Characterization of crystallographic orientation and lattice disorder in hydroxyapatite thin films by Raman scattering

This report presents the way of evaluating the crystallographic orientation and crystallinity of hydroxyapatite (HAp) thin films by Raman scattering spectroscopy. According to the Raman selection rule, the symmetric stretching mode (ν1) of P−O bonds in an isolated PO43− units is Raman active, whereas the asymmetric stretching mode (ν3) is not. Nevertheless, a weak ν3 signal is usually observed in the Raman spectra of HAp powder crystals. We will show that the ν3 signal is entirely absent for uniaxially oriented HAp films, whereas it appears in the case of randomly-oriented ones. This principle is feasible for discriminating the uniaxial orientation from a random one in HAp crystals. Another simple approach for evaluating crystallinity is exploiting the line width of the ν1 PO43− Raman signal. If the full-width at half maximum (FWHM) of the ν1 signal is smaller than 7.4 cm−1, the HAp crystal is uniaxially oriented in one direction, whereas if the FWHM value is larger than 7.4 cm−1, the film is composed of randomly-oriented crystallites.