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 [churchite(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 dehydration of these phases is different and can be correlated to the strength of the hydrogen bonds. The minimum distance between water O and phosphate O atoms is considered to be a measure of this strength. The mean length of the hydrogen bonds decreases with increasing temperature of dehydration, from gypsum (2.856 Angstrom) to CaSeO4.2H(2)O (2.845 Angstrom) and weinschenkite (2.726 Angstrom); hence the strength of the hydrogen bonds increases.