Design and Optical Properties of Rare Earth-Based Mixed-Anions (O, S, F) Compounds
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Design and Optical Properties of Rare Earth-Based Mixed-Anions (O, S, F) Compounds Damien Pauwels, Alain Demourgues and Alain Tressaud. Institut de Chimie de la Matiere Condensee de Bordeaux (ICMCB-CNRS), 87, avenue du Dr. Albert Schweitzer, Pessac cedex, 33608 FRANCE.
ABSTRACT Rare earth-based mixed-anions (O, S, F) compounds have been prepared by solid state route. The structures have been determined on the basis of powder diffraction (X-ray/neutrons) refinement and/or single crystal analysis. They have been described as a stacking of [Sx]2x- layers and fluorite type blocks. The evaluation of the rare earth environment (bondlength, number of neighbors) can be directly related to the bandgap and to the UV-visible absorption in the Labased compounds.
INTRODUCTION The rare earth-based mixed anions compounds containing sulfur atoms have been largely investigated for their specific properties in absorption and luminescence field. The rare earth oxysulfides Ln2O2S are mainly used as luminescent materials [1, 2]. The rare earth fluorosulfides LnSF are good candidates as new inorganic color pigments [3]. They exhibit good chromatic properties and high stability in acidic medium. The presence in the vicinity of rare earth ions (4f and 5d shells) of more than two anions (p shell) with various polarizabilities leads to an alteration of the energetic parameters such as the bandwidth and the position of 3p (S) and 5d (Ln) bands, as well as those of 4f (Ln) levels, thus giving these unique optical properties. Because of their various sizes and charges, anions can occupy various crystallographic sites. For instance, the Ln2O2S and LnSF structures exhibit a stacking of layers due to the occurrence of sulfur and oxygen/fluorine atoms in two different sites. The Ln2O2S compounds have a hexagonal symmetry, closely related to the La2O3 type and the LnSF compounds crystallize with the PbFCl type structure. A new series of mixed anions compounds containing rare earth, Ln3OF3S2 (Ln = La, Ce), La2O1.5FS and La4O2F2S3 have been prepared by solid state routes. The structures have been determined and refined on the basis of single crystal and powder X-ray/ neutron diffraction analysis. The diffuse reflectance spectra have been recorded for several compounds and the optical absorption of these samples has been discussed taking into account the nature of rare earth and the crystal field and nephelauxetic effects related to the rare earth environment. EXPERIMENTAL Rare earth α-LnSF, Ln2CaF4S2 fluorosulfides, Ln2O2S oxysulfides, Ln3OF3S2, Ln4O2F2S3 and Ln2O1.5FS oxyfluorosulfides were synthesized by reaction of stoichiometric quantities of high purity rare earth sulfides in their α form, rare earth fluorides, calcium fluoride and rare earth oxides. These starting materials were mixed under a dry argon atmosphere in a glove box
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because of their sensitivity to oxygen and moisture. For α-LnSF and Ln2O2S (Ln = La-Gd), the reactions were carried out in a platinum crucible placed in a quartz tube, which was sealed under vacuum a
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