Effect of the Lanthanide Ionic Radius on the Raman Spectroscopy of Lanthanide Agardite Minerals

Raman microscopy has been used to study the Raman spectra of a series of rare earth element agardites ACu6(AsO4)3(OH)6.3H2O where A = lanthanide3+. These minerals have a porous framework similar to that of zeolites with a structure based upon (A3+ )1-x(A2+)xCu6(OH)6(AsO4)3-x(AsO3OH)x . Two sets of AsO stretching vibrations were found and assigned to the vibrational modes of AsO4 and HAsO4 units. The position of selected bands was found to be sensitive to the lanthanide ionic radius. Raman spectra of the low wavenumber region are complex. Bending modes of the AsO4 and HAsO4 units were found to be sensitive to the ionic radius of the lanthanide ion. Linear relationships between positions of the Raman bands were derived, with correlation coefficients > 0.90