A comparative study of carbonate bands from nanocrystalline carbonated hydroxyapatites using FT-IR spectroscopy in the transmission and photoacoustic modes

Nanocrystalline carbonated calcium hydroxyapatites (CHA) of the mixed type AB, containing ca. 1 wt.% of the A-type carbonates (substituted for OH−) and ca. 5–8 wt.% of the B-type carbonates (substituted for PO 4 3 - ), were studied using the IR spectroscopy in the transmission (TRANS) and photoacoustic (PAS) modes. The respective ν2 and ν3 carbonate spectral regions were examined and the TRANS and PAS bands were assigned and compared. For the first time, different A-carbonate configurations, A1 and A2, have been discovered in nanocrystalline CHA. New bands were found at 1387 and 884 cm−1 and assigned to carbonate ions in aqueous compartments (adsorbed water and/or water trapped in crystal grains) and located on the crystal surface, respectively. The comparison of the TRANS and PAS IR spectra provides valuable information on distribution of carbonate ions in the apatite crystal lattice and their separation between the crystal interior and the crystal surface.