Synthesis and characterization of metastable crystalline st12 germanium
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-08-02 |
| Journal | Acta Crystallographica Section A Foundations and Advances |
| Authors | Bianca Haberl, Mary-Ellen Donnelly, Yan Wu, Emily Kroll, Matthias Frontzek |
| Institutions | Oak Ridge National Laboratory |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research characterizes the metastable, crystalline st12 phase of germanium (Ge) recovered after high-pressure synthesis, focusing on its structural and vibrational properties using neutron scattering techniques.
- Core Achievement: Successful synthesis and comprehensive characterization of the simple tetragonal st12 structure of Ge (P43212), a phase typically difficult to recover in bulk quantities.
- Synthesis Parameters: The material was pressurized to above 15 GPa using a Paris-Edinburgh press and double-toroidal diamond anvils to ensure full conversion from the diamond-cubic phase to the metallic beta-Sn phase (I41/amd).
- Characterization: A combination of in situ neutron diffraction (WAND2) confirmed the phase transition, while inelastic neutron scattering (ARCS) on recovered samples provided a fine resolution phonon density of states (DOS).
- Key Finding: The measured phonon DOS closely matches Density Functional Theory (DFT) predictions, validating the structural model and opening avenues for detecting subtle differences in the materialâs behavior.
- Value Proposition: Metastable Si and Ge phases are promising for next-generation technologies, potentially offering ideal band gap characteristics for solar power conversion, improved thin-film properties, or even high-temperature superconductivity when formed as a hydride.
- Methodological Advance: This work represents the first use of double-toroidal anvils on the WAND2 beamline and the first time pressures above 10 GPa were achieved at the HFIR facility for such experiments.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Target Phase Structure | st12 (P43212) | N/A | Simple tetragonal metastable germanium |
| Starting Phase Structure | Fd-3m | N/A | Standard diamond cubic germanium |
| Intermediate Phase Structure | beta-Sn (I41/amd) | N/A | Metallic high-pressure polymorph |
| Minimum Conversion Pressure | ~11 | GPa | Required for Fd-3m to beta-Sn transition at room temperature |
| Synthesis Pressure (Experiment) | > 15 | GPa | Maximum pressure applied to ensure full conversion |
| Synthesis Duration | Several | Hours | Time held at maximum pressure |
| Inelastic Scattering Energy 1 | 30 | meV | Incident energy used on ARCS beamline |
| Inelastic Scattering Energy 2 | 50 | meV | Incident energy used on ARCS beamline |
| Inelastic Scattering Energy 3 | 70 | meV | Incident energy used on ARCS beamline |
Key Methodologies
Section titled âKey MethodologiesâThe st12 germanium phase was synthesized and characterized using a multi-step process combining high-pressure synthesis with advanced neutron scattering techniques at Oak Ridge National Laboratory (ORNL).
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Sample Preparation and Pressurization:
- Small pieces of a standard Ge wafer (diamond-cubic structure) were used as the starting material.
- Synthesis was performed using a Paris-Edinburgh press equipped with double-toroidal diamond anvils.
- The sample was pressurized to a maximum pressure of above 15 GPa.
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Phase Conversion:
- The sample was held at the maximum pressure ( > 15 GPa) for several hours to ensure complete conversion of the diamond-cubic Ge to the metallic high-pressure beta-Sn phase.
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In Situ Structural Confirmation (WAND2):
- In situ high-pressure neutron diffraction was conducted on the WAND2 beamline (High Flux Isotope Reactor, HFIR).
- Rietveld refinement confirmed that all diamond-cubic material had successfully converted to the metallic beta-Sn phase.
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Recovery and Ex Situ Characterization:
- The pressure was slowly released (decompression) to recover the metastable st12 phase.
- Multiple recovered pellets were prepared for subsequent measurements.
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Vibrational Characterization (ARCS):
- Inelastic neutron scattering was performed on the recovered st12 pellets using the ARCS beamline (Spallation Neutron Source, SNS).
- Measurements were taken using three distinct incident energies (30, 50, and 70 meV).
- The data were combined to generate a fine resolution phonon density of states (DOS), which was compared against DFT predictions.
Commercial Applications
Section titled âCommercial ApplicationsâThe characterization of metastable st12 germanium is relevant to several high-value engineering and materials science sectors, particularly those focused on advanced electronic and energy materials.
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Advanced Semiconductor Manufacturing:
- The st12 phase offers potential for novel semiconductor materials with unique electronic properties distinct from standard diamond-cubic Ge.
- Relevant for high-performance transistors and integrated circuits where new material structures can enhance speed or reduce power consumption.
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Solar Power Conversion:
- Metastable Si and Ge phases are theorized to possess ideal band gap characteristics.
- This research supports the development of highly efficient photovoltaic devices by utilizing these recovered high-pressure phases.
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Thin-Film Technology:
- The metastable phases could lead to improved thin-film characteristics, crucial for flexible electronics, display technology, and advanced coatings.
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High-Temperature Superconductivity:
- When formed as a hydride, these metastable structures (Si and Ge) are predicted to become useful materials for very high-temperature superconductivity.
- Relevant for energy transmission, magnetic resonance imaging (MRI), and high-field magnet applications.
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Materials Modeling and Validation:
- The experimental phonon DOS data provides critical validation for Density Functional Theory (DFT) models of high-pressure and metastable phases.
- This improves the predictive capability of computational materials science for designing new functional materials.
View Original Abstract
This work uses neutron scattering to characterize a metastable crystalline phase of germanium recovered from high pressure.Such metastable phases of silicon and germanium exhibit interesting functionality.They could potentially yield a Si or Ge structure with ideal band gap characteristics for solar power conversion, improved thin-film characteristics or -in the form of a hydride -even become a useful material for very high temperature superconductivity.Several of such metastable, crystalline phases can be recovered from the metallic high-pressure polymorph of Si and Ge, the so-called β-Sn phase (I4[sub]1[/sub]/amd).This metallic polymorph forms upon room temperature compression to ~11 GPa from the standard diamond cubic Si or Ge (Fd-3m).Upon decompression, this transition is not reversible and instead these metastable phase form.The exact crystal structure that is nucleated is thereby dependent on the exact decompression parameters such as temperature, rate or hydrostaticity.However, the need for synthesis pressures above ~10 GPa has typically limited the recoverable sample volumes.Hence, the majority of studies have been conducted computationally and fewer experimental characterizations have been performed.Thus, there are many open questions on the behavior and characteristics of these metastable phases.Here, we focus on the simple tetragonal st12 structure of Ge (P4[sub]3[/sub]2[sub]1[/sub]2) by combining synthesis capabilities of the SNAP diffractometer of the Spallation Neutron Source with in situ high pressure diffraction on the WAND[sup]2[/sup] beamline of the High Flux Isotope Reactor and inelastic neutron scattering on recovered samples on the ARCS beamline of the Spallation Neutron Source.The st12 structure is synthesized using double-toroidal diamond anvils in a Paris-Edinburgh press from small pieces of a Ge wafer.The sample is pressurized to above ~15 GPa and kept at maximum pressure for several hours to ensure full conversion to the β-Sn phase.The transition pathways is confirmed by in situ diffraction on WAND[sup]2[/sup].Rietveld refinement of the Ge material under pressure confirms that all diamond-cubic material was indeed converted to the metallic β-Sn phase.It is noteworthy that this experiment represents the first use of the double-toroidal anvils on the WAND2 beamline and that pressures above 10 GPa were achieved for the first time at the HFIR facility.Several such pellets were then measured on ARCS using incident energies of 30, 50, and 70 meV, which were combined to provide a fine resolution phonon density of states.The result closely matches the DFT predictions, although subtle differences may be detected.Thus, in summary, these findings yield new insights into the potential use of the st12 phase as future semiconductor material and also open avenues for further characterization of such metastable phases of Si and Ge.