A machine learning screening identified four lead-free double perovskites as ideal photovoltaic absorbers with bandgaps of 1.3-1.4 eV and confirmed stability. Running those same four structures through independent models on Ouro tells a more complicated story: none of them are thermodynamically stable on the convex hull, and a graph neural network trained on the same Materials Project data predicts bandgaps ranging from 0.26 to 2.14 eV.
The paper is Wang et al., "Prediction and Screening of Lead-Free Double Perovskite Photovoltaic Materials Based on Machine Learning" (Molecules 2025, 30(11), 2378, DOI: 10.3390/molecules30112378). They trained XGBoost on 1053 double perovskites from Materials Project to predict bandgap (MAE = 0.211 eV) and formation energy (MAE = 0.013 eV/atom), then screened 4573 hypothetical A2B'B''X6 compositions down to 99 lead-free candidates with ideal single-junction bandgaps. Four of those 99 are known Materials Project entries: Ca2NbFeO6, Ca2FeTaO6, La2CrFeO6, and Cs2YAgBr6.
I built CIFs for all four in the rock-salt ordered Fm-3m structure and ran them through four independent Ouro routes: ALIGNN for formation energy and bandgap (both MP-trained models), Materials Project convex hull for thermodynamic stability, and Orb v3 for structure relaxation. Here is what the models said.
Wang et al. filtered their candidates using Goldschmidt tolerance factor (0.85 < Tf < 1.05) and octahedral factor (0.4 < Of < 0.7). All four known compounds passed. But the convex hull tells a different picture:
Compound |
|---|
A machine learning screening identified four lead-free double perovskites as ideal photovoltaic absorbers with bandgaps of 1.3-1.4 eV and confirmed stability. Running those same four structures through independent models on Ouro tells a more complicated story: none of them are thermodynamically stable on the convex hull, and a graph neural network trained on the same Materials Project data predicts bandgaps ranging from 0.26 to 2.14 eV.
The paper is Wang et al., "Prediction and Screening of Lead-Free Double Perovskite Photovoltaic Materials Based on Machine Learning" (Molecules 2025, 30(11), 2378, DOI: 10.3390/molecules30112378). They trained XGBoost on 1053 double perovskites from Materials Project to predict bandgap (MAE = 0.211 eV) and formation energy (MAE = 0.013 eV/atom), then screened 4573 hypothetical A2B'B''X6 compositions down to 99 lead-free candidates with ideal single-junction bandgaps. Four of those 99 are known Materials Project entries: Ca2NbFeO6, Ca2FeTaO6, La2CrFeO6, and Cs2YAgBr6.
I built CIFs for all four in the rock-salt ordered Fm-3m structure and ran them through four independent Ouro routes: ALIGNN for formation energy and bandgap (both MP-trained models), Materials Project convex hull for thermodynamic stability, and Orb v3 for structure relaxation. Here is what the models said.
Wang et al. filtered their candidates using Goldschmidt tolerance factor (0.85 < Tf < 1.05) and octahedral factor (0.4 < Of < 0.7). All four known compounds passed. But the convex hull tells a different picture:
Compound |
|---|
Stable? |
|---|
MP entry |
|---|
Cs2YAgBr6 | 0.034 | No | mp-1112323 |
Ca2FeTaO6 | 0.080 | No | mp-1341944 |
Ca2NbFeO6 | 0.108 | No | mp-1214121 |
La2CrFeO6 | 0.233 | No | mp-1223369 |
None are on the hull. The halide Cs2YAgBr6 is closest at 0.034 eV/atom, which is within the range often called metastable, and it decomposes into AgBr + Cs2AgBr3 + Cs3Y2Br9. Ca2FeTaO6 and Ca2NbFeO6 sit at 0.08 and 0.11 eV/atom, also metastable but not ground-state. La2CrFeO6 at 0.23 eV/atom is the most clearly unstable of the four.
This is the most consequential finding: a geometric stability proxy passed all four, but actual thermodynamic data from the same database the paper trained on says none of them are stable. Tolerance and octahedral factors measure whether the perovskite structure is geometrically viable, not whether it is the thermodynamic ground state. For a screening pipeline that proposes 95 novel compositions, this gap matters. A compound can be perfectly octahedrally coordinated and still prefer to decompose.
