A direct observation of tunnelling states in molecular solids with random internal strains

Inelastic-neutron-scattering experiments on ternary salts (NH4)(0.02)RbxK0.98-xI for x = 0.0, 0.05, 0.1 and 0.5 have been carried out at 4 K. The gradual buildup of intensity in the range 0 to 0.6 meV with increasing values of x suggest that, beyond a certain Value of x, these systems might show a low-temperature two-level system behaviour and that the tunnelling defects responsible for this might well be the NH4+ ions present in them. This has been confirmed by specific-heat measurements down to 0.7 It on the maximally strained (x approximate to 0.5) system.