Analysis of Melting Phenomena of the Alkali Halides: What Causes the Low Melting Points of the Lithium Halides?

The alkali halides, like other polar crystals, melt at high temperatures, but the melting points ( T m ) of the lithium halides are conspicuously lower (by about 300 ± 70 K) than expected from reasonable comparisons with the other alkali halides. For LiCl, LiBr, and LiI, the relatively low T m values have previously been explained by the facile conversion of the 6:6 coordinated crystal to 4:4 coordination in the liquid. Here we present a different perspective which also includes LiF. After collecting data for a number of bulk properties, it is found that two related properties in particular show a strong correlation with T m , namely, their static dielectric constants ( ɛ , as was proposed previously from diffusional force theory) and the transverse optical frequencies υ TO (or ω TO ) of the lattice. More precisely, a strong correlation is found between T m and μω 2 TO (previously referred to as the combined force constant, μ is reduced mass of an ion pair). This follows from the observation that υ TO for the lithium halides are significantly smaller than expected from comparative considerations (phonon softening). Related to these lower frequencies is the observation that the Lindemann parameter ρ (which is a measure of the vibrational amplitude) is very large (0.33) for the lithium halides, compared with the generally accepted value of 0.22 for a melt, which interestingly is close to the value (0.24 ± 0.02) obtained for the other halides. Thus, we argue that these large vibrational amplitudes cause the lithium halide molecules to “shake” themselves loose at lower‐than‐expected temperatures.