Raman spectroscopic studies of structural properties of solid and molten states of the magnesium chloride – alkali metal chloride system

Abstract

Raman spectra have been recorded for solids and melts of composition MgCl2 + nACl (n = 0–4 and A = Cs, Rb, K, Na, Li). Characteristic spectra have been observed for each of the solids predicted from phase diagram studies and an additional two compounds formulated as Rb3MgCl5 and K3MgCl5 were detected. The compounds Cs2MgCl4, Cs3MgCl5, and Rb3MgCl5 contain the discrete MgCl42− tetrahedral ion which has been characterized by its Raman spectrum. These compounds melt with retention of the MgCl42− ion. Most other solids appear to contain distorted network octahedra with face-, edge-, or corner-shared chlorides. These solids melt to give the more stable tetrahedral MgCl42− ion. In melts of high MgCl2 concentration a new peak was detected which appears to be characteristic of the dimeric Mg2Cl73− ion which exists in equilibrium with the MgCl42− ion. The results are compared to similar studies on the better characterized CsCl–AlCl3 system.