Raman scattering in melts

From the physico-chemical point of view, molten salts are a class of liquids having many microscopic and macroscopic properties similar to the corresponding properties of other (molecular, atomic) liquids. However, the experimental and theoretical evidence accumulated during the past 50 years shows that molten salts exhibit individual or group peculiarities that complicate the understanding of their morphology. 

The polarizability of ions forming the melt gives rise to light scattering either due to electrostatic field fluctuation around the ion and/or to polarizability changes due to bond formation within the melt (i.e. complex formation). 

An isolated n-atom molecule has 3n degrees of freedom and in—6 vibration degrees of freedom. The collective motions of atoms, moving with the same frequency and which in phase with all other atoms, give rise to normal modes of vibration. In principle, the determination of the form of normal modes for any molecule requires the solution of equation of motion appropriate to the n-symmetry. Methods of group theory are important in deriving the symmetry properties of the normal modes. With the aid of the character tables for point groups and the symmetry properties of the normal modes, the selection rules for Raman and IR activity can be derived. For a molecule with a center of symmetry, e.g. AXe, octahedral molecule, a non-Raman active mode is also IR active, whereas for the BX4 tetrahedral molecule, some modes are simultaneously IR and Raman active. 

The vibration properties of a group of isolated molecules change drastically when these molecules are condensed to form a crystalline solid. The influence of the neighboring molecules and the surrounding crystalline lattice will alter the vibration modes of the molecules. The long-range order correlates the atoms in the crystal, and the vibrations are described in terms of lattice waves rather than by free molecular modes. 

Upon melting, the long-range order and space symmetry of the solids are destroyed. In principle, the vibration modes of the liquids can be considered as a long-wavelength limit of the solid vibrations and thus certain internal and/or external modes may be present in the vibration spectrum of the melt. The internal modes in melts have been investigated mainly by Raman spectroscopy in a variety of melt mixtures. The objective of these studies is the determination and characterization of possible discrete species (i.e. complexes ) in the melt. 

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