  In $A_{3}$Cr$_{2}$O$_{8}$, where $A$ = Sr or Ba, the Cr$^{5+}$ ions
surrounded by oxygen ions in a tetrahedral coordination are Jahn-Teller active.
The Jahn-Teller distortion leads to a structural transition and a related
emergence of three twinned monoclinic domains below the structural phase
transition. This transition is highly dynamic over an extended temperature
range for $A$ = Sr. We have investigated mixed compounds
Ba$_{3-x}$Sr$_{x}$Cr$_{2}$O$_{8}$ with $x=2.9$ and $x=2.8$ by means of X-ray
and neutron diffraction, Raman scattering and calorimetry. Based on the
obtained evolution of the phonon frequencies, we find a distinct suppression of
the orbital-lattice fluctuation regime with increasing Ba content. This stands
in contrast to the linear behaviour exhibited by unit cell volumes, atomic
positions and intradimer spin-spin exchange interactions.
