Effect of the lanthanide ionic radius on the Raman spectroscopy of lanthanide agardite minerals

Raman microscopy was 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 on (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.