This article deals with infrared spectroscopic studies, in the range 400-4OOOcm-~, of some calcium phosphates having an apatitic structure. The bands are assigned and small differences between some spectra ale explained.Occurrence of free OH-groups in the apatitic cell is confirmed; acidic phosphate groups, linked together by hydrogen bonds, have been detected in the spectra of compounds having a C a p value lower than the hydroxylapatite one.Heat effect upon the constitution of these compounds has also been studied; these infrared data get information which could not be obtained by X rays spectrography: i.e. the occurrence of pyrophosphate groups in solid solution in the apatihc cell.The experimental data are satisfactorily explained by the assumption that H+ ions are able to (
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