Vibrational Frequencies and Force Constants for Tetrahedral MgX4−2 (X=Cl, Br, and I) in MgX2–KX Melts

Abstract

Raman spectra are reported for the species present in ${\mathrm{MgX}}_2–\mathrm{KX}$ melts (X=Cl, Br, and I) with X⁻¹/Mg⁺² mole ratios near 4.0. The observation of one polarized band and three depolarized bands for each halide melt is consistent with the existence of the tetrahedral complexes ${\mathrm{MgCl}}_4^{\text{−2}}$ , ${\mathrm{MgBr}}_4^{\text{−2}}$ , and ${\mathrm{MgI}}_4^{\text{−2}}$ . All the observed frequencies are calculated within 1 cm⁻¹ using a Urey—Bradley force field which consists of an $\mathrm{Mg}–\mathrm{X}$ stretching constant, an $\mathrm{X}–\mathrm{Mg}–\mathrm{X}$ bending constant, and an $\mathrm{X}\mathrm{···}\mathrm{X}$ repulsion constant.