Role of acid phosphate in hydroxyapatite lattice expansion

Summary

Questions remain about which subcomponents of human tooth enamel (TE) are responsible for its crystallographica axis being nearly 0.02Å longer than that of pure hydroxyapatite (OHAp) and contracting to that of OHAp on heating. From infrared spectroscopic and X-ray diffraction studies of a synthetic OHAp containing HPO₄ and “structural” H₂O, it has been concluded that HPO₄ expands thea axis at the rate of ∼ 0.0015Å/ wt % but that this accounts for substantially less than one-half of the total observable contraction. The remaining, more than one-half of thea axis change, may be only partially ascribable to “structural” H₂O and partially to P₂O₇ (formed from the HPO₄), coming out of solid solution in the apatite. Some 90% of the HPO₄ observed with infrared is lost in the 160–240° temperature range and more than one-half of the P₂O₇ observed as a separate phase is developed above that temperature and continues to increase all the way up to the 500°C, the limit of the experiments. The loss of HPO₄ is accompanied by reduction of disorder or variety in the structural OH ion sites, consistent with the view that initially some of the PO₄ groups neighboring the OH ions were actually HPO₄ groups.