Structural phase transition in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">Ca</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>at high pressures
Bibliographic record
Abstract
The structural and vibrational properties of $\mathrm{Ca}{\mathrm{H}}_{2}$ have been examined up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ at room temperature. Under ambient conditions, $\mathrm{Ca}{\mathrm{H}}_{2}$ has a $Pnma$ (cotunnite-type) structure. A structural phase transformation was observed around $15\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and completed at $20\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The high pressure structure is identified as hexagonal $P{6}_{3}∕mmc$. First-principles calculations reproduced the first-order nature of the transition. Since $P{6}_{3}∕mmc$ is a supergroup of $Pnma$ the structural change can be traced back to gradual displacements of the hydrogen atoms from the $4c$ positions in the cotunnite structure to the special $2a$ and $2d$ positions in the hexagonal structure. The observed phase transition pressure is much lower than that predicted for $\mathrm{Mg}{\mathrm{H}}_{2}$.
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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 | 0.002 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.002 |
| Open science | 0.002 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.209 | 0.003 |
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; both teacher heads agree on what is shown here.
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".