Unusually high values of the ultrasonic absorption observed at frequencies below 1 GHz in some ionic-molecular crystals the strontium, baryum and lead nitrates is a consequence of the existence of complex ions in the lattice. Acousto-optical interaction performed in the frequency range 0.1 GHz - 1 GHz and completed by data obtained at about 20 GHz by Brillouin scattering, allowed a very complete study from room temperature to 4 K of the dynamics of energy transfer between coupled internal and external vibrational modes in these compounds
