Collective modes in molten alkaline-earth chlorides. II. Inelastic neutron scattering from molten SrCl2

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Abstract
For pt.I see ibid., vol.17, p.4705 (1984). The authors have measured the inelastic neutron scattering spectra of three samples of molten SrCl2 with different isotopic chlorine compositions using the IN6 time-of-flight spectrometer at the Institut-Langevin in Grenoble. de Gennes narrowing of the quasielastic peak is observed at the position of the first peak in S++(Q). The power spectra of the total longitudinal current correlation functions are more complex than those for the molten alkali halides. Using second-order differences in scattering between the three samples (the first use of second-order isotopic substitution in this type of inelastic scattering experiment) they have identified broad acoustic (mass-current) and optic (charge-current) modes similar to those found in molten RbCl and CsCl. The energies of these modes are comparable with those in the crystal. In addition there are two well defined modes (A1, A2) that are neither acoustic nor optic in nature but seem to be related to the structural disorder of anions. A quasidispersion curve derived from the peak positions shows that for h(cross) omega >10 meV these two branches are approximately parallel with a slope equal to the sound velocity. At lower energies one of the modes (A1) lies outside the (Q, omega ) range of the spectrometer while the other mode (A2) has a vertical dispersion at Q approximately=2.1 AA-1 and is identified with diffusive broadening of the first peak in S--(Q). The A modes are found to occur upon disordering of the anion sublattice in the superionic phase of the solid state and so may be modes of the vacancy/interstitial clusters that have been proposed to account for the superionic behaviour. This would imply that such clustering effects also occur in molten SrCl2.

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