Experimental study of the 4fn → 4fn and 4fn → 4fn-15d1 transitions of the lanthanide diiodides LnI2 (Ln=Nd, Sm, Eu, Dy, Tm, Yb).

The diffuse reflectance and photoluminescence (PL) spectra of NdI2, SmI2, EuI2, DyI2, TmI2 and YbI2 were measured between 225 and 12 500 nm in order to determine their 4f(n) -> 4f(n-1)5d(1) optical bandgaps. The results were compared with those obtained using an empirical model of the electronic structure of LnI(2). The results can be used to explain the lanthanide valency and crystalline structure changes of other lanthanide diiodides such as PrI2.