Periodicity in the band gap variation of Ln2X3 (X = O, S, Se) in the lanthanide series

Abstract Band gap E g variation is considered across the lanthanides in rare earth oxides, sulphides and selenides of the type Ln 2 X 3 . A periodicity of the E g variation for the Ln 2 O 3 series is found. The minima are observed at the beginning (Ce. Tb) and at the end (Eu, Yb) of the two halfs of the lanthanide series. The greatest feature of the dependence of E g on atomic number Z of a lanthanide is a deep minimum for Ce 2 O 3 . In the sulphide and selenide series the peculiarities are smoother. The analysis of the dependences E g ( E ) leads to the conclusion that there are two quite different reasons for the decrease of E g . The position of the 4f-band between the valence and conduction bands determines the anomalies of E g at the beginning of the halfs of the lanthanide series. For Eu and Yb oxides the smaller values of E g are determined by the smaller gap between the valence and conduction bands.