Tuning the orbital-lattice fluctuations in the mixed spin-dimer system Ba${}_{3-x}$Sr${}_x$Cr${}_2$O${}_8$

Abstract

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.

6 pages, 5 figures