Strontium
titanate (STO), with a wide spectrum of emergent properties
such as ferroelectricity and superconductivity, has received significant
attention in the community of strongly correlated materials. In the
strain-free STO film grown on the SrRuO3 buffer layer,
the existing polar nanoregions can facilitate room-temperature ferroelectricity
when the STO film thickness approaches 10 nm. Here we show that around
this thickness scale, the freestanding STO films without the influence
of a substrate show the tetragonal structure at room temperature,
contrasting with the cubic structure seen in bulk form. The spectroscopic
measurements reveal the modified Ti–O orbital hybridization
that causes the Ti ion to deviate from its nominal 4+ valency (3d0 configuration) with excess delocalized 3d electrons. Additionally,
the Ti ion in TiO6 octahedron exhibits an off-center displacement.
The inherent symmetry lowering in ultrathin freestanding films offers
an alternative way to achieve tunable electronic structures that are
of paramount importance for future technological applications.