Thermodynamic ground state of MgB6 predicted from first principles structure search methods

Area of Science:

• Materials Science
• Solid-State Physics
• Computational Chemistry

Background:

• Previous theoretical models for magnesium hexaboride (MgB6) proposed a primitive cubic structure.
• These models suggested MgB6 might possess semiconducting properties and permanent dipole moments.

Purpose of the Study:

• To determine the true thermodynamic ground state of magnesium hexaboride (MgB6).
• To investigate the electronic and structural properties of MgB6.
• To clarify the conductive nature and stability of MgB6.

Main Methods:

• Utilized unbiased structure searching algorithms.
• Employed first-principles density functional calculations.
• Assessed structural stability under varying pressures.

Main Results:

• Identified a stable orthorhombic Cmcm structure as the ground state for MgB6.
• The Cmcm structure features both localized B6 octahedra and extended B∞ boron ribbons.
• Calculations indicate a metallic ground state due to delocalized electrons in boron ribbons, contradicting previous semiconducting predictions.
• The predicted structure is stable up to 18.3 GPa.
• MgB6 exhibits weaker electron-phonon coupling than MgB2, suggesting no high-temperature superconductivity.

Conclusions:

• The ground state of MgB6 is an orthorhombic Cmcm structure, not previously considered.
• MgB6 is a metal with unique boron clustering, challenging prior assumptions of its semiconducting nature.
• The material's properties suggest limited potential for high-temperature superconductivity.