PERIODICITY IN THE BAND-GAP VARIATION OF LN(2)X(3) (X=O, S, SE) IN THE LANTHANIDE SERIES

Band gap E(g) variation is considered across the lanthanides in rare e arth oxides, sulphides and selenides of the type Ln(2)X(3). A periodic ity of the E(g) variation for the Ln(2)O(3) series is found. The minim a are observed at the beginning (Ce, Tb) and at the end (Eu, Yb) of th e two halfs of the lanthanide series. The greatest feature of the depe ndence of E(g) on atomic number Z of a lanthanide is a deep minimum fo r Ce2O3. In the sulphide and selenide series the peculiarities are smo other. The analysis of the dependences E(g)(Z) 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 d etermines the anomalies of E(g) at the beginning of the halfs of the l anthanide series. For Eu and Yb oxides the smaller values of E(g) are determined by the smaller gap between the valence and conduction bands .