Line-broadening effects in the powder infrared spectrum of apatite

The crystallinity of natural and synthetic apatite samples is often determined from the broadening of ν₄ PO₄ infrared absorption bands. However, various physical mechanisms contribute to the observed linewidth. In the present study, the factors determining the linewidth in the powder spectrum of synthetic fluorapatite and hydroxyapatite samples are investigated. The temperature dependence of the infrared spectrum (10-270 K) is used to assess the respective contributions of homogeneous broadening, related to the decay of phonons through anharmonic coupling, and heterogeneous broadening related to elastic strain and macroscopic electrostatic effects. This latter contribution is dominant in the investigated samples and depends on the shape of powder particles. It is discussed under the light of the theoretical modeling of the low-frequency dielectric properties of apatite based on first-principles density functional theory calculations. The linewidth of the weak ν₁ PO₄ absorption band provides a reliable information on microscopic sources of broadening, i. e., apatite crystallinity. In comparison, the other more intense PO₄ bands are more sensitive to long-range electrostatic effects.