Neutron scattering studies of(CN)−defects in KBr

We have studied both the librational and the tunneling excitations of ${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}^{-}$ ions in a KBr matrix through the interaction of these modes with the acoustic phonons of the KBr host. From our measurements on a sample containing 0.00034 mole fraction of KCN in KBr, we find the $A_{1g}-T_{2g}$ tunnel splitting to be 0.28 ± 0.05 meV (2.3 ± 0.4 ${\mathrm{cm}}^{-1\mathrm{}}$). From measurements on a more concentrated system, 0.0045 mole fraction of KCN in KBr, the $A_{1g}-E_g$ librational excitation energy is found to be 1.6 ± 0.1 meV (13.0 ± 1.0 ${\mathrm{cm}}^{-1\mathrm{}}$). These values are consistent with the results of optical measurements.