Is Pbam-32 thermodynamically stable compared with diamond and graphite under variable pressure and temperature...

Area of Science:

• Materials Science
• Solid State Physics
• Computational Chemistry

Background:

• Pbam-32 is a newly discovered carbon allotrope with sp3 hybridization.
• It exhibits high Young's modulus and Vickers hardness, suggesting super-hard material applications.
• Thermodynamic properties, stability, and production pathways of Pbam-32 remain largely unexplored.

Purpose of the Study:

• To systematically investigate the structural, thermodynamic, and mechanical properties of Pbam-32.
• To compare Pbam-32 with diamond and graphite using ab initio simulations.
• To clarify the stability and potential applications of Pbam-32.

Main Methods:

• Ab initio simulations were employed to explore Pbam-32's structure and properties.
• Phonon dispersion, entropy, and enthalpy changes were calculated and compared to diamond and graphite.
• Mechanical properties like Young's modulus and Vickers hardness were assessed.

Main Results:

• Pbam-32 is a meta-stable allotrope with five- and seven-membered rings and no imaginary phonon frequencies.
• An acoustic band gap in the terahertz range suggests potential as a phononic crystal.
• Pbam-32 possesses entropy and enthalpy similar to diamond and exhibits high pressure resistance.
• However, its free energy is higher than diamond's across various conditions, placing it outside the equilibrium phase diagram.

Conclusions:

• Pbam-32 is a meta-stable carbon allotrope with promising mechanical properties but higher free energy than diamond.
• Its absence in the equilibrium phase diagram necessitates kinetically controlled production methods.
• Understanding Pbam-32's thermodynamic properties is crucial for its synthesis and potential applications.