The vibrational spectrum of hydroxyl groups in hydroxyapatites

The infrared spectra of four samples of hydroxyapatites were measured using thin wafers. One sample had nearly the stoichiometric composition and one had a deficiency of Ca²⁺; the other samples contained small amounts of substitutional F^- and Cu²⁺, respectively. Spectra were taken following different pretreatment procedures and at several temperatures. Particular attention was given to absorptions attributable to hydroxyl groups. A weak band at 3660 cm^-1 in all samples was characterized as a surface OH group by its exchange behavior with D₂O and back exchange with NH₃. Internal POH groups in the non-stoichiometric sample ( Ca P = 1.58) gave rise to a band at 880 cm^-1, which was unchanged following deuterium exchange, but which disappeared when heated above 400° due to condensation reactions leading to the formation of P₄O₇^4- groups which absorbed at 715 cm^-1. The spectral characteristics of the stretching vibrations of lattice OH groups (and their OD analogues) were determined at temperatures from 25 to 600°C. At room temperature the absorptions at 3572 cm^-1 (and 2634 cm^-1) were symmetrical, approximately Lorentzian in shape and narrower in the non-stoichiometric than the stoichiometric ( Ca P = 1.67) material. At 600°C these frequencies were decreased to 3565 cm^-1 (and 2628 cm^-1) and new absorptions, not apparent in room temperature spectra, were observed at 3537 cm^-1 for the OH form (and 2610 cm^-1 for the OD form). In both instances the new absorptions comprised 30 % of the respective total band areas. The changes, brought about by temperature increase, were fully and instantaneously reversible on cooling, the new bands being of intermediate intensity at intermediate temperature. It is suggested that they result from an orders-disorder transition (of enthalpy 2.7 kcal mol^-1) in the hydroxyl (and deuteroxyl) columns of the lattice.