Volumetric Properties of Molten and Crystalline Alkali Fluoroborates

Abstract

The densities of the molten alkali fluoroborates were each measured over a temperature interval of approximately 150°C using a dilatometric method. The molar volumes and expansivities derived from these measurements exhibit the order predicted from the periodic table. At equal corresponding temperatures, molar volumes of molten alkali fluoroborates were found to be slightly larger than the molar volumes of molten alkali iodides. For LiBF₄, RbBF₄, and CsBF₄, melting points and solid‐state transition temperatures were determined. At the solid‐state transition temperature, volume changes are predicted on the basis of analogous behavior of MBF₄ and MClO₄. The indices of refraction of the crystalline compounds were also measured; an electronic polarization averaging 2.77 ų for BF₄⁻ was calculated from these measurements. The polarization of BF₄⁻ appears to decrease with increasing alkali cation size.