Order-disorder phase transition and orientational defect formation in a hybrid organic-inorganic piezoelectric material

Abstract

Hybrid organic-inorganic materials offer an alternative to Pb-based ceramics for ferroelectric and piezoelectric applications. However, these materials suffer from ferroelectric-to-paraelectric phase transitions above which the piezoelectric response disappears. For most hybrid materials, this occurs below room temperature. Here, we investigate a notable exception, TMCMCdCl 3 (TMCM = trimethylchloromethyl ammonium), which features an order-disorder phase transition at 400 K. By training a machine learning force field based on the scalable Allegro architecture and performing large scale molecular dynamics simulations, we show that the phase transition temperature can be predicted in close agreement with experiments, provided that dispersion interactions are included, and that the high temperature phase is dominated by dynamical disorder of the molecular cations. We uncover an unexpected molecular orientation in the high temperature phase, not following the expectations from the symmetry relations between ferroelectric and paraelectric phases, which contributes to entropic stabilisation of the high temperature phase. This demonstrates the shortcomings of previous model Hamiltonian based approaches, which did not include the unexpected molecular orientations. Finally, we show that orientational defects of the molecular cations are present in significant concentrations in the low temperature phase, suggesting that these can act as nucleation sites for polarisation switching. This paves the way for accurate modelling of ferroelectric-to-paraelectic phase transitions and design of hybrid organic-inorganic materials with exceptional room temperature piezoelectric response.