Modelling Orientational Disorder and Phase Transitions in Hybrid Piezoelectric Materials from First Principles

Piezoelectric materials interconvert electrical and mechanical energy and find applications in diverse areas as sensors, actuators, and high precision motors. However, current state-of-the-art Pb-based piezoelectric ceramics pose significant environmental issues and preclude design of biocompatible devices. In recent years, solution processable, flexible and potential biocompatible hybrid organic-inorganic piezoelectric materials rivalling the performance of lead-based ceramics have been discovered [1,2].<br/><br/>The most promising candidates are based on low-dimensional inorganic frameworks with perovskite inspired structures, and they feature order-disorder phase transitions close to room temperature. Above these phase transitions, disordered, centrosymmetric phases appear, destroying their piezoelectric performance. Furthermore, conventional computational methods fail to predict the large piezoelectric response of this materials category [3]. Thus, modelling of this materials class must include entropic effects to phase stability [4], and nanoscale effects for response properties.<br/><br/>Here, we present the development of coarse-grained model Hamiltonians for the correlated disorder of dipolar organic cations in the archetypical 1D hybrid hexagonal perovskite, TMCMCdCl₃ (TMCM=trimethylchloromethyl ammonia) from first principles [5]. Coupled with Monte Carlo simulations, we predict and rationalise its order-disorder phase transition, showing that vibrational entropic contributions are key for quantitative agreement with the experimental phase transition temperature. This highlights the importance of vibrational entropy in describing phase stability for a materials class seemingly driven by configurational entropy.<br/><br/>Based on an observed easy stabilisation of anisotropic, two-dimensional disorder in the system, we calculate defect formation energies of isolated and extended orientational defects, which are shown to be easily formed at ambient conditions. Switching of these defects is suggested to be the origin the strong, unconventional piezoelectric response [5].<br/><br/>[1] Y.-M. You, W.-Q. Liao, D. Zhao et al., <i>Science</i>, 2017, <b>357</b>, 306-309<br/>[2] W.-Q. Liao, D. Zhao, Y.-Y. Tang et al., <i>Science</i>, 2019, <b>363</b>, 1206–1210.<br/>[3] P. S. Ghosh, S. Lisenkov, I. Ponomareva, <i>Phys. Rev. Lett.</i>, 2020, <b>125</b>, 207601<br/>[4] K. Tolborg, J. Klarbring, A. Ganose, A. Walsh, <i>Digital Discovery</i>, 2022, <b>1</b>, 586-595<br/>[5] K. Tolborg, A. Walsh, <i>J. Mater. Chem. C</i>, 2023, accepted, DOI: 10.1039/D3TC01835K