Sodium’s elusive tI50 phase predicted to be a body-centered tetragonal electride
Bibliographic record
Abstract
Sodium exhibits an anomalous melting curve with a minimum at high pressure, where several complex crystalline phases coinciding with this minimum have been observed but remain unresolved. Solving these structures would help better understand sodium’s unusual melting behavior. In this study, we determine the crystal structure of one such phase, the tI50 phase. Using a data-derived potential-assisted structure search, we identify the tI50 phase as a body-centered tetragonal structure with 50 atoms per unit cell, crystallizing in the I4/m space group. The predicted lattice parameters deviate by less than 0.38% from experimental values, and enthalpy calculation confirms this structure as the ground state for sodium across a ~20 GPa pressure range. The structure features a cage-like polyhedral network with interstitial electride states and remains weakly metallic, unlike the insulating hP4 phase. Resolving this unique structure advances understanding of sodium’s diverse structural behavior under high pressure. Sodium exhibits an anomalous melting curve with a minimum at high pressure, but the crystalline phases coinciding with this minimum remain unresolved. Here, using a data-derived potential-assisted structure search, the authors predict the crystal structure of one of those phases, tI50, and show that it exhibits weak metallic character.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".