Infrared studies of apatites. II. Preparation of normal and isotopically substituted calcium, strontium, and barium hydroxyapatites and spectra-structure-composition correlations

Procedures are described for preparation of calcium, strontium, and barium hydroxyapatites, their deuterated analogs, and calcium hydroxyapatite enriched with isotopic calcium with particular emphasis on purity, stoichiometry, crystal perfection, and gravimetrically monitoring the apatite composition. The effects of low temperature and differences in physical properties of the apatites on the infrared vibrational frequencies are discussed. Shifts in the internal phosphate modes to higher frequencies, in the barium-strontium--calcium hydroxyapatite sequence and also on cooling, are attributed primarily to increased interphosphate repulsion concomitant with lattice contraction. Causes for variation in the frequencies and intensities of the OH modes are considered from several viewpoints including the presence of weak OH-O bonding which is neither conclusively established nor rejected. Finally, marked increases in OH band intensities in the barium--strontium--calcium hydroxyapatite sequence are attributed to differences in OH bonding rather than to other effects.