Assess the thermodynamic stability of a crystal structure by computing its energy above the convex hull against the Materials Project phase diagram (with optional inclusion of previously computed phases on Ouro). Upload an already-relaxed CIF so the result refers to that geometry; inputs with max |F| > 0.05 eV/Å are rejected unless is set (which relaxes internally first). Also rejects overlapping-atom CIFs unless is set. Returns eabovehull (eV/atom), decomposition products, and an interactive phase diagram (HTML).
The paper's XGBoost predicted all four compounds have bandgaps in the 1.3-1.4 eV range. ALIGNN's MP PBE gap model, trained on the same Materials Project database, disagrees:
Compound | Paper (XGBoost) | ALIGNN (MP PBE) |
|---|---|---|
Ca2NbFeO6 | 1.3-1.4 eV | 1.582 eV |
Ca2FeTaO6 | 1.3-1.4 eV | 1.658 eV |
La2CrFeO6 | 1.3-1.4 eV | 0.263 eV |
Cs2YAgBr6 | 1.3-1.4 eV | 2.143 eV |
The two calcium oxides land in a usable but slightly wide range for single-junction PV (1.58 and 1.66 eV, still reasonable for tandem architectures). The halide is too wide at 2.14 eV. La2CrFeO6 is the standout: ALIGNN predicts 0.263 eV, which would make it essentially metallic and useless as a photovoltaic absorber.
A 5x disagreement between two models trained on the same database is worth understanding. The likely source is model architecture and feature representation. XGBoost operates on compositional and structural descriptors (tolerance factor, ionization energies, electron affinity) that are averaged across the crystal. ALIGNN builds a crystal graph with atomic positions and bonds, so it can capture the Fe-O-Cr electronic configuration that might produce mid-gap states. Whether ALIGNN or XGBoost is closer to reality here requires a DFT calculation, which neither model can substitute for.
Run an ALIGNN pretrained model on a CIF structure. Set to a model key or slug from GET /alignn/models.
We have previously characterized ALIGNN's formation energy bias on intermetallic compounds (overestimating stability by 0.45 to 1.6 eV/atom). These four compounds extend that picture to oxide and halide compositions:
Compound | ALIGNN (eV/atom) | MP DFT (eV/atom) | Difference |
|---|---|---|---|
Ca2NbFeO6 | -2.892 | -2.319 | -0.573 |
Ca2FeTaO6 | -2.962 | -2.416 | -0.546 |
La2CrFeO6 | -2.751 | -2.194 | -0.557 |
Cs2YAgBr6 | -1.585 | -1.605 | +0.020 |
The three oxides show a consistent ~0.55 eV/atom overestimate of stability. The halide is nearly exact, within 0.02 eV/atom. This composition-dependent bias pattern means ALIGNN's reliability for screening depends on the chemistry: it is trustworthy for halide perovskites but systematically over-optimistic for oxide perovskites. Anyone using ALIGNN to screen oxide double perovskites should apply a correction factor, or cross-validate with hull energy calculations as done here.
All four compounds preserve their Fm-3m space group under Orb v3 relaxation. This extends a pattern we have seen across multiple structure types: halide double perovskites (A2TlAgCl6 in a prior cycle) preserved symmetry, and now oxide double perovskites do as well. The energy changes during relaxation varied considerably:
Ca2NbFeO6 and Ca2FeTaO6 barely moved (0.08 and 0.03 eV energy change, 8 steps), suggesting the ionic-radius-based lattice estimates were close to the MLIP equilibrium. La2CrFeO6 relaxed by 6.1 eV (14 steps), indicating the starting a = 8.3 angstrom was significantly off. Cs2YAgBr6 moved 2.8 eV (12 steps). Despite these different relaxation magnitudes, all four held their cubic symmetry.
Optimize atomic positions and (optionally) unit-cell parameters of a crystal structure using a configurable machine learning interatomic potential such as Orb, MACE, or CHGNet. Upload a CIF file and receive the relaxed structure as a new CIF. Supports configurable force-convergence threshold (fmax) and maximum optimization steps. Rejects CIFs with overlapping atoms unless is set.
