Observation of a guest-free Si46 clathrate-I framework from Ba8-xSi46 upon in situ vacuum heating
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-10-17 |
| Journal | Nature Communications |
| Authors | Yi Zhou, Qing Zhang, Ălvaro Mayoral, Peter R. Spackman, Takashi Matsumoto |
| Institutions | Seikagaku Corporation (Japan), Curtin University |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research details the first experimental observation and characterization of the guest-free clathrate-I Si46 framework, a highly sought-after silicon allotrope with promising electronic properties.
- Core Achievement: The stable, pure Si46 clathrate-I framework was successfully generated and observed experimentally in the thin regions (4-6 nm) of Ba8-xSi46 nanocrystals using in situ vacuum heating.
- Evacuation Mechanism: Barium (Ba) evacuation is a stepwise, thermodynamically driven process. Bal atoms (from the smaller Si20 cages) are removed preferentially at 500 °C, forming Ba6Si46, followed by the removal of Ba2 atoms (from the larger Si24 cages) to yield Si46.
- Electronic Properties: DFT calculations confirm that the pure Si46 framework is an insulator with a quasi-direct bandgap of 1.89 eV (HSE06), significantly higher than the 1.17 eV indirect bandgap of standard diamond-Si (d-Si).
- Structural Stability: The guest-free Si46 framework remains stable at room temperature and under low-vacuum conditions after heating.
- Defect Control: An interesting defect-healing phenomenon was observed at 400 °C, suggesting that annealing can be used as a method to reduce planar defects in these clathrate materials.
- Methodology: Advanced Cs-corrected STEM (ADF, ABF, qDPC) combined with in situ heating and EELS/EDS mapping was critical for resolving the atomic structure and dynamic evacuation process.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Starting Composition | Ba7.434Si46 | Formula | Refined structure from SCXRD |
| Initial Synthesis Conditions | 3 GPa, 800 °C | Pressure, Temperature | Preparation of bulk Ba8-xSi46 |
| Critical Evacuation Temperature | 500 | °C | Complete Bal removal under high vacuum |
| Guest-Free Framework Thickness | 4-6 | nm | Observed Si46 region at crystal edge |
| Si46 Bandgap (HSE06) | 1.89 | eV | Calculated quasi-direct bandgap |
| Si46 Bandgap (PBE) | 1.27 | eV | Calculated quasi-direct bandgap |
| Ba7.434Si46 Lattice Constant | 10.3223(14) | Angstrom | Cubic cell (Pm-3n) |
| Si46 Calculated Lattice Constant | 10.16 | Angstrom | DFT prediction for bulk Si46 |
| Superconducting Transition | 7.6 | K | Tc measured for Ba7.434Si46 |
| Bal Occupancy (2a site) | 0.765 | Ratio | In Si20 cages (pristine crystal) |
| Ba2 Occupancy (6d site) | 0.984 | Ratio | In Si24 cages (pristine crystal) |
| Defect Healing Temperature | 400 | °C | Observed restoration of original structure |
| STEM Resolution | 80-86 | pm | Achieved using Cs-corrected ADF/ABF/qDPC |
Key Methodologies
Section titled âKey MethodologiesâThe experimental observation relied on a multi-modal approach combining high-resolution imaging with in situ thermal control and theoretical modeling.
- Sample Preparation:
- Bulk Ba8-xSi46 (synthesized at 3 GPa, 800 °C) was crushed into fine powder.
- Nanocrystals were dispersed onto ultrathin carbon-coated copper grids for TEM analysis.
- In Situ Heating and Imaging:
- Experiments were conducted using a JEM-ARM300F microscope equipped with a cold FEG and double Cs correctors.
- In situ heating was performed using Fusion Thermal E-chips and the Fusion Select system under constant high vacuum.
- Heating rates were controlled: 20 °C s-1 up to 300 °C, then slowed to 0.2-0.5 °C s-1 at higher temperatures (400-500 °C) to minimize sample drift.
- Atomic Resolution Characterization:
- STEM Imaging: Annular Dark Field (ADF), Annular Bright Field (ABF), and quantitative Differential Phase Contrast (qDPC) were used to image Ba and Si atoms and analyze Ba deficiency (Z-contrast in ADF is proportional to Z(2x), x=0.3-0.7).
- Chemical Mapping: Electron Energy-Loss Spectroscopy (EELS) and Energy-Dispersive Spectroscopy (EDS) were used to confirm the Ba and Si distribution and valence states.
- Structural and Electronic Modeling:
- SCXRD: Single-crystal X-ray diffraction was used to solve the pristine Ba7.434Si46 structure and refine Ba occupancies.
- DFT Calculations: Density Functional Theory (DFT) using PBE and HSE06 functionals was employed to:
- Calculate the band structure and bandgap of Si46, Ba6Si46, and Ba8Si46.
- Determine the relative thermodynamic stability and kinetic barriers for Bal versus Ba2 evacuation, supporting the preferential loss of Bal.
Commercial Applications
Section titled âCommercial ApplicationsâThe successful synthesis and characterization of stable, wide-bandgap Si46 clathrate-I opens new avenues for silicon-based devices that traditionally rely on more expensive or less abundant materials.
- Optoelectronics and Photonics:
- Silicon-based Light-Emitting Diodes (LEDs) and lasers.
- Photosensitive devices and detectors, leveraging the quasi-direct bandgap for efficient light interaction.
- High-Performance Electronics:
- High-speed transistors and advanced semiconductor components, utilizing the insulating nature of the pure Si46 framework.
- Renewable Energy:
- Next-generation solar cell technologies, where the wide, quasi-direct bandgap (1.89 eV) offers potential efficiency improvements over standard crystalline silicon.
- Materials Science and Engineering:
- Guidance for the rational atomic material manufacturing of silicon allotropes, particularly concerning guest behavior and defect engineering in clathrate structures.
- Nanoscale Device Fabrication:
- The observation of Si46 stability in thin, epitaxial-like films (4-6 nm) suggests potential for integration into nanoscale device architectures.
View Original Abstract
Silicon and its composites are key materials owing to their extensive use in the semiconductor industry. While the diamond-structured form dominates, other allotropes with superior properties at ambient conditions remain of interest. Toward this, Si<sub>46</sub> clathrate type-I crystals containing alkali/alkaline-earth metals have been extensively studied, but the experimental observation of a guest-free Si<sub>46</sub> structure has been challenging. Using advanced electron microscopy, we show experimental evidence of guest-free clathrate-I Si<sub>46</sub> framework from Ba<sub>8-x</sub>Si<sub>46</sub> under in situ heating. We reveal the stepwise Ba evacuation process, starting with loss of Ba1 from the smaller cages to form Ba<sub>6</sub>Si<sub>46</sub>, followed by removal of Ba2 in larger cages to reach Si<sub>46</sub> that appears in the thin region of the nanocrystal with a thickness around 4-6 nm at 500 °C. Calculations give a quasi-direct bandgap of 1.89 eV and support the preferential evacuation of Ba1. The observation of this guest-free Si<sub>46</sub> framework opens up possibilities for applications in high-speed transistors, optoelectronic devices or solar cell technologies.