Density Functional Theory (DFT): A tool for rational de-sign of crystalline piezoelectrics
Geetu Kumari1, Sarah Guerin2
Geetu.Kumari@ul.ie ; Sarah.guerin@ul.ie
1Department of Physics, University of Limerick, Castletroy, Limerick, Republic of Ireland, V94T9PX.
Abstract: Crystallizing biomolecules creates a network of unit cell dipoles identical to the mechanisms of classical inorganic piezoelectrics, which allows for biological single crystals to easily fulfill the role of piezoceramics, e.g., in stack actuation. Biomolecular piezoelectric materials are considered a strong candidate for biomedical applications due to their robust piezoelectricity, biocompatibility, and low dielectric property. A combination of modeling and characterization can provide much-needed insight into how piezoelectric properties are modulated by unit cell properties, such as dipole moments, molecular packing, and composition.
Keywords: Piezoelectricity, DFT, Biomolecules, Pb-FREE, material design.

Introduction
By interconverting electrical and mechanical ener-gy they enable medical device, infrastructure, au-tomotive and aerospace industries, but with a huge environmental cost. The majority of piezoelectric sensors contain Lead Zirconium Titanate (PZT), the fabrication of which requires toxic lead oxide. Prominent lead-free alternatives are heavily pro-cessed, and rely on expensive, non-renewable ma-terials such as Niobium.
Biological materials such as amino acids and peptides have emerged as exciting new piezoe-lectrics. Biomolecular-crystal assemblies can be grown at room temperature with no by-products, and do not require an external electric field to in-duce piezoelectricity, unlike PZT and other pie-zoceramics.

Results and Discussion
Convergence in calculated energy, volume, and lattice parameter values with respect to the vdW cut-off radius used for dispersion corrected DFT calculations. The excellent quantitative correspondence of DFT predicted stiffness, permittivity and piezoelectricity tensors with those obtained from experiment conducted in the present study and in the literature highlights the potential that DFT calculations now demonstrate to identify materials with significant piezoelectric response, and to estimate the expected magnitudes of individual piezoelectric constants.
The DFT values are calculated at 0 K, so it should be noted that systems could exhibit a deviation between predicted DFT values and properties measured at room temperature .
Comparison of piezoelectric charge coefficients calculated with Finite Differences and Density Functional Perturbation Theory (DFPT) methods.

       (a)                            (b)

Figure 1: a. Unit cell of a. Anhydrous Asparagine, b. Arginine dihydrate.
Conclusions
High polarization, low elastic stiffness, and low dielectric permittivity are the three main perfor-mance indicators that can be used to break down high piezoelectric responsiveness. As a result, this approach will make it easier to create a convolu-tional neutral network that can analyze any crystal structure and forecast the likelihood of a strong piezoelectric response. TensorFlow will be the first software of choice for this work, but as the project moves forward, we'll try to use the most recent breakthroughs in the machine learning field.
References
[1] Sarah Guerin, Ning Liu, Tewfik Soulimane, Syed A. M. Tofail, Damien Thompson, “Control of piezoelectricity in amino acids by supramolecular packing”, Nature materials, 17, 180–186, 2018
[2] De Jong, Maarten, et al. "A database to enable discovery and design of piezoelectric materials." Scientific data 2.1 2015: 1-13.
[3] Kiely, Evan, et al. "Density functional theory predictions of the mechanical properties of crystalline materials." CrystEngComm” 23.34 2021: 5697-5710
Acknowledgements
S G and G.K. acknowledge ongoing support from the Irish Centre for High-End Computing (ICHEC), and are funded by the European Union.