HP-MoO₂: A High-Pressure Polymorph of Molybdenum Dioxide

Area of Science:

• Materials Science
• Solid-State Chemistry
• High-Pressure Physics

Background:

• Molybdenum dioxide (MoO2) exists in various polymorphs, with the monoclinic α-MoO2 being the thermodynamically stable phase at ambient conditions.
• Previous theoretical studies suggested the possibility of new high-pressure polymorphs of MoO2.

Purpose of the Study:

• To synthesize and characterize a high-pressure polymorph of molybdenum dioxide (HP-MoO2).
• To determine the crystal structure and physical properties of the synthesized HP-MoO2.
• To investigate the stability and phase transitions of HP-MoO2 using theoretical methods.

Main Methods:

• High-pressure synthesis using a multianvil press at 18 GPa and 1073 K.
• Single-crystal X-ray diffraction for crystal structure determination.
• Density-functional theory (DFT) calculations for structure optimization and stability analysis.

Main Results:

• Successful synthesis of a new high-pressure molybdenum dioxide (HP-MoO2) polymorph.
• The new polymorph crystallizes isotypically with HP-WO2 in the orthorhombic system (space group Pnma).
• HP-MoO2 was found to be diamagnetic and slightly metastable at ambient pressure (ΔEm = 2.2 kJ·mol⁻¹).

Conclusions:

• A novel orthorhombic polymorph of molybdenum dioxide has been synthesized and structurally characterized.
• Theoretical calculations confirm the existence of HP-MoO2 and its metastability relative to the stable α-MoO2 phase.
• This discovery expands the understanding of molybdenum dioxide phase diagram under high pressure.