A Powder Neutron Diffraction Investigation of the Two Rhombohedral NASICON Analogues: γ-Na<sub>3</sub>Fe<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> and Li<sub>3</sub>Fe<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>
Bibliographic record
Abstract
The crystal structures of polycrystalline rhombohedral γ-Na 3 Fe 2 (PO 4 ) 3 and Li 3 Fe 2 (PO 4 ) 3 obtained through Na ↔ Li ion exchange have been determined for the first time, from neutron diffraction data recorded at 473 and 300 K, respectively. Na 3 Fe 2 (PO 4 ) 3 crystallizes in the space group R 3̄ c at 473 K (γ-form, a = 8.7270(2) Å, and c = 21.8078(5) Å). A progressive transfer of Na + ions from the M(1) (6-coordinate) to the M(2) (8-coordinate) sites occurs upon raising the temperature from ambient (α-form, τ M(1) = 1) to 393 K (β-form, τ M(1) = 0.91) and then to 473 K (γ-form, τ M(1) = 0.85). This is associated with a significant increase of Fe−Fe distances through the M(1) site and with relaxation of the framework such that the FeO 6 octahedra become less distorted. Ion exchange from Na 3 Fe 2 (PO 4 ) 3 leads to the rhombohedral form of Li 3 Fe 2 (PO 4 ) 3 (space group R 3̄; a = 8.3162(4) Å, and c = 22.459(1) Å). The M(1) and M(2) sites of the NASICON structure are empty in Li 3 Fe 2 (PO 4 ) 3 which is isotypic with Li 3 In 2 (PO 4 ) 3 . Removal of Na + from M(1) results in a strong increase of the c parameter due to stronger repulsions between adjacent FeO 6 octahedral faces along [001]. Lithium is located on one single-crystallographic site, M(3), with four Li−O distances ranging between 1.91 and 2.09 Å which span a regular tetrahedral geometry and gives rise to one signal in the 6 Li spectrum at 152 ppm. Two crystallographic sites are clearly distinguished for iron: Fe(1)O 6 shares three of its vertexes only with LiO 4 tetrahedra and is far more distorted than Fe(2)O 6 which shares three edges with LiO 4 tetrahedra.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| 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 | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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 source (direct Gemma or distilled Codex), 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".