-cell-pm-3m-vacancy-ordered-heusler) | Pm-3m | [new composition](https://ouro.foundation/actions/01a1d78a-42bd-715d-adea-d66dcf73ed8c) | [24.5](https://ouro.foundation/actions/01a1d781-ab89-7b34-9e36-16b2942ee754) | [33.1](https://ouro.foundation/actions/01a1d790-fd88-7f10-9490-d934192f425d) | FeCo₂Ge Heusler with ¼ of its Fe sites empty |
| [Fe₉CoB₆](https://ouro.foundation/files/mmoderwell/fe9cob6-orb-relaxed-cell-i4-m-cr5b3-type) | I4/m | [new composition](https://ouro.foundation/actions/01a1d78d-ae69-7112-9452-1f408989cbc6) | [27.3](https://ouro.foundation/actions/01a1d77d-10c9-7aa8-a38a-ec0dd8116e31) | [31.9](https://ouro.foundation/actions/01a1d791-0794-7269-8bf4-e54bd84d698a) | Fe/Co ordering of the Cr₅B₃ type |
| ZrFe₈Co₄ | I4/mmm | [new composition](https://ouro.foundation/actions/01a1d78d-df77-7a37-b0b0-cd034e2c2190) | [31.1](https://ouro.foundation/actions/01a1d77d-fafc-7196-a695-30ea54524fb2) | [49.8](https://ouro.foundation/actions/01a1d791-0c4f-7d6e-b70c-ee79d4ba1480) | Fe/Co ordering of the ThMn₁₂ type |

Over the whole batch: 2 of 555 cells sit on the Orb hull (0.36 %), 38 within 25 meV and 87 within 50. Of the 38: 10 are decorations, 24 are Fe/Co orderings of a type already in the databases, 2 are stackings from the same Laves family, 1 is already known (Zr₃Fe₇Co₂, Alexandria agm005622237), and 1 is Fe₃Co₈Ge₄. Zr systems gave 16 of the 38, Ti 9 and Nb 4. Fe-Co-B, -C, -P, -Sn and binary Fe-Co gave nothing within 25 meV; the closest were B at 27, C at 257 and Fe-Co at 101.

For a while Fe₃Co₈Ge₄ looked like the one real new structure, since its all-Fe parent has 15 atoms and it matched no binary type. Its sites give it away. Fe sits on 3c, Co on 8g and Ge on 3d + 1b, with a = 5.65 Å, and the 1a site is empty: it is the FeCo₂Ge Heusler, which is known, with one atom in sixteen removed. Look for the hole at the cube corner:

> Embedded file: [Fe₃Co₈Ge₄: Orb-relaxed cell (Pm-3m, vacancy-ordered Heusler)](https://ouro.foundation/files/mmoderwell/fe3co8ge4-orb-relaxed-cell-pm-3m-vacancy-ordered-heusler)

Fe₉CoB₆ is the only tetragonal cell within 30 meV that is not a decoration. It is (Fe,Co)₅B₃ on the Cr₅B₃ type, built from the made compound Cr₉FeB₆, and it sits about 30 meV above Fe₂B + FeB + CoB on both surfaces:

> Embedded file: [Fe₉CoB₆: Orb-relaxed cell (I4/m, Cr₅B₃ type)](https://ouro.foundation/files/mmoderwell/fe9cob6-orb-relaxed-cell-i4-m-cr5b3-type)

## What the hypotheses did

- **H10: what lands on the hull is mostly Fe/Co orderings of a binary type. Supported in kind, refuted on rate.** We predicted 1–5 % on the hull, on the grounds that the Fe-Co-X hull has few ternaries to compete with; we got 0.36 %. We predicted at least 70 % orderings; within 25 meV, 34 of 38 cells were decorations, orderings or stackings.
- **H11: Fe-Co with a tetrel or early metal holds a true ternary type on the hull. Refuted for this batch.** We gave it 40 %. No true-type cell came within 25 meV, and the best near-miss turned out to be a defective Heusler.
- **H12: prototypes whose type has a made member give more true-type hits. Inconclusive.** Too few hits to compare (1 against 4 within 50 meV).

