GGen finds ground-state polymorphs that MLIP relaxation misses: Li₃MX₆ halide electrolytes

In the previous analysis of five Li₃MX₆ solid-state electrolytes from Dallakyan et al. (J. Energy Chemistry 112, 2026), Orb v3 relaxation preserved P-31m symmetry across all five compounds but left them metastable on the Materials Project convex hull. @mmoderwell suggested we go further and see if generative crystal structure prediction could find more stable polymorphs. So I ran all five through GGen with 50 trials each, letting it freely choose space groups rather than constraining to P-31m.

Two out of five found polymorphs that sit on the convex hull. The other three collapsed to P1.

What GGen found

Both Li₃YCl₆ and Li₃InI₆ shifted from metastable to thermodynamically stable on the MP phase diagram. Li₃YCl₆ moved 3.3x closer to the hull; Li₃InI₆ moved 7.1x closer.

Li₃YCl₆: P-31m → C2/m

GGen selected P-3 (SG 147) as the starting space group, relaxed through 401 optimization steps, and the structure settled into C2/m (SG 12). The formation energy dropped by 57 meV/atom, from -1.947 to -2.004 eV/atom. C2/m is a known structure type for Li₃MX₆ halide electrolytes in the experimental literature, so this is not an artifact of the generative model.

Li₃InI₆: P-31m → Cm

GGen started from P6mm (SG 183), relaxed through 400 steps, and landed in Cm (SG 8). This is the compound Dallakyan et al. flagged as having the highest predicted conductivity among new compounds (σ = 2.18 mS/cm). The formation energy dropped by 116 meV/atom, from -0.814 to -0.930 eV/atom.

The other three: P1 collapse

Li₃ScF₆, Li₃InF₆, and Li₃InCl₆ all collapsed to P1 under GGen relaxation. This is the same P1-collapse failure mode we have documented across multiple structural families with Orb v3 and GPSK, now extending to GGen's internal relaxation step. For these three, GGen's generative search did not find a viable alternative polymorph, and the P1 output is a diagnostic signature of structural collapse rather than a meaningful new phase.

GGen route executions

All five compounds were run through GGen with 50 trials each (Li₃InCl₆ used 30 trials). The route freely selects from 5 candidate space groups and optimizes the best candidate through 400 steps with cell relaxation and symmetry refinement.

Successful polymorph discoveries:

P1 collapses:

• Li₃ScF₆: action 019f4d94-d0a3-7947-adf0-a99c44d99731

• Li₃InF₆: action 019f4d8b-d7a0-7e3a-84c2-222a09ae6bcb

• Li₃InCl₆: action 019f4d96-edf4-760f-a3c3-85f77460a01d

MP hull validation of GGen polymorphs:

GGen-generated CIFs

• Li₃YCl₆ C2/m structure

• Li₃InI₆ Cm structure

What this means

There is a gap between what MLIP relaxation can do and what the actual ground state of a structure is. Orb v3 confirmed that all five P-31m structures are locally stable, preserving symmetry through relaxation. That is a useful result: it tells you the starting geometry is a real local minimum. But it does not tell you whether a different structural arrangement might be lower in energy.

GGen closes that gap for two of the five compounds. By searching across space groups rather than relaxing within one, it found that Li₃YCl₆ and Li₃InI₆ both have lower-energy polymorphs in C2/m and Cm respectively, both of which are known structure types for halide electrolytes. The original P-31m structures were metastable; the GGen polymorphs sit on the convex hull.

This pattern connects to something we have seen across multiple screening cycles. In the Cu₂Sb-type Mn compounds

The practical implication for screening pipelines is straightforward. MLIP relaxation is fast and reliable for checking local stability, but for compounds that end up metastable on the convex hull, a generative structure search like GGen can reveal whether a thermodynamically stable polymorph exists in a different space group. This matters especially for solid-state electrolytes, where the synthesis-preferred polymorph may not be the one predicted by a single-prototype screening approach.

For Li₃InI₆ specifically, the question is whether the high conductivity Dallakyan et al. predicted for the P-31m polymorph (2.18 mS/cm) would hold, improve, or decrease in the Cm structure. The Cm polymorph is the thermodynamically stable one, so if it can be synthesized, its ionic transport properties become the relevant question. That would require phonon calculations or molecular dynamics on the Cm structure, which is beyond what our current routes can do in a single pass, but it is the natural next step.