8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Fri 911:10-14:00P1.13Detection of collective optic excitations in molten NaIShinya Hosokawa, 1 Masanori Inui, 1 T. Bryk, 1 I. Mryglod, 2 Wolf-Christian Pilgrim, 3Y. Kajihara, 1 K. Matsuda, 4 Y. Ohmasa, 5 Satoshi Tsutsui, 6 and Alfred Q. R. Baron 71 Hiroshima Institute of Technology, 2-1-1 Miyake, Saeki-ku, 7315193, Hiroshima,Japan2 National Academy of Sciences of Ukraine, Lviv, Ukraine3 Philipps University of Marburg, Marburg, Germany4 Kyoto University, Kyoto, Japan5 Hiroshima Institute of Technology, Hiroshima, Japan6 JASRI/SPring-8, Hyogo, Japan7 RIKEN SPring-8 Center, Hyogo, JapanThe existence of optic excitation modes in molten salts was predicted by Hansen and McDonald[1] more than three decades ago by a pioneering molecular dynamics (MD) simulation, whichrevealed the existence of propagating short wavelength charge fluctuations as in the solid state.The computer studies motivated several groups to investigate ion dynamics in molten salts byinelastic neutron scattering. However, the experiments were subject to the kinematic constraints ofthe energy (¯hω)-momentum (Q) transfer relation of the neutrons. Inelastic x-ray scattering (IXS)has nearly no such kinematic constraints and tends to have reduced backgrounds. The existence ofhigh-ω excitation modes was reported from several inelastic neutron scattering investigations onliquid binary mixtures, such as liquid Li 4 Pb, Na x Sn1 − x, or DF. Due to the kinematic constraints,however, such high-ω modes were found only in the high Q range beyond ∼ 10 nm −1 . In thisQ region, the high- and low-ω branches of binary liquids are interpreted by reflecting mainlypartial dynamics of light and heavy elements, respectively. Thus observation of modes at low Qdemonstrates the presence of the true out-of-phase motion, which is characteristic of a longitudinaloptic (LO) mode like in a solid. The S(Q, ω) spectra of molten NaI were measured at 680 ◦ C atBL35XU/SPring-8. Small and damped indications of LO excitation modes were observed on thetails of the longitudinal acoustic modes at small Q ∼ 5 nm −1 . The measured spectra are in goodagreement, in both frequency and linewidth, with ab initio MD simulations. The observation ofthese modes at small Q and the good agreement with the simulation permits clear identification ofthese as collective optic modes with well defined phasing between different ionic motions.[1] J. -P. Hansen and I. R. McDonald, Phys. Rev. A 11, 2111 (1975).13