  SrTiO3 is an incipient ferroelectric that is believed to exhibit a prototype
displacive, soft mode ferroelectric transition when subjected to mechanical
stress or alloying. We use high-angle annular dark-field imaging in scanning
transmission electron microscopy to reveal local polar regions in the
room-temperature, paraelectric phase of strained SrTiO3 films, which undergo a
ferroelectric transition at low temperatures. These films contain
nanometer-sized domains in which the Ti columns are displaced. In contrast,
these nanodomains are absent in unstrained films, which do not become
ferroelectric. The results show that the ferroelectric transition of strained
SrTiO3 is an order-disorder transition. We discuss the impact of the results on
the nature of the ferroelectric transition of SrTiO3.
