Formation of Distinctive Nanostructured Metastable Polymorphs Mediated by Kinetic Transition Pathways in Germanium

Nanoscience Metallic Materials Chemistry Materials Science Materials Chemistry Condensed Matter Physics Physics
nanostructures metastable germanium high-pressure science kinetic transition

Academic Background

Germanium (Ge), as one of the Group IV elements, holds significant importance in both fundamental science and technological applications. Its metastable polymorphs have garnered considerable attention due to their unique nanostructures and excellent electronic and optical properties. However, the phase transition mechanisms of germanium under high pressure and the formation processes of its metastable polymorphs remain unclear, especially the synthesis methods for controlling its nanostructures through kinetic pathways. This study aims to reveal the mechanisms behind the formation of different nanostructured metastable polymorphs of high-pressure β-Sn germanium during decompression and explore its phase transition kinetic pathways.

Source of the Paper

This paper was co-authored by Mei Li, Xuqiang Liu, Sheng Jiang, and others, with contributors from the Center for High Pressure Science and Technology Advanced Research (China), the Shanghai Advanced Research Institute of the Chinese Academy of Sciences, and Argonne National Laboratory (USA), among other institutions. The paper was published on April 17, 2025, in the journal Matter and Radiation at Extremes, titled “Formation of Distinctive Nanostructured Metastable Polymorphs Mediated by Kinetic Transition Pathways in Germanium.”

Research Process and Results

1. Experimental Design and Methods

The study explored the phase transition pathways of β-Sn germanium under different decompression rates through high-pressure experiments and rapid decompression techniques. The specific experimental procedures are as follows:
- Preparation of High-Pressure β-Sn Germanium: Diamond cubic (DC) germanium was compressed above 14 GPa to obtain high-pressure β-Sn germanium.
- Rapid Decompression Experiments: Rapid decompression of β-Sn germanium was performed at different rates (from 0.001 GPa/s to 4 TPa/s) to observe its phase transition processes.
- High-Resolution Transmission Electron Microscopy (HRTEM) Analysis: The decompressed samples were characterized using HRTEM to analyze their nanostructures.
- In Situ X-Ray Diffraction (XRD) and X-Ray Absorption Fine Structure (XAFS) Measurements: Synchrotron radiation was used to monitor real-time changes in crystal and electronic structures during decompression.

2. Main Results

The study found that β-Sn germanium forms three distinct metastable polymorphs under different decompression rates:
- St12 Germanium: At extremely low decompression rates (<0.001 GPa/s), β-Sn germanium transforms into long-range ordered St12 germanium, with a complete crystal structure and larger grain sizes (30-90 nm). - **BC8/R8 Germanium**: At relatively high decompression rates (~40 GPa/s), β-Sn germanium transforms into BC8/R8 germanium, with smaller grain sizes (3-17 nm) and amorphous grain boundaries. - **Amorphous Germanium (a-Ge)**: At very high decompression rates (>4 TPa/s), β-Sn germanium directly transforms into amorphous germanium, with nanocluster sizes of 0.8-2.5 nm.

Through XAFS analysis, the study revealed that the formation of St12 germanium is closely related to changes in electronic structure. During decompression, the involvement of d orbitals leads to a rapid decrease in electron density along the c-axis, triggering the formation of the St12 phase. In contrast, the formation of BC8/R8 germanium is characterized by simultaneous changes in electronic and crystal structures.

3. Phase Transition Kinetic Mechanisms

Based on classical nucleation theory, the study proposed three nucleation mechanisms:
- Heterogeneous Nucleation: At low decompression rates, the nucleation rate is low, leading to the formation of large-grained St12 germanium.
- Homogeneous Nucleation: At moderate decompression rates, the nucleation rate is high, resulting in the formation of small-grained BC8/R8 germanium.
- Nucleation Catastrophe: At critical decompression rates, nucleation events occur suddenly and uniformly, leading to the formation of amorphous germanium.

Research Conclusions and Significance

This study reveals the kinetic pathways for the formation of different nanostructured metastable polymorphs of high-pressure β-Sn germanium during rapid decompression, elucidating the combined effects of decompression rate, temperature, and stress on the phase transition process. The research not only deepens the understanding of germanium’s phase transition mechanisms but also provides new insights into the synthesis of nanostructured materials through kinetic pathway control. Furthermore, the findings can be extended to other material systems, offering a theoretical framework for designing and developing metastable materials with tailored functional properties.

Research Highlights

  • First realization of the transformation of β-Sn germanium into amorphous germanium at room temperature through rapid decompression experiments.
  • Revealed the correlation between the formation of St12 germanium and changes in electronic structure, providing a new perspective for understanding its phase transition mechanisms.
  • Proposed a phase transition model based on nucleation kinetics, offering a theoretical basis for controlling the synthesis of nanostructured materials.

Other Valuable Information

The study also found that stress plays a significant role in the phase transition process. Under non-hydrostatic conditions, β-Sn germanium is more likely to transform into St12 germanium, indicating that stress can reduce the kinetic barrier for phase transitions. This finding provides an important reference for future research on material phase transitions under high pressure.

Through this study, scientists can not only better understand the phase transition mechanisms of germanium but also develop new strategies for designing and synthesizing nanostructured materials with specific functional properties. This achievement holds significant theoretical and applied value in the fields of materials science and high-pressure physics.