Structure-spectra relationships of carbonate defects in hydroxyapatite revealed by first-principles infrared...

Carbonate substitution in hydroxyapatite alters its vibrational response, yet experimental infrared (IR) assignments remain ambiguous due to overlapping features from distinct defect configurations. We present a systematic first-principles study of the IR signatures of carbonate defects in hydroxyapatite using density functional theory molecular dynamics. A dataset of 30 structurally distinct carbonate-substituted models, including bulk and surface substitutions with various charge-compensation schemes, was constructed. IR spectra were obtained from finite-temperature dipole autocorrelation functions. Comparative analysis of peak positions, intensities, and line shapes reveals quantitative structure-spectra relationships that link specific carbonate environments to their vibrational fingerprints. Substitution of phosphate by carbonate leads to variation in features of the 1100-1500 cm^-1 and 3600 cm^-1 regions whose positions and relative intensities depend on the number of substituted phosphate groups and proton/hydroxyl compensation. These results provide a physically grounded reference for interpreting experimental IR spectra of carbonated apatites and demonstrate the utility of first-principles vibrational spectroscopy for resolving defect chemistry in complex inorganic solids.