Vibrational spectra and structure of rare earth orthophosphates, weinschenkite type

Of the weinschenkite-type rare earth orthophosphates, only the Dy, Er, Y and Yb compounds are known. However, all elements from Tb to Yb inclusive exist in this form. Monoclinic LnPO(4) . 2H(2)O (when Ln is Tb, Dy, Ho, Y, Er, Tm or Yb) has been prepared and investigated by IR absorption spectroscopy (4000-400 cm(-1)), Raman scanning spectroscopy (1400-100 cm(-1)), X-ray diffractometry and thermal gravimetric analysis. The IR spectra 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. Raman spectra of LnPO(4) . 2H(2)O (where Ln is Dy, Y, Er, Tm or Yb) and IR spectra of all the compounds have been obtained. These spectra have been interpreted in a manner based on the known structure of CaSO4 . 2H(2)O, which is isostructural with the weinschenkite-type compounds. Unit cell parameters for LnPO(4) . 2H(2)O (where Ln is Tb, Ho or Tm) have been determined from the X-ray diffraction patterns. (C) 1997 Elsevier Science B.V.