Raman spectroscopic studies of manganese chloride-alkali metal chloride systems

Shabana, Ahmed Amin Aly Ahmed (1985) Raman spectroscopic studies of manganese chloride-alkali metal chloride systems. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Raman and visible spectra of MnCl₂ - ACl systems (A= Li, Na, K, Rb and Cs) were investigated in the solid and the molten phases to identify possible species formation. The many solid compounds precipitated from the melts were identified by their characteristic spectra. The agreement with reported phase diagrams was good. The results indicated that the MnCl₄ ²⁻ ions were present in solid Cs₂MnCl₄ and Cs₃MnCl₅ but in the other compounds the manganese was octahedrally coordinated with bridged chlorides. -- The Raman spectra of the solids were discussed in terms of factor group analysis. The data for the melts suggested that a discrete MnCl₄ ²⁻ species was the predominant species in the molten phase of composition MnCl₂.nACl (n ≥ 2). For melts of composition MnCl₂.nACl (n < 2) a tetrahedral species with corner sharing was proposed but no new peaks due to new discrete species could be identified. Melts of high MnCl₂ content appeared to have a very rapid equilibrium of ions with most Mn retaining tetrahedral coordination but some octahedral coordination was also suggested from assessment of visible spectra. A rapidly fluctuating local structure around the Mn²⁺ with lifetime of the aggregate on the time scale of 10⁻¹² seconds would be consistent with the composition and temperature studies of the bandshape of the Mn-Cl stretching region. -- The Raman spectra of MgCl₂ - CsCl and MgCl₂ - KCl systems were investigated in the molten phase for comparison. The results confirm the presence of discrete MgCl₄ ²⁻ ions as the predominant species in melts even of low MgCl₂ concentration, but a distinct polynuclear species Mg₂Cl₇ ³⁻ was also formed when the MgCl₂ concentration was increased. In contrast to the manganese system the equilibrium species present in the magnesium system have lifetimes greater than 10⁻¹² seconds. The results also indicated that by increasing the temperature the polynuclear species was broken down and more MgCl₄ ²⁻ ions were formed.

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