Models of orientational disorder and phase transitions in hybrid piezoelectric materials Kasper Tolborg, Aron Walsh Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ UK Piezoelectric materials interconverting electrical and mechanical energy find applications in diverse areas as sensors, actuators, and high precision motors. Current state-of-the-art piezoelectric materials are made from lead- based ceramics, which pose significant environmental issues and precludes design of biocompatible devices. Recent progress in molecular and hybrid organic-inorganic piezoelectric materials have led to discovery of new materials with properties rivalling the lead-based ceramics, and with the additional prospects of being solution processable, flexible, and potentially biocompatible [1,2]. However, conventional computational methods fail to predict their large piezoelectric response [3] , and phase transitions to disordered, centrosymmetric phases appear near or above room temperature, which may hamper their practical applications. Thus, proper computational modelling of this materials class must include nanoscale effects for response properties, and entropic effects to phase stability [4] . Here, we present the development of ab initio models for the ordering of dipolar molecules in the high- performance hybrid piezoelectric material TMCMCdCl 3 (TMCM = trimethyl chloromethyl ammonia) [1] . Based on a model Hamiltonian fitted to density functional theory (DFT) data and Monte Carlo simulations, we predict and rationalise its order-disorder ferroelectric-to-paraelectric phase transition. We show that a model Hamiltonian based on internal energies calculated from DFT fails to quantitatively reproduce phase transition temperatures, whereas inclusion of vibrational free energy improves the agreement with experiment significantly. This highlights the importance of vibrational entropy in describing phase stability for a materials class seemingly driven by configurational entropy. Besides prediction of phase stability, our model Hamiltonian and defect calculations provide insights into the exceptional piezoelectric response, which has been reported to be influenced by metastable phases [3] . References
1. Y.-M. You, W.-Q. Liao, D. Zhao et al., Science , 2017, 357 , 306-309 2. W.-Q. Liao, D. Zhao, Y.-Y. Tang et al., Science , 2019, 363 , 1206–1210. 3. P. S. Ghosh, S. Lisenkov, I. Ponomareva, Phys. Rev. Lett. , 2020, 125 , 207601 4. K. Tolborg, J. Klarbring, A. Ganose, A. Walsh, Digital Discovery , 2022, 1 , 586-595
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