Structure-Spectra Relationships of Carbonate Defects in Hydroxyapatite Revealed by First-Principles Infrared Spectroscopy

Abstract

Carbonate substitution in hydroxyapatite modifies its vibrational response, yet infrared (IR) assignments remain challenging due to overlapping contributions from distinct defect configurations. Here, we present a systematic first-principles investigation of the IR signatures of carbonate defects based on density functional theory molecular dynamics. A set of 33 structurally distinct models, including bulk and surface substitutions with different chargecompensation mechanisms, was constructed. Finite-temperature IR spectra were obtained from dipole autocorrelation functions, enabling direct comparison with experiment. Analysis of peak positions, intensities, and line shapes reveals clear structure-spectra relationships linking local carbonate environments to their vibrational fingerprints. Substitution of phosphate by carbonate produces distinct, environment-dependent features in the 1100-1500 cm⁻¹ and 3600 cm⁻¹ regions, governed by the number of substituted phosphate groups and proton or hydroxyl compensation. These results provide a consistent framework for interpreting experimental IR spectra of carbonated apatites and clarify the signatures of different defect configurations.