## What changed

- The literature already pointed this way for B, C and Si. A 2026 genetic-algorithm + DFT search of Fe-Co-{B,C,P,Si,S} ([JMMM 646, 173978](https://doi.org/10.1016/j.jmmm.2026.173978)) put every Fe-Co-C cell it found 110–140 meV above the hull and Fe-Co-Si 50–100 meV above, and Fe₃CoB₂, which has been made, about 23 meV above. This loop extends that "no" to Ge, Ga, Al, Sn, Ti, Zr, Nb, V and Mo, for prototypes up to 24 atoms.
- Fe and Co behave as one element near the hull. A new compound type in this chemistry would need a third element that orders against both, the way Ge does in the Heuslers, and the Heuslers are already known.
- The published-PBE surface put every surviving cell 5–19 meV higher than Orb. Loop 5 showed this surface is a fair guide to our own PBE numbers, so no DFT was bought here.

## Instruments

Two of our false-positive filters leak. (1) The all-Fe parent check cannot see vacancies, and that is how the defective Heusler got through. (2) An Fe/Co ordering matcher that only looks at MP binaries within 50 meV misses Alexandria's Laves-type stackings, so every Zr-Fe-Co cell first looked new. With the matcher widened to every MP and Alexandria binary and ternary, all of them matched. Both fixes go into the funnel from here on.

## Next

1. **Co on the sites of this campaign's two compounds.** U₆Co₁₂Ge₄C, the made parent of Zr₆Fe₁₂Si₄C, is itself a Co compound. Zr₆(Fe,Co)₁₂Si₄C and Co in the filled G phase test Fe-Co inside a type that is already stable here, where an Fe/Co site preference has something to act on. The Orb screen is free.
2. **Add the two filter fixes, then rebuild from made types only**, across all symmetries for Fe-rich cells, so later counts can be trusted. Free.
3. **A PBE single point on Fe₉CoB₆ (about $0.5)**, only if you want Orb tested on borides one more time. Orb rated Ti₃Fe₅B₂ too low (loop 3), so this would probably move the wrong way.

Earlier loop: [loop 5, two new cubic carbides](https://ouro.foundation/posts/mmoderwell/stable-unique-novel-fe-rich-compounds-loop-5-two-new-zr-fe-si-carbides-hold-up-in-dft).

## Assets in this post

- [Fe₃Co₈Ge₄: Orb-relaxed cell (Pm-3m, vacancy-ordered Heusler)](https://ouro.foundation/files/mmoderwell/fe3co8ge4-orb-relaxed-cell-pm-3m-vacancy-ordered-heusler) (file, id `51d7d3d6-896a-449f-850b-49837941480b`)
- [Fe₉CoB₆: Orb-relaxed cell (I4/m, Cr₅B₃ type)](https://ouro.foundation/files/mmoderwell/fe9cob6-orb-relaxed-cell-i4-m-cr5b3-type) (file, id `db77a054-5a4d-4fb5-9620-8d39b49bbd23`)
- [Calculate energy above the convex hull](https://ouro.foundation/routes/mmoderwell/calculate-energy-above-the-convex-hull) (route, id `75fe7f4b-22f5-4883-aa1d-378fd42da47c`)
- [Check whether a crystal structure is already known](https://ouro.foundation/routes/mmoderwell/check-whether-a-crystal-structure-is-already-known) (route, id `3dced825-cfea-4711-89a9-1d7998242001`)
- [Stable, unique, novel Fe-rich compounds, loop 5: two new Zr–Fe–Si carbides hold up in DFT](https://ouro.foundation/posts/mmoderwell/stable-unique-novel-fe-rich-compounds-loop-5-two-new-zr-fe-si-carbides-hold-up-in-dft) (post, id `01a1d607-dd12-7b11-a5e1-e2ed43be1744`)