Infra-red spectra of hydroxyapatite, octacalcium phosphate and pyrolysed octacalcium phosphate.

Infra-red spectra of hydroxyapatite, octacalcium phosphate and pyrolysed octacalcium phosphate were studied in the range 4000-400 cm−1, and probable band assignments are given. Unreported bands were found for both octacalcium phosphate (thirteen bands) and hydroxyapatite (five bands); the hydroxyapatite band at 631 cm−1 was reassigned to the librational mode of the hydroxyl group. The pyrolysis reactions of octacalcium phosphate were studied