Interplay between local structure, vibrational and electronic properties on CuO under pressure.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Chemistry

Background:

• Copper oxide (CuO) possesses a crystal structure with CuO4 square planar units and elongated apical Cu-O bonds.
• Understanding CuO's response to external pressure is crucial for its potential applications in electronics and materials science.

Purpose of the Study:

• To investigate the electronic and local structural properties of CuO under high pressure (up to 17 GPa).
• To elucidate the pressure-induced phase transitions and their impact on CuO's physical properties.

Main Methods:

• Utilized X-ray absorption spectroscopy (XAS) at the Cu K edge to probe local atomic and electronic structure.
• Performed ab initio calculations to complement experimental findings and model material behavior under pressure.
• Analyzed EXAFS Debye-Waller factors (σ²) to assess local dynamic disorder.

Main Results:

• CuO4 square planar units remain stable, while apical Cu-O distances continuously decrease with pressure.
• Anomalous increase and subsequent drastic reduction in the mean square relative displacement (σ²) of apical Cu-O bonds indicate dynamic disorder.
• A transition from 4-fold square planar to 4+2 Jahn-Teller distorted octahedral Cu2+ local structure occurs above 13 GPa.
• Predicted anomalous rise in permittivity and modest piezoelectricity in the 5-13 GPa range.
• XAS near-edge features show a discontinuity at 5 GPa, linked to electronic effects, charge migration, and band gap changes.

Conclusions:

• Pressure induces significant changes in CuO's local structure and electronic properties, including dynamic disorder and a phase transition.
• The observed phenomena are attributed to the Cu2+ ion's tendency to form favorable interactions and softening of specific vibrational modes.
• CuO exhibits pressure-dependent dielectric and piezoelectric properties, opening avenues for tunable electronic applications.