Role of acid phosphate in hydroxyapatite lattice expansion

Questions remain about which subcomponents of human tooth enamel (TE) are responsible for its crystallographic alpha axis being nearly 0.02A 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 the alpha axis at the rate of approximately 0.0015A/wt% but that this accounts for substantially less than one-half of the total observable contraction. The remaining, more than one-half of the alpha 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 degrees 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 degrees 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.