Soft mode behavior in SrTiO3/DyScO3 thin films: Evidence of ferroelectric and antiferrodistortive phase transitions

Infrared reflectance, terahertz transmittance, and microwave resonance measurements show that SrTiO3 films, strained by ∼1% in biaxial tension by growing them on (110) DyScO3 substrates, undergo a pronounced phonon softening near 270 K. This in-plane soft-mode drives the ferroelectric transition. The appearance of two new low-frequency modes and splitting of the high-frequency TO4 mode provide evidence of an antiferrodistortive phase below ∼180 K.