revealed by hybrid functional study*

Stoichiometric SrTiO₃ in the absence and presence of antiferrodistortive (AFD) distortion were calculated with multiple exchange correlation functionals, including hybrid functional. The reliability of the calculations was reinforced by the calculations of strain and AFD dependence.
In contrast with previous works, we show that the ferroelectric phase is more stable in the absence of quantum and thermal fluctuations than the paraelectric phase, even for the stress-free case, and clarify the properties of these FE phases.
The energy gain of stress-free FE, in comparison with the thermal and quantum fluctuation energy, indicates that a paraelectric phase emerges at room temperature by thermal fluctuations. But the gain near 0 K is marginally close to the FE phase, which aligns with the experimental incipient FE. This implies that the paraelectricity of stress-free SrTiO₃ in experiments contains incoherent atomic-scale FE regions. These results are consistent with the FE microregions (FMR), signatures of polar disorders, and the emergence of FE in SrTiO₃ due to impurities and defects. The spontaneous polarization could reach 10 μC/cm² in the absence of fluctuations, even for the stress-free case.
Y. Watanabe, Phys. Rev. B 99, 064107(2019)