STRUCTURE AND RAMAN-SPECTROSCOPY OF CZOCHRALSKI-GROWN BARIUM YTTRIUM AND BARIUM YTTERBIUM FLUORIDES CRYSTALS

Large Bay2F8 and BaYb2F8 single crystals were grown by the Czochralski method. The structure of the former was determined by X-ray diffracti on at room temperature and both crystals were studied by Raman spectro scopy from room temperature down to 20 K. At room temperature, all the results are consistent with the monoclinic structure C2/m as already proposed for Bay2F8, but the structure refinements suggest a slight di sorder around one of the fluorine sites. Most of the normal modes of v ibration are tentatively identified using the relationships between th e irreducible representations of the point group C2h and those of the supergroups D2h and D3d. For BaYb2F8, anomalous electronic lines appea r in the Raman spectra below 80 K. The magnetic susceptibility of this compound was also studied in the range 4K-280K. The results are consi stent with a large zero-field splitting of Yb3+, as also deduced from the Raman study.