Ab Initio Thermodynamics of Ferroelectrics: The case of PbTiO3*

We report a molecular dynamics study of the ferroelectric phase transition in PbTiO₃ using deep neural networks, trained on first-principles SCAN-DFT data, to represent potential and polarization surfaces [1,2]. Our approach includes anharmonic effects beyond the limitations of reduced models and of the linear approximation for the polarization. An external isotropic pressure of ~28 kbar was applied to correct for the overestimate of the tetragonality by the SCAN functional. The calculated enthalpy, spontaneous polarization, specific heat and dielectric susceptibility agree well with experiments on single domain crystals. In addition, we studied how the free energy depends on the polarization with enhanced sampling methods, further supporting the first-order character of the transition.

[1]L. Zhang, J. Han, H. Wang, R. Car, and W. E, Phys.Rev. Lett., 120, 143001 (2018)

[2]L. Zhang, M. Chen, X. Wu, H. Wang, W. E, and R. Car, Phys. Rev. B, 102, 041121 (2020)