Piezoelectric Response of Plastic Ionic Molecular Crystals: Role of Molecular Rotation

Abstract

Plastic ionic molecular crystals are a novel class of materials that have attracted recent interest due to the discovery of ferroelectric and piezoelectric properties together with an orientationally disordered mesophase with high plasticity. Despite the growing interest, little is known about the mechanisms that underpin their piezoelectric properties. To address this knowledge gap, we use density functional theory calculations with van der Waals density functionals to study the dielectric, piezoelectric, and elastic properties of 11 plastic ionic molecular crystals. The piezoelectric coefficients were found to reach values comparable to inorganic piezoelectrics. Further, some plastic crystals have strikingly large piezoelectric anisotropies. For HQReO4 (quinuclidinium perrhenate) an anisotropy of |d 16/d 33| = 118 was found, 11 times that of LiNbO3, a phase pure inorganic noted for its anisotropy. Our study links the anisotropy to the rotational motion of the constituent molecules in response to shear stress. The large shear piezoelectric coefficients, yet modest dielectric permittivity, results in coupling coefficientsa measure of its suitability for energy harvestingwith values up to 0.79. Our study points to the engineering of the rotational response of plastic ionic crystals as key to realizing the outstanding functional properties of these compounds.

Ferroelectric Crystals of Globular Molecules: Cambridge Structural Database Mining and Computational Assessment

“…Molecules in plastic crystals often pack in complex arrangements, and the direction of the individual molecular dipoles does not necessarily align with the direction of the spontaneous polarization . In Tables and , the “dipolar” direction relative to the polarization direction is listed.…”

“…For both relaxation and polarization calculations, the vdW-DF-cx functional was employed, 60 based on the accurate lattice constants determined in our earlier benchmarking study of exchange− correlation functionals for ferroelectric plastic crystals. 16…”

“…Molecules in plastic crystals often pack in complex arrangements, and the direction of the individual molecular dipoles does not necessarily align with the direction of the spontaneous polarization. 16 In Tables 1 and 2, the "dipolar" direction relative to the polarization direction is listed. Often, the unit cell holds several equivalent molecules that align at the same angle with the polarization axis, as given by space group symmetry.…”

“…In some materials, rotational switching can also occur in combination with displacive mechanisms . These materials can show rapid switching with frequencies of up to 263 kHz. – Moreover, molecular rotations in response to an applied electric field or mechanical force can result in large shear piezoelectric responses . These types of molecular crystals can also crystallize in pseudo high-symmetry structures, thus allowing multiaxial polarization, with as many as 24 equivalent axes. – Multiaxial polarization allows the spontaneous polarization to be aligned in a desired direction in polycrystalline materials and could allow for multibit storage in single crystals …”

Perspectives on recent advancements in energy harvesting, sensing and bio-medical applications of piezoelectric gels

“…However, excluding a few cases, most examples displayed rigid and complex device architectures that limit their use in flexible and wearable self-powered electronics applications. [365][366][367][368][369][370][371] PiezoGels based on amino acids or short peptides could be able to solve these challenges due to their excellent merits of transparency, softness, tissue adherence, excellent mechanical properties and the added benefit of working at low mechanical pressure, such as human-body movements. One of the most studied short aromatic dipeptides is Fmoc-FF due to its ability to form various self-assembled ordered nanostructures (i.e., nanotubes, nanorods, nanospheres, nanocylinders, nanofibers, nanofibrils, etc.).…”