Piezoelectric Transition in a Nonpyroelectric Gyroidal Metal-Organic Framework

Area of Science:

• Materials Science
• Crystallography
• Solid-State Physics

Background:

• The 432 crystallographic point group lacks inversion symmetry but is typically nonpiezoelectric.
• Gyroidal structures, belonging to the 432 point group, possess unique isotropic network properties.
• Metal-organic frameworks (MOFs) offer tunable structures and properties for advanced applications.

Purpose of the Study:

• To investigate the structural and physical properties of a gyroidal cobalt oxalate MOF.
• To explore the phase transition and emergent piezoelectricity in this material.
• To understand the role of sulfate tetrahedra ordering in inducing electric dipole moments.

Main Methods:

• Synchrotron X-ray diffraction on a single crystal to identify structural phase transitions.
• Pyroelectric current measurements on polycrystalline samples to detect electric polarization.
• Analysis of crystallographic point group changes and molecular ordering.

Main Results:

• A cubic-to-cubic structural phase transition was observed at 120 K.
• The point group changes from nonpiezoelectric 432 to piezoelectric 23 below 120 K.
• Ordering of distorted SO4 tetrahedra leads to a 3D helical arrangement of electric dipole moments, inducing piezoelectricity dependent on pressure.

Conclusions:

• The gyroidal cobalt oxalate MOF exhibits a pressure-dependent piezoelectric response below 120 K due to symmetry breaking and helical dipole ordering.
• This material provides a platform for studying unique dielectric properties arising from molecular ordering and structural flexibility.
• The study highlights the potential of MOFs in developing novel piezoelectric materials.