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Rare earth fluorosulfides LnSF and Ln2AF4S2 (A=Ca, Sr) have been prepared and characterized by X-ray diffraction. The structures can be described as the succession of various sheets of rare earth or alkaline earth, fluorine and sulfur. Thus in these compounds, rare earth is at the center of a distorted square antiprism with four F atoms on one base, four S in the other and a fifth Ln–S bond parallel to the c axis. These compounds exhibit interesting colour from yellow (Sm, Gd) to red (Ce). The chromatic properties have been correlated to structural features and mechanisms at the origin of the colour have been proposed. The variation of energy of absorption edge as a function of rare earth has been explain in terms of ionization energies. |
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