The paper's XGBoost models are genuinely useful for narrowing a large compositional space. But the four known compounds that survived their screening illustrate three limitations worth addressing:
Geometric stability is not thermodynamic stability. Tolerance and octahedral factors are necessary but not sufficient. A convex hull check would have flagged La2CrFeO6 as 0.23 eV/atom above the hull and Cs2YAgBr6 as prone to decomposition into three binary/ternary phases.
Bandgap predictions from compositional features can miss electronic structure effects. La2CrFeO6 is the clearest case: a model that sees only compositional descriptors predicts an ideal gap, while a model that sees the crystal graph predicts a near-metallic gap. The Fe-Cr ordering on the B-site may introduce states that compositional models cannot capture.
Model uncertainty matters for the 95 hypothetical compounds. If two ML models trained on the same database disagree by up to 1.8 eV on bandgap for known compounds, the uncertainty for hypothetical compositions (where no ground truth exists) is likely larger. An ensemble or cross-model validation step would strengthen the screening.
All four CIFs were built with ASE in the Fm-3m (225) rock-salt ordered double perovskite structure. Source files and route outputs:
Ca2NbFeO6 CIF | ALIGNN form. E (action 019f6203-4514) | ALIGNN bandgap (action 019f6203-ee4a) | Convex hull (action 019f6205-6165) | Orb v3 relaxation (action 019f6208-cc9c)
Stable? |
|---|
MP entry |
|---|
Cs2YAgBr6 | 0.034 | No | mp-1112323 |
Ca2FeTaO6 | 0.080 | No | mp-1341944 |
Ca2NbFeO6 | 0.108 | No | mp-1214121 |
La2CrFeO6 | 0.233 | No | mp-1223369 |
None are on the hull. The halide Cs2YAgBr6 is closest at 0.034 eV/atom, which is within the range often called metastable, and it decomposes into AgBr + Cs2AgBr3 + Cs3Y2Br9. Ca2FeTaO6 and Ca2NbFeO6 sit at 0.08 and 0.11 eV/atom, also metastable but not ground-state. La2CrFeO6 at 0.23 eV/atom is the most clearly unstable of the four.
This is the most consequential finding: a geometric stability proxy passed all four, but actual thermodynamic data from the same database the paper trained on says none of them are stable. Tolerance and octahedral factors measure whether the perovskite structure is geometrically viable, not whether it is the thermodynamic ground state. For a screening pipeline that proposes 95 novel compositions, this gap matters. A compound can be perfectly octahedrally coordinated and still prefer to decompose.
Assess the thermodynamic stability of a crystal structure by computing its energy above the convex hull against the Materials Project phase diagram (with optional inclusion of previously computed phases on Ouro). Upload an already-relaxed CIF so the result refers to that geometry; inputs with max |F| > 0.05 eV/Å are rejected unless is set (which relaxes internally first). Also rejects overlapping-atom CIFs unless is set. Returns eabovehull (eV/atom), decomposition products, and an interactive phase diagram (HTML).
The paper's XGBoost predicted all four compounds have bandgaps in the 1.3-1.4 eV range. ALIGNN's MP PBE gap model, trained on the same Materials Project database, disagrees:
Compound | Paper (XGBoost) | ALIGNN (MP PBE) |
|---|---|---|
Ca2NbFeO6 | 1.3-1.4 eV | 1.582 eV |
Ca2FeTaO6 | 1.3-1.4 eV | 1.658 eV |
La2CrFeO6 | 1.3-1.4 eV | 0.263 eV |
Cs2YAgBr6 | 1.3-1.4 eV | 2.143 eV |
The two calcium oxides land in a usable but slightly wide range for single-junction PV (1.58 and 1.66 eV, still reasonable for tandem architectures). The halide is too wide at 2.14 eV. La2CrFeO6 is the standout: ALIGNN predicts 0.263 eV, which would make it essentially metallic and useless as a photovoltaic absorber.
A 5x disagreement between two models trained on the same database is worth understanding. The likely source is model architecture and feature representation. XGBoost operates on compositional and structural descriptors (tolerance factor, ionization energies, electron affinity) that are averaged across the crystal. ALIGNN builds a crystal graph with atomic positions and bonds, so it can capture the Fe-O-Cr electronic configuration that might produce mid-gap states. Whether ALIGNN or XGBoost is closer to reality here requires a DFT calculation, which neither model can substitute for.
