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

► Nanocrystalline carbonated apatites (CHA) of the mixed type AB. ► Comparison of the IR spectra in the transmission and photoacoustic modes. ► Different A-carbonate configurations, A1 and A2, discovered in nanocrystalline CHA. ► New bands from CO 3 2− ions in aqueous compartments and on the crystal surface. ► Information on distribution of CO 3 2− ions between various material sites. 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 ν 2 and ν 3 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.