Luminescence of calcium hydroxyapatite at Xray irradiation

Abstract

Synthetic calcium hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ is very similar to the main mineral component of the hard tissues of living organisms. Thus, it is actively used in innovative medicine as a filler for artificial bones, for the manufacture of implants, in dentistry, etc. Mostly, such objects are in the aqueous environment, and small clusters of water have recently been discovered inside nanostructured calcium hydroxyapatite. Since the presence of water can significantly affect physical and chemical properties of minerals, information on the interaction of nanostructured calcium hydroxyapatite with water molecules can be useful from a practical point of view. Luminescence spectra of two different samples of calcium hydroxyapatite were studied under Xray excitation at temperatures 295 K and 85 K. At room temperature (295 K), an Xray luminescence band was registered for both samples with a maximum at about 550 nm. At a low temperature (85 K), the spectra of the first and second samples differ significantly from each other. The spectrum of the first sample contains a band with a maximum at 550 nm, but with a higher intensity than that at room temperature. In the spectrum of the second sample, another wide band appears in the range 300500 nm, which was absent at room temperature. This difference can be explained by the different degrees of hydration of the samples — in the first sample, a large number of incorporated water molecules can be present acting as quenching centers of Xray luminescence. In the second sample, there are fewer water molecules (and/or hydroxyl groups). Therefore, two bands are recorded in the spectrum. For the second sample, it was possible to register phosphorescence and thermally stimulated luminescence after the Xray irradiation at 85 K.