Vapor pressures, mass spectra, and thermodynamic properties of LaI3

Vaporization Studies of Lanthanum Trichloride, Tribromide, and Triiodide

“…The phase diagram of the system LaI 3 -La, studied by differential thermal analysis (Corbett et al, 1961), showed the existence of stable LaI 2 (black, melting congruently at 1103 K) and a stoichiometric intermediate compound LaI 2.4 (with an incongruent melting point at 1023 K). On this basis, in addition to the principal sublimation reaction as mass-spectrometrically observed by Hirayama et al (1976) (the dimer form La 2 I 6 (g) is negligible), we believe that at temperatures below 1023 K a small amount of lanthanum triiodide partially decomposes to give LaI 2.4 . According to the phase diagram, at about 1023 K this compound melts (this explains the amorphous phase) and decomposes to I 2 (g) and LaI 2 , which probably is the final residue observed at the end of each experiment.…”

“…With regard to LaI 3 , the only vapor pressure data are those measured by Shimazaki and Niwa (1962) using the Knudsen method and those mass-spectrometrically evalu-ated by Hirayama et al (1976), but the reported data are in disagreement.…”

Vaporization thermodynamics of lanthanide trihalides

“…There have been six published studies of triiodide vaporizations from two laboratories (16)(17)(18)(19)(20)34). The third-law values are consistently lower than the second-law results, the differences being as great as 10 kcal/mol in some cases.…”

“…The data of Shimazaki and Niwa (34) are suspect because of possible reaction of the triiodide vapors with the fused silica effusion cell (16). It is not certain whether the second law-third law differences in the results of Hirayama and co-workers (16,17,20) are due to experimental errors, to errors in free-energy functions for the gaseous and condensed compounds, or both. Possible sources of error in the free-energy functions which would lead to low third-law results include too great an estimated contribution of iodide to the entropy of the condensed state and/or too high estimates for the vibrational frequencies, particularly for the bending modes, in the gaseous molecules.…”

“…On the other hand, the second-law results could be too high as a result of temperature-dependent errors. Hirayama and co-workers (17) have observed the La2l3+ in the mass spectrum of the vapor above Lal3 near 1000 K and calculate an enthalpy of dimerization, using the second law, of -46 kcal/mol. If the entropy of dimerization at 1000 K is assumed to be the same as for All3, -29 cal/(mol K), the enthalpy of dimerization is calculated to be -39 ±10 kcal/mol, assuming (15) P(dlmer) = 0.7[/(La2l5+)/ /(Lal"+)]P(monomer)…”