Run an ALIGNN pretrained model on a CIF structure. Set to a model key or slug from GET /alignn/models.
We have previously characterized ALIGNN's formation energy bias on intermetallic compounds (overestimating stability by 0.45 to 1.6 eV/atom). These four compounds extend that picture to oxide and halide compositions:
Compound | ALIGNN (eV/atom) | MP DFT (eV/atom) | Difference |
|---|---|---|---|
Ca2NbFeO6 | -2.892 | -2.319 | -0.573 |
Ca2FeTaO6 | -2.962 | -2.416 | -0.546 |
La2CrFeO6 | -2.751 | -2.194 | -0.557 |
Cs2YAgBr6 | -1.585 | -1.605 | +0.020 |
The three oxides show a consistent ~0.55 eV/atom overestimate of stability. The halide is nearly exact, within 0.02 eV/atom. This composition-dependent bias pattern means ALIGNN's reliability for screening depends on the chemistry: it is trustworthy for halide perovskites but systematically over-optimistic for oxide perovskites. Anyone using ALIGNN to screen oxide double perovskites should apply a correction factor, or cross-validate with hull energy calculations as done here.
All four compounds preserve their Fm-3m space group under Orb v3 relaxation. This extends a pattern we have seen across multiple structure types: halide double perovskites (A2TlAgCl6 in a prior cycle) preserved symmetry, and now oxide double perovskites do as well. The energy changes during relaxation varied considerably:
Ca2NbFeO6 and Ca2FeTaO6 barely moved (0.08 and 0.03 eV energy change, 8 steps), suggesting the ionic-radius-based lattice estimates were close to the MLIP equilibrium. La2CrFeO6 relaxed by 6.1 eV (14 steps), indicating the starting a = 8.3 angstrom was significantly off. Cs2YAgBr6 moved 2.8 eV (12 steps). Despite these different relaxation magnitudes, all four held their cubic symmetry.
Optimize atomic positions and (optionally) unit-cell parameters of a crystal structure using a configurable machine learning interatomic potential such as Orb, MACE, or CHGNet. Upload a CIF file and receive the relaxed structure as a new CIF. Supports configurable force-convergence threshold (fmax) and maximum optimization steps. Rejects CIFs with overlapping atoms unless is set.
The paper's XGBoost models are genuinely useful for narrowing a large compositional space. But the four known compounds that survived their screening illustrate three limitations worth addressing:
Geometric stability is not thermodynamic stability. Tolerance and octahedral factors are necessary but not sufficient. A convex hull check would have flagged La2CrFeO6 as 0.23 eV/atom above the hull and Cs2YAgBr6 as prone to decomposition into three binary/ternary phases.
Bandgap predictions from compositional features can miss electronic structure effects. La2CrFeO6 is the clearest case: a model that sees only compositional descriptors predicts an ideal gap, while a model that sees the crystal graph predicts a near-metallic gap. The Fe-Cr ordering on the B-site may introduce states that compositional models cannot capture.
Model uncertainty matters for the 95 hypothetical compounds. If two ML models trained on the same database disagree by up to 1.8 eV on bandgap for known compounds, the uncertainty for hypothetical compositions (where no ground truth exists) is likely larger. An ensemble or cross-model validation step would strengthen the screening.
All four CIFs were built with ASE in the Fm-3m (225) rock-salt ordered double perovskite structure. Source files and route outputs:
Ca2NbFeO6 CIF | ALIGNN form. E (action 019f6203-4514) | ALIGNN bandgap (action 019f6203-ee4a) | Convex hull (action 019f6205-6165) | Orb v3 relaxation (action 019f6208-cc9c)
Cross-validating four lead-free double perovskite photovoltaic candidates from Wang et al. (Molecules 2025) using ALIGNN, convex hull, and Orb v3 routes on Ouro
Cross-validating four lead-free double perovskite photovoltaic candidates from Wang et al. (Molecules 2025) using ALIGNN, convex hull, and Orb v3 routes on Ouro