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>
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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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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 | 0.003 | 0.001 |
| Meta-epidemiology (narrow) | 0.004 | 0.005 |
| Meta-epidemiology (broad) | 0.005 | 0.002 |
| Bibliometrics | 0.001 | 0.003 |
| Science and technology studies | 0.002 | 0.004 |
| Scholarly communication | 0.001 | 0.004 |
| Open science | 0.003 | 0.002 |
| Research integrity | 0.003 | 0.003 |
| Insufficient payload (model declined to judge) | 0.000 | 0.001 |
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 it