STRUCTURE OF A Y-1-X(GD,DY,ER)(X)PO4.2H2O MICROCRYSTAL USING SYNCHROTRON-RADIATION

The gypsum-type structure of the title compound, weinschenkite [church ite(Y)], is confirmed. The H-atom positions could not be located. The structures of gypsum [Cole & Lancucki (1974). Acta Cryst. B30, 921-929 ], CaSeO4.2H(2)O [Kruger & Abriel (1991). Acta Cryst. C47, 1958-1959] and weinschenkite are isotypic. However, the temperature of dehydratio n of these phases is different and can be correlated to the strength o f the hydrogen bonds. The minimum distance between water O and phospha te O atoms is considered to be a measure of this strength. The mean le ngth of the hydrogen bonds decreases with increasing temperature of de hydration, from gypsum (2.856 Angstrom) to CaSeO4.2H(2)O (2.845 Angstr om) and weinschenkite (2.726 Angstrom); hence the strength of the hydr ogen bonds increases.