Entropy-Driven Polar Ordering in Single-Phase Ferroelectrics

Landau theory of phase transitions as one of the most influential phenomenological theory predicts a guaranteed reduction of order parameter with increasing temperatures, corroborating the intuition that a system becomes less ordered driven by entropy and numerous experimental evidence. Herein, combining the Landau theory with quasi-harmonic phonon theory, we demonstrate that the vibrational entropy can drive a more polar ordered state even in a single-phase material. Supported by first-principles calculations, our theory reveals the intrinsic origin of the thermally enhanced spontaneous electric polarization in ferroelectric hydrogen-bonded molecular crystal croconic acid: the seemingly counterintuitive entropy loss from polar ordering is overcompensated by the entropy gain from molecular rotational disordering, highlighting that the Landau order parameter is not necessarily a viable measure of Boltzmann’s entropy in ferroelectrics.