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 | Hiroshima University, Seikagaku Corporation (Japan) |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Guest-Free $\text{Si}_{46}$ Clathrate Framework
Section titled âTechnical Documentation & Analysis: Guest-Free $\text{Si}_{46}$ Clathrate FrameworkâExecutive Summary
Section titled âExecutive SummaryâThis research successfully demonstrates the experimental observation and stabilization of the guest-free clathrate-I $\text{Si}_{46}$ framework, a novel silicon allotrope with significant potential for advanced semiconductor applications. 6CCVD leverages its expertise in high-ppurity MPCVD materials to support and extend research in wide-bandgap semiconductors and defect engineering.
- Core Achievement: Experimental confirmation of the stable, guest-free clathrate-I $\text{Si}{46}$ framework via in situ vacuum heating of $\text{Ba}{8-x}\text{Si}_{46}$ nanocrystals up to 500 °C.
- Electronic Properties: The resulting $\text{Si}_{46}$ allotrope is calculated to possess a quasi-direct bandgap of 1.89 eV (HSE06), positioning it as a promising material for silicon-based optoelectronics, high-speed transistors, and solar cell technologies.
- Mechanism Revealed: The Ba evacuation process is stepwise and thermodynamically driven, starting with the preferential loss of $\text{Ba}{1}$ from the smaller $\text{Si}{20}$ cages, followed by $\text{Ba}{2}$ loss from the larger $\text{Si}{24}$ cages, achieving pure $\text{Si}_{46}$ in thin regions (4-6 nm).
- Structural Control: Advanced Cs-corrected STEM (ADF, ABF, qDPC) was critical for resolving atomic structure, confirming Ba deficiency (0.765 occupancy for $\text{Ba}_{1}$), and observing structural defect healing at 400 °C.
- 6CCVD Relevance: This work underscores the critical need for ultra-pure, structurally controlled wide-bandgap materials, a domain where 6CCVDâs Single Crystal Diamond (SCD) and Boron-Doped Diamond (BDD) offer superior, scalable performance solutions.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Starting Material Synthesis Pressure | 3 | GPa | High-pressure synthesis of $\text{Ba}{8-x}\text{Si}{46}$ |
| Starting Material Synthesis Temperature | 800 | °C | High-temperature synthesis of $\text{Ba}{8-x}\text{Si}{46}$ |
| As-Synthesized Formula | $\text{Ba}{7.434}\text{Si}{46}$ | N/A | Refined structure from SCXRD |
| $\text{Ba}{1}$ Occupancy (2a site, $\text{Si}{20}$ cage) | 0.765 | N/A | Indicates high initial vacancy rate |
| $\text{Ba}{2}$ Occupancy (6d site, $\text{Si}{24}$ cage) | 0.984 | N/A | Near full occupancy |
| Superconducting Transition ($\text{T}_{c}$) | 7.6 | K | Measured for $\text{Ba}{7.434}\text{Si}{46}$ sample |
| $\text{Si}_{46}$ Bandgap (HSE06 Calculation) | 1.89 | eV | Quasi-direct bandgap, insulating phase |
| $\text{Si}_{46}$ Framework Thickness Observed | 4-6 | nm | Thickness of guest-free region at 500 °C |
| Defect Healing Temperature | 400 | °C | Observed temperature for planar defect restoration |
| SCD Unit Cell Parameter (Calculated $\text{Si}_{46}$) | 10.16 | Ă | DFT calculation (PBE) |
| SCD Unit Cell Parameter (Calculated $\text{Ba}{8}\text{Si}{46}$) | 10.27 | Ă | DFT calculation (PBE) |
| STEM Resolution (qDPC) | 80 | pm | Achieved atomic resolution |
| In Situ Heating Vacuum Environment | Ultra-High | Vacuum | Required for complete Ba evacuation |
Key Methodologies
Section titled âKey MethodologiesâThe successful observation of the guest-free $\text{Si}_{46}$ framework relied on a combination of high-precision synthesis, advanced in situ characterization, and computational modeling:
- High-Pressure Synthesis: The starting material, $\text{Ba}{8-x}\text{Si}{46}$ clathrate-I, was prepared under extreme conditions (3 GPa, 800 °C) to achieve the desired crystal structure.
- Structural Refinement: Single-crystal X-ray diffraction (SCXRD) was used on micro-sized crystals (2.5 x 2.0 x 1.1 ”m³) to determine the initial structure and Ba occupancy ($\text{Ba}{7.434}\text{Si}{46}$).
- In Situ STEM Imaging: Cs-corrected Scanning Transmission Electron Microscopy (STEM) was performed using an in situ heating holder (Fusion Select system) under high vacuum. This allowed dynamic observation of the Ba evacuation process along key crystallographic directions ([001] and [110]).
- Multimodal Atomic Resolution: High-resolution imaging utilized Annular Dark Field (ADF), Annular Bright Field (ABF), and quantitative Differential Phase Contrast (qDPC) to distinguish Ba and Si atoms and analyze local deficiencies.
- Chemical Mapping: Electron Energy-Loss Spectroscopy (EELS) and Energy-Dispersive Spectroscopy (EDS) confirmed the atomic resolution chemical composition and Ba deficiency variations across the crystal.
- Electronic Structure Calculation: Density Functional Theory (DFT) using PBE and HSE06 functionals was employed to predict the quasi-direct bandgap (1.89 eV) of the pure $\text{Si}{46}$ phase and model the thermodynamic stability of Ba sites, supporting the observed preferential $\text{Ba}{1}$ evacuation.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThis research highlights the engineering challenges inherent in creating and stabilizing novel wide-bandgap semiconductors with controlled defects. 6CCVDâs MPCVD diamond materials offer robust, scalable alternatives and superior substrates for similar advanced materials research.
| Research Requirement / Challenge | 6CCVD Solution & Capability | Value Proposition for Researchers |
|---|---|---|
| Need for Wide Bandgap Semiconductors (Targeting 1.89 eV quasi-direct bandgap) | Optical Grade Single Crystal Diamond (SCD): Intrinsic SCD provides the ultimate wide bandgap (5.5 eV) and superior thermal conductivity (22 W/cm·K), far exceeding Si allotropes for high-power and UV optoelectronics. | Immediate access to materials that surpass the electronic and thermal performance requirements for high-speed transistors and UV/deep-UV devices. |
| Controlled Doping and Conductivity (Metallic $\text{Ba}{8}\text{Si}{46}$ vs. Insulating $\text{Si}_{46}$) | Boron-Doped Diamond (BDD): 6CCVD offers highly conductive, p-type BDD (SCD or PCD) with precisely controlled boron concentration, enabling predictable electrical properties for electrode and sensor applications. | Provides a stable, chemically inert framework for electrochemical and high-conductivity applications, eliminating the instability issues associated with guest-atom clathrates. |
| Ultra-High Purity and Defect Management (Observing defect healing at 400 °C) | High-Purity MPCVD Growth: Our proprietary growth recipes minimize impurities and defects. We offer SCD with extremely low nitrogen content, critical for quantum and optical applications. | Ensures that material properties are intrinsic to the structure, not masked by impurities, facilitating accurate defect engineering and annealing studies. |
| Custom Dimensions and Thin Films (Need for 4-6 nm thin regions, large-scale integration) | Custom Dimensions & Thickness Control: We supply SCD and PCD plates up to 125 mm in diameter, with thickness control from 0.1 ”m to 500 ”m, supporting both thin-film and bulk device architectures. | Enables seamless transition from laboratory-scale nanocrystal studies to scalable, inch-size device fabrication and integration. |
| Device Integration and Metalization (Required for transistors and solar cells) | In-House Metalization Services: 6CCVD provides custom deposition of standard contacts (Au, Pt, Pd) and refractory metals (Ti, W, Cu) directly onto diamond substrates. | Accelerates the device prototyping cycle by providing ready-to-use, metalized substrates optimized for high-temperature or high-power operation. |
Engineering Support
Section titled âEngineering Supportâ6CCVDâs in-house team of PhD material scientists specializes in optimizing MPCVD growth parameters to meet stringent specifications for electronic, optical, and quantum applications. We offer consultation services to assist researchers in selecting the optimal diamond material (SCD, PCD, or BDD) and surface preparation (Ra < 1 nm polishing) required for advanced projects involving optics, high-speed transistors, and defect engineering.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
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.