  Octahedral tilting is most common distortion process observed in
centrosymmetric perovskite compounds (ABO$_{3}$). Indeed, crucial physical
properties of this oxide stem from the tilts of BO$_{6}$ rigid octahedra. In
microwave ceramics with perovskite-type structure, there is a close relation
between the temperature coefficient of resonant frequency and tilt system of
the perovskite structure. However, in many cases, limited access facilities are
needed to assign correctly the space group, including neutron scattering and
transmission electron microscopy. Here, we combine the Raman scattering and
group-theory calculations to probe the structural distortion in the perovskite
(Ba$_{1-x}$Sr$_{x}$)$_{3}$CaNb$_{2}$O$_{9}$ solid solution, which exhibits a
structural phase transition at $x$ $\geq$ 0.7, from D$_{3d}^{3}$ trigonal to
C$_{2h}^{3}$ monoclinic cell. Both phases are related by an octahedral tilting
distortion ($a^{0}b^{-}b^{-}$ in Glazer notation). Low temperature Raman
spectra corroborate the group-theoretical predictions for
Sr$_{3}$CaNb$_{2}$O$_{9}$ compound, since 36 modes detected at 25 K agree well
with those 42 (25A$_{g}$ $\oplus$ 17B$_{g}$) predicted ones.
