Models of orientational disorder in hybrid organic-inorganic piezoelectric materials

Hybrid organic-inorganic materials show potential as flexible and Pb-free piezoelectric materials, but ferroelectric-to-paraelectric phase transitions are responsible for a loss of properties at increased temperatures. Here, we develop a model Hamiltonian from first-principles calculations for the archetypical hybrid organic-inorganic piezoelectric TMCMCdCl3 (TMCM = trimethylchloromethyl ammonium, Me3NCH2Cl+) to model the order-disorder phase transition. We show that the configurational entropy from molecular orientational disorder is alone unable the quantitatively reproduce the phase transition temperature, but inclusion of vibrational entropy leads to good quantitative agreement with experiments. This highlights the importance of vibrational contributions for the phase stability of soft and flexible materials. We furthermore show that defective structures and intergrowths are readily formed at ambient temperature which is suggested to be responsible for the exceptional piezoelectric response of these materials, which cannot be explained from the conventional small-displacement limit.