  The sequence of ground states for SrTiO3 film subjected to epitaxial strain
as well as to mechanical stress along the [001] and [110] axes is calculated
from first principles within the density functional theory. Under the
fixed-strain boundary conditions, an increase in the lattice parameter of a
substrate results in the $I4cm \to I4/mcm \to Ima2 \to Cm \to Fmm2 \to
Ima2$(II) sequence of ground states. Under the fixed-stress boundary
conditions, the phase sequence is different and depends on how the stress is
applied. It is revealed that the simultaneous presence of competing
ferroelectric and antiferrodistortive instabilities in SrTiO3 gives rise to the
appearance of metastable phases, whose number increases dramatically under the
fixed-stress conditions. In the metastable phases, the octahedral rotation
patterns are shown to differ substantially from those in the ground state. It
is suggested that in systems with competing instabilities, each polar phase has
its optimal octahedral rotation pattern which stabilizes this phase and creates
a potential barrier preventing this phase to be transformed into other
structures.
