THE INTERPRETATION OF VIBRATIONAL-SPECTRA OF IONIC MELTS

Computer simulations of the short-time, vibrational dynamics of the ne twork-forming ionic melts, LaCl3 and ZnCl2, and of their mixtures with network-breaking alkali halides are described. In the mixtures, high frequency peaks in the vibrational density of states are shown to be d escribable in terms of the normal coordinates of vibration of transien t molecular ion species, like LaCl63- and ZnCl42-. Novel simulation me thods are presented which allow this association to be established. In the pure melts, the vibrational motions retain a strong aspect of thi s local polyhedral unit vibrational character, but the effects of netw ork-induced coupling between the vibrations of different units become pronounced, particularly in ZnCl2. The calculated vibrational spectra are compared with extensive Raman data on these systems, and with infr ared and neutron spectra in pure ZnCl2. For the mixtures, remarkably g ood agreement with experiment is found, confirming the high quality of the representation of the interionic interactions obtained with the p olarizable ion model potentials used. For the melts, there are discrep ancies between the peak frequencies observed in the vibrational DOS an d the Raman spectra. These discrepancies are likely to be due to the n etwork-induced couplings, whose effect on the Raman (and infrared) spe ctra is not fully included in the calculated DOS. (C) 1997 American In stitute of Physics.