ELECTRON WORK FUNCTION OF LANTHANIDE TRIIODIDES

Desorption enthalpies of LnI(4)(-) and Ln(2)I(7)(-) associative ions (Ln = La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, and Lu) and the enthalpy of sublimation of LnI(3) molecules were determined by Knudsen effusion mass spectrometric technique. These data were used to calculate the effective values of electron work function phi(e) of polycrystalline samples of lanthanide triiodides LnI(3) for the first time. The calculation methodology is based on the study of thermochemical cycles, which include atoms, molecules, ions, and electrons being in thermodynamic equilibrium with the LnI(3) crystal inside the effusion cell. The values obtained for different lanthanides turned out to be close. They lie in the range of about 2.4 - 4.4 eV with an average value in the series: phi(e) = 3.2 +/- 0.3 eV. The latter value is close to those for previously studied lanthanide tribromides. No secondary periodicity of phi(e) was found within the calculated errors along the lanthanide series. The results obtained are in quantitative agreement with the theoretical calculation of the values of the band gap of lanthanide triiodides. Comparison of phi(e) with other classes of lanthanide compounds such as oxides, hexaborides, and lanthanide metals shows relatively high electron emission ability yielding only to alkali and alkali-earth metals.