Synthesis of Na3WH9 and Na3ReH8 Ternary Hydrides at High Pressures
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2024-10-31 |
| Journal | Inorganic Chemistry |
| Authors | TomĆ”s MarqueƱo, Israel Osmond, Mikhail A. Kuzovnikov, Hannah A. Shuttleworth, Samuel GallegoāParra |
| Institutions | University of Edinburgh, Center for High Pressure Science and Technology Advanced Research |
| Citations | 1 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled āExecutive SummaryāThis research details the high-pressure, high-temperature synthesis and characterization of two novel ternary hydrides, Na3WH9 and Na3ReH8, focusing on their structural stability and electronic properties for engineering applications.
- Novel Synthesis: Na3WH9 and Na3ReH8 were successfully synthesized using Diamond Anvil Cells (DAC) and infrared laser heating at extreme conditions (T = 1400 K, P > 7.8 GPa).
- Exceptional Stability: Both compounds exhibit remarkable stability, remaining intact upon decompression down to pressures <1 GPa (0.1 GPa for Na3WH9 and 0.3 GPa for Na3ReH8).
- High Hydrogen Content: The materials host homoleptic 18-electron complex anions, [WH9]3- and [ReH8]3-, confirming high hydrogen-to-metal ratios beneficial for storage applications.
- Structural Polymorphism: Upon decompression, both hydrides undergo analogous phase transitions: Distorted fcc Heusler (IIā) ā Symmetric fcc (II) ā Distorted hexagonal (I).
- Dynamic Disorder: Molecular Dynamics (MD) simulations confirmed that the fcc phases (II) feature rotationally disordered hydrogen atoms occupying nonisotropic shells around the transition metal (TM).
- Electronic Properties: DFT calculations classify both materials as electrical insulators with wide bandgaps, predicting metallicity only at extremely high pressures (>180 GPa), ruling them out for high-Tc superconductivity at attainable pressures.
Technical Specifications
Section titled āTechnical Specificationsā| Parameter | Value | Unit | Context |
|---|---|---|---|
| Synthesis Temperature (T) | 1400 | K | Achieved via infrared laser heating in DAC. |
| Na3WH9 Minimum Synthesis P | >7.8 | GPa | Pressure threshold for Na3WH9 formation. |
| Na3ReH8 Minimum Synthesis P | >10.1 | GPa | Pressure threshold for Na3ReH8 formation. |
| Na3WH9 Stability Range | 0.1 to 42.1 | GPa | Observed stability window across pressure cycling. |
| Na3ReH8 Stability Range | 0.3 to 32.5 | GPa | Observed stability window across pressure cycling. |
| Na3WH9-IIā Structure (High P) | P21/c (Monoclinic) | Space Group | Distorted fcc Heusler structure observed above 10 GPa. |
| Na3WH9-II Transition P | 6.4 to 10 | GPa | Transition to symmetric fcc Heusler structure. |
| Na3WH9-I Transition P | ~3 | GPa | Transition to hexagonal structure (P63/m). |
| Na3ReH8-II Transition P | ~17 | GPa | Transition to symmetric fcc Heusler structure. |
| TM-H Stretching Mode (Raman) | 1900-2500 | cm-1 | Characteristic vibrational signature of [TMHn]3- anions. |
| Na3WH9-IIā Lattice Parameters | a = 4.951, b = 10.488, c = 8.570 | Angstrom | Refined parameters at 23.5 GPa (P21/c). |
| Na3WH9 Predicted Metallicity | >180 | GPa | Pressure required for band gap closure (insulator below this P). |
| Na3ReH8 Predicted Metallicity | >350 | GPa | Pressure required for substantial band gap decrease. |
Key Methodologies
Section titled āKey MethodologiesāThe synthesis and characterization relied on a combination of high-pressure experimental techniques and advanced computational modeling.
- High-Pressure Synthesis (DAC): Samples (NaH, W/Re powder) were loaded into a Diamond Anvil Cell (DAC) under excess hydrogen (H2) gas at initial pressures around 0.2 GPa.
- Infrared Laser Heating: Reactants were heated to high temperatures (T ā 1400 K) while maintained at high pressure (P > 7.8 GPa) to drive the solid-state reaction.
- Synchrotron X-ray Diffraction (XRD): Used in situ during compression and decompression to determine crystal structures, lattice parameters, and identify phase transitions (Rietveld refinement used for structural fitting).
- Raman Spectroscopy: Used to characterize the vibrational modes of the complex anions, specifically the TM-H bending (1000-1300 cm-1) and stretching (1900-2500 cm-1) modes, confirming the presence of [TMHn]3- units.
- Density Functional Theory (DFT): Employed for calculating relative enthalpy curves (ĪH vs P) to predict thermodynamic stability and phase transition pressures, showing good agreement with experimental observations.
- Molecular Dynamics (MD) Simulations: Used to model the dynamic behavior of hydrogen atoms, confirming the pseudorotational character and rotational disorder of the [TMHn]3- anions in the fcc phases.
Commercial Applications
Section titled āCommercial ApplicationsāThe unique stability and high hydrogen content of these ternary hydrides suggest relevance in several advanced materials engineering fields, particularly those focused on energy storage and specialized solid-state components.
- Chemical and Physical Hydrogen Storage:
- The stability of Na3WH9 and Na3ReH8 down to ambient-like pressures (<1 GPa) makes them promising candidates for safer, high-density hydrogen storage materials, potentially overcoming the decomposition issues faced by binary hydrides.
- Solid-State Phase Change Materials (ss-PCMs):
- The reversible transitions between rotationally disordered (fcc) and ordered (distorted) phases, driven by temperature or pressure, are analogous to mechanisms utilized in ss-PCMs for thermal energy storage and management systems.
- Specialized Dielectrics/Insulators:
- As confirmed electrical insulators with wide bandgaps, these materials could be explored for use in high-pressure electronic components or specialized dielectric applications where chemical and structural stability under extreme conditions is required.
- Catalysis and Chemical Synthesis:
- The synthesis of novel, high-coordination transition metal hydride complexes ([WH9]3-, [ReH8]3-) provides new chemical targets and potential precursors for industrial hydrogenation/dehydrogenation catalysts.
View Original Abstract
The Na-W-H and Na-Re-H ternary systems were studied in a diamond anvil cell through X-ray diffraction and Raman spectroscopy, supported by density functional theory and molecular dynamics calculations. Na<sub>3</sub>WH<sub>9</sub> can be synthesized above 7.8 GPa and 1400 K, remaining stable between at least 0.1 and 42.1 GPa. The rhenium analogue Na<sub>3</sub>ReH<sub>8</sub> can form at 10.1 GPa upon laser heating, being stable between at least 0.3 and 32.5 GPa. Na<sub>3</sub>WH<sub>9</sub> and Na<sub>3</sub>ReH<sub>8</sub> host [WH<sub>9</sub>]<sup>3-</sup> and [ReH<sub>8</sub>]<sup>3-</sup> anions, respectively, forming homoleptic 18-electron complexes in both cases. Both ternary hydrides show similar structural types and pressure dependent phase transitions. At the highest pressures they adopt a distorted fcc Heusler structure (Na<sub>3</sub>WH<sub>9</sub>-IIā and Na<sub>3</sub>ReH<sub>8</sub>-IIā) while upon decompression the structure symmetrizes becoming fcc between ā¼6.4 and 10 GPa for Na<sub>3</sub>WH<sub>9</sub>-II and at 17 GPa for Na<sub>3</sub>ReH<sub>8</sub>-II. On further pressure release, the fcc phases transform into variants of a (quasi-) hexagonal structure at ā¼3 GPa, Na<sub>3</sub>WH<sub>9</sub>-I and Na<sub>3</sub>ReH<sub>8</sub>-I.