Inelastic neutron scattering study of the rotational excitations in(KBr)1−x(KCN)xin the paraelastic and structural glass state

The coupled rotational-translational excitations in ${\left(\mathrm{KBr}\right)}_{1-x}{\left(\mathrm{KCN}\right)}_x$ were studied by inelastic neutron scattering for concentrations $0.008\le x\le 0.20$. We followed the $A_{1g}-T_{2g}$ tunneling transition and the $A_{1g}-E_g$ librational excitation through the transition from the paraelastic to the structural glass state. We found that these two excitations and their coupling to the lattice strains exhibit a very different temperature dependence in the glass state. While the tunneling transition, which triggers reorientations of the ${\mathrm{CN}}^{-}$ ions, shows a drastic reduction of the $T_{2g}$ rotation-translation coupling, the librational excitations conserving the CN alignment are less influenced. These results suggest that for $T\le T_F$ the CN system is gradually blocked in clusters, thereby reducing the density of free-ion tunneling states in favor of cluster-reorientation modes effectively decoupled from the lattice. This leads to a metastable low-temperature state with extremely long relaxation times.