Impact of leach on lead vanado-iodoapatite [Pb5(VO4)3I]: An infrared and Raman spectroscopic study

Structural changes in lead vanado-iodoapatite [Pb5(VO4)3I], leached in KOH/KCO3H buffer solution at 90 °C, were investigated using infrared (IR) and Raman spectroscopy. The untreated material shows characteristic phonon bands in three wavenumber regions (50–250, 300–500 and 750–870 cm−1). In comparison with a natural apatite, the replacement of phosphate by vanadium in the apatite structure leads to stretching and bending vibrations of the structural tetrahedra shift to lower frequencies. The leached samples all show extra bands between 3000 and 3600 cm−1, characterised by two features: a broad tail and two sharp absorption bands near 3490 and 3538 cm−1. The former is attributed to the H2O species absorbed by the gains or located between grain boundaries, whereas the latter two bands, which have the similar frequencies as those of hydroxyls in fluorapatite, are assigned as OH species which substitutes iodine in the leached materials. The leached samples also show an extra Raman band near 1058 cm−1 and IR bands near 1370 and 1420 cm−1, which are the characteristic vibrations of CO3 units. The uncorrelated relative intensities between CO3 and OH bands and the absence of the characteristic COH vibrations (near 1017 and 1300 cm−1) of the HCO3 ion in the Raman spectra indicate that hydrogen- and carbon-related species mainly exist in the form of CO3 and OH rather than HCO3. CO3 ions could be incorporated into the tetrahedral sites by substitution of VO4 (B-type substitution) as a result of the leached-induced loss of vanadium.