Fig. 4.

The insets (A–G) show the transformation process of ACP to HAP. The peak fit of selected spectra shows that the observed signal (blue dots) between 150 and 1060 min (Inset B – F) is a convolution (red line) of two interfering peaks at 950 (ACP, black line) and 960 cm⁻¹ (HAP, grey line). The transformation process is generally accompanied by a clear peak sharpening recorded in a decreasing FWHM (Inset A–G). The Reference spectrum of well-crystallized HAP (Sigma Aldrich) shows a well-defined sharp peak highlighted by a lowe FWHM (Inlet H).