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 Å3 for BF4− was calculated from these measurements. The polarization of BF4− appears to decrease with increasing alkali cation size.

Anomalously Large Formula Unit Volume and Its Effect on the Thermal Behavior of LiBF4

“…It should be noted that either LiBF4 or LiPF6 does not exhibit any phase transition up to the decomposition temperatures. 14,15,35 The Tdec_calc of LiBF4 in Table 4 is certainly higher than that of LiPF6, which agrees with the experimental data. [18][19][20][21][22]35 Since the difference between ΔdecH°298 values of the two compounds is not large compared with the difference between ΔdecS°298 values, the contribution of the latter is expected to be dominant.…”

“…8 Lithium salts of bulky anions such as CF3SO3and N(SO2CF3)2are also structurally characterized from their powder diffraction data. 9,10 Although LiBF4 is known as one of the common electrolytes for lithium batteries and the large number of reports are available on the structures of other MBF4-type (M = univalent cation) salts, [11][12][13][14][15] all the attempts to determine the crystal structure of LiBF4 have been unsuccessful. [15][16][17] In this study, we report the first successful results of the crystal structure determination of LiBF4.…”

Enthalpy of alkali metal fluoroborates from 298-1000.deg.K. Enthalpies and entropies of fusion and transition

“…We have measured the high temperature enthalpies of NaBF4, KBF4, RbBF4, and CsBF4 to obtain information for reactor technology as well as to explore the interesting transition behavior of the solid fluoroborates. The melting points and solid transition temperatures, densities and molar volumes, and predicted volume changes on transition of the alkali fluoroborates have been reported (3). We now propose to examine the effect of the differences in structure and cation size on the enthalpy and entropy of fusion and transition in the alkali fluoroborates.…”

The cohesive energy of molten salts and its density

“…c From references [16,20]. d From reference [25]. e From reference [26] for Li acetate and reference [27] for Na and K acetates.…”