The Densities and the Surface Tensions of Fluoride Melts

Abstract

The densities and the surface tensions of molten pure fluorides were measured by the Archimedean method and the maximum bubble pressure method, respectively. It was shown that the physico-chemical properties of pure alkaline metal and alkali-earth metal fluorides mainly depend on the Coulomb forces experienced by foreign ions, I (=Z_aZ_c/(r_a+r_c)²). However, magnesium fluoride shows slightly different behavior from other fluorides, that may result from the reduction of the cation-anion attractive force, I by shielding effect of larger fluorine ions to smaller magnesium cation. Surface tensions of pure fluoride melts were changed as a function of KZ_aKZ_c/(V_M)^2/3, where V_M and K are the molar volume of a melt and a packing parameter of anions on the cleavage plane (for NaCl structure, K=1, for Rutile structure K=√(1+√2)/2 and for CaF₂ structure K=4/√3). It suggests that the surface structure of a melts refers to that of the corresonding solid cleavage plane.