Structural features of new rare earth-based mixed anions (O,S,F) compounds: relationships between optical absorption and rare earth environment

摘要

In the structure of alpha-LnSF and Ln_2AF_4S_2(A=Ca, Sr) compounds which can be considered as formed of layer stacking, the rare earth and fluorine atoms form [Ln_2F_2]~(4+) or [Ln_2AF_4]~(4+) fluorite-type blocks which alternate with double [S_2]~(4-) layers along the c-axis. Then, it appears possible to modify the size of the fluorite-type block without any modification of its charge. In the case of rare earth oxyfluorosulfides, two new classes of compounds Ln_3OF_3S_2 (Ln=La, Ce) and La_2O_(1.5)FS were identified. The structures of these compounds are related to alpha-LnSF and Ln_2O_2S networks. In these frameworks, the charge of the blocks containing the rare earth ions can be 2+ or 4+. These blocks alternate with single or double layers of sulfur atoms. The local geometry (number of neighbors, bond distances and angles) around the rare earth varies as a function of the number of the sulfur sheets. Absorption properties in UV-visible range are correlated with the structural features.