  Very recently there was a report of the discovery using piezoelectric force
microscopy (PFM) of a switchable ferroelectric polarization in 1.6 % tensile
strained TiO2 thin film with anatase crystal structure (see N. Deepak et al.
Adv. Funct. Mater. 2014, 24, 2844). The polarization disappeared only above 450
K, which was assigned as a Curie temperature, Tc. Here we have performed X-ray
diffraction, second-harmonic generation (SHG) and infrared investigations of
the same films. Phonon frequencies exhibit less than a 10 % shift down with the
tensile strain and no anomaly near expected TC. SHG experiment did not reveal
any signal characteristic for inversion symmetry breaking, as expected in
ferroelectric phase, and the c-lattice parameter exhibits no anomaly on heating
near expected TC. Based on these results, we can summarize that we were not
able to confirm the previously discovered ferroelectricity in tensile-strained
anatase-TiO2 thin films.
