  Electron paramagnetic resonance studies of the impurity Fe$^{3+}$ and
Mn$^{4+}$ centers indicate that the symmetry of the crystal structure of
millimeter-scale strontium titanate plates above 105 K lowers in the bulk from
cubic to tetragonal, different from that of the conventional low-temperature
antiferrodistortive phase. It is shown that the effect does not originate from
the residual stress; its magnitude (observed tetragonal distortion) and
particular manifestations depend on plate orientation, surface quality and the
geometry of a sample. Peculiarities of the observed phenomenon are presented
and possible scenario of its realization is discussed.
