Features of the Phase Preferences, Long- and Short-Range Order in Ln 2 (WO 4) 3 (Ln = Gd, Dy, Ho, Yb) with Their Relation to Hydration Behavior.

The effect of synthesis conditions on the features of the long- and short-range order of Ln 2 (WO 4 ) 3 (Ln = Gd, Dy, Ho, Yb) powders synthesized via coprecipitation of salts has been studied by a complex of physico-chemical techniques including synchrotron X-ray powder diffraction, X-ray absorption spectroscopy, Raman and infrared spectroscopy, and simultaneous thermal analysis. It was found that crystallization of amorphous precursors begins at 600 °C/3 h and leads to the formation of the monoclinic structure with sp. gr. C12/c1(15) for Ln 2 (WO 4 ) 3 (Ln = Gd, Dy) and with sp. gr. P12 1 /a1(14) for Ln = Yb, whereas crystallization of Ho precursor requires even higher temperature. After annealing at 1000 °C, the P12 1 /a1(14) phase becomes the dominant phase component for all heavy lanthanoid types except for Ln = Gd. It was shown that the Ln (Ln = Dy, Ho, and Yb) tungstates with the P12 1 /a1(14) monoclinic structure correspond to trihydrates Ln 2 (WO 4 ) 3 ·3H 2 O formed due to a rapid spontaneous hydration under ambient conditions. It was concluded that the proneness to hydration is due to a specific structure of the P12 1 /a1(14) phase with large voids available to water molecules. Modifications in the local structure of Ln-O coordination shell accompanying the structure type change and hydration are monitored using EXAFS spectroscopy. [ABSTRACT FROM AUTHOR]