Novel metastable metallic and semiconducting germaniums

Area of Science:

• Condensed Matter Physics
• Materials Science
• Computational Materials Science

Background:

• Group-IVa elements like silicon and germanium are vital semiconductors.
• Germanium's high-pressure phases include metallic and superconducting states (e.g., β-tin, tI4).
• Recent experiments challenge the established germanium phase diagram under ambient conditions.

Purpose of the Study:

• To explore the lower-pressure phase diagram of germanium.
• To identify novel germanium structures potentially stable near room conditions.
• To reconcile theoretical predictions with recent experimental observations of germanium's metallicity.

Main Methods:

• Utilized ab initio metadynamics simulations for structure prediction.
• Investigated the pressure-dependent phase transitions of germanium.
• Analyzed the electronic properties (semiconducting/metallic) of predicted phases.

Main Results:

• Discovered a new monoclinic germanium phase (mC16) with four-membered rings, less dense than diamond.
• Identified a tetragonal bct-5 phase existing between the diamond and β-tin (tI4) structures.
• Found mC16 to be a narrow-gap semiconductor and bct-5 to be metallic and potentially superconducting at low pressures.

Conclusions:

• The newly predicted mC16 and bct-5 phases offer plausible explanations for recent experimental findings.
• These phases necessitate a revision of the germanium phase diagram at lower pressures.
• The metallic bct-5 phase could exhibit superconductivity, warranting further investigation.