Can a tensile tensile strained TiO₂ thin film be ferroelectric?

A series of papers about discovery of new ferroelectrics using PFM microscopy has recently been published. On the example of strained TiO₂ thin films, which exhibit ferroelectric-like hysteresis loops, we have shown that PFM response can come from electrochemical effects like migration of defects.

Additional experiments like temperature dependent second harmonic generation, IR spectroscopy of phonons and X-ray diffraction are always necessary for unambiguous confirmation of ferroelectric phase transitions [ S. Skiadopoulou et al, Adv. Funct. Matt. 26, 642 (2016) ].

Figure: (a) Schema of anatase crystal structure of TiO₂ under tensile strain. (b) Dielectric loss spectra calculated from temperature-dependent reflectance spectra of 20 nm thin film of TiO₂ reveal no temperature shifts of phonon frequencies, because crystal structure is stable. PFM studies detected loss of “ferroelectric” polarization at 460 K.