Combined vibrational spectra of natural wardite

Vibrational spectra (IR, Raman, inelastic neutron scattering) were measured of natural wardite (ideal formula NaAl3(OH)4(PO4)2·2 H2O) from Trauira, Brazil, with the main impurities Fe and Ca. The spectra are discussed on the basis of a symmetry analysis restricted to one layer in the four-layer structure. The band pattern in the n(OH) region is due to two different Al2OH groups and their correlation coupling; their
deformations d and g are assigned based on IR and INS spectra. Contributions of the hydrogen-bonded H2O molecules are discussed, as are the vibrations of the AlO6 octahedra dominating the Raman spectrum. From the fundamentals n(OH) of the OH groups and their overtones anharmonicity constants have been estimated.