A small angle neutron scattering investigation of the concentration fluctuations in the solutions of potassium in molten potassium bromide

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Abstract
In order to investigate the possible interaction between the concentration fluctuations related to the liquid-liquid immiscibility and the non-metal to metal transition observed for the metals in molten salts systems, we have undertaken a neutron small angle scattering study of the solutions of potassium in potassium bromide over a rather large temperature and concentration range around the liquid-liquid critical point. The experimental conditions for the critical regime being satisfied, we have determined with accuracy the γ and ν critical indices. All our results are consistent with a tridimensional Ising behaviour for this system, the volumic fraction being the correct order parameter. This is different from the mean field behaviour obtained over the same reduced temperature range for the analogous solutions of metals in liquid ammonia. We interpret this discrepancy with the help of the Ginzburg criterion and the large difference in the value of the characteristic length for the fluctuations, ξ 0. ξ0 is probably dependent on the nature of the metal or the salt chosen. In any case, for K in KBr the interaction between electronic localization processes and concentration fluctuation is likely to be very weak or non-existent