Inelastic Neutron Scattering Study of the Structural Glass Transition in a K (Br,CN) Mixed Crystal

The coupled rotational-translational excitations in ${\mathrm{}\left(\mathrm{KBr}\right)\mathrm{}}_{0.992}$ ${\mathrm{}\left(\mathrm{KCN}\right)\mathrm{}}_{0.008}$ were studied by inelastic neutron scattering. For the first time the $A_{1g}-T_{2g}$ tunneling transition and the $A_{1g}-E_g$ librational excitation were followed at the transition from the paraelastic to the structural glass state. The drastic reduction of the $T_{2g}$ rotation-translation coupling suggests that the number of free-ion tunneling states is reduced in favor of cluster reorientation modes which are effectively decoupled from the lattice.