Supporting evidence for complex ion formation in MgCl 2 alkali chloride melts was sought from a single crystal X-ray analysis of the title compounds. Crystals of K 2 MgCl 4 are tetragonal, a = 4.94, c = 15.58 Å, Z = 2, space group I 4/mmm. The compound is isostructural with K 2 NiF 4 , and was determined with MoK α diffractometer data and refined by full-matrix least-squares to R = 0.046 for 180 reflections.Crystals of Cs 2 MgCl 4 are orthorhombic, a = 9.777, b = 7.514, c = 13.234 Å,Z = 4, space group Pnma. The compound is isostructural with β-K 2 SO 4 , and was determined with MoKα diffractometer data and refined by full-matrix least-squares to R = 0.074 for 1084 reflections. The K 2 MgCl 4 structure contains an infinite two-dimensional array of MgCl 6 octahedra sharing edges. The Cs 2 MgCl 4 structure contains discrete MgCl 4 2− anions which likely continue to exist in the melt close to the melting point.
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Gibbons et al. (1975) studied this question.