Tuning the orbital-lattice fluctuations in the mixed spin-dimer system Ba3−xSrxCr2O8

In A3Cr2O8, where A = Sr or Ba, the Cr5+ 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 Ba3−xSrxCr2O8 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 behavior exhibited by unit cell volumes, atomic positions, and intradimer spin-spin exchange interactions.