Defect-Related Low-Temperature Luminescence of Undoped Biogenic Carbonate-Containing Hydroxyapatite Under VUV Synchrotron Excitation

Defect-related self-activated luminescence of unintentionally undoped biogenic carbonate-containing hydroxyapatite (HAp) was studied at 12 K under tunable vacuum-ultraviolet excitation at the P66 beamline of PETRA III. X-ray diffraction confirmed the hexagonal P63/m apatite framework and an apparent coherent-domain size of approximately 57.7 nm. Raman spectroscopy verified the phosphate framework, B-type carbonate near 1072 cm−1, and a perturbed structural-OH environment, while not identifying defect charge states. Four empirical emission families were resolved at 2.530, 2.995, 3.305, and 3.630 eV. PLE monitored at 504, 426, 375, and 346 nm covers D1–D4 and reveals common excitation regions near 5.3–5.5, 6.1, and 7.06–7.10 eV, together with D2-selective and D4-selective features near 6.50 and 9.83 eV. The D3-centred 375 nm profile is dominated by the 7.10 eV component. Hybrid-functional and G0W0 calculations place the ideal-HAp edge near 7.1–7.5 eV; therefore, the common near-7.1 eV PLE feature is treated as the best-supported host-related near-edge/excitonic marker, whereas lower-energy features are assigned cautiously to localized sub-gap absorption. Because PLE is not a direct absorption measurement, the pathway scheme is presented as a conceptual hypothesis rather than a uniquely resolved kinetic model.