Vibrational spectra of hydrated rare earth orthophosphates

Weinschenkite-type LnPO(4). 2H(2)O (Ln is Gd, Tb, Dy, Ho, Y, Er, Tm or Yb) and rhabdophane-type, LnPO(4).H2O (Ln is La, Ce, Pr, Nd, Sm, Eu, Gd, Tb or Dy) have been investigated by IR absorption spectroscopy (4000-400 cm(-1)) and Raman scanning spectroscopy (1400-100 cm(-1)). The IR spectra of weinschenkite-type LnPO(4). 2H(2)O (Ln is Gd --> Yb) are characterized by a band at 750 +/- 6 cm(-1) and the occurrence of a doublet in the region of the HOH bending vibrations, the low-frequency component exceeding the first high-frequency component in intensity. This rather peculiar pattern has already been observed in other compounds of similar chemical composition and is interpreted as arising from the presence of water molecules coordinated to the same metal cation. The Raman and IR spectra of these compounds have been interpreted in a manner based on the known structure of CaSO4. 2H(2)O, which is isostructural with the weinschenkite-type compounds. The Raman and IR spectra of rhabdophane-type LnPO(4).H2O is analyzed on the basis of the knowledge of the space group of rare earth orthophosphates rhabdophane-type. Its relation with the spectra of rare earth orthophosphates weinschenkite-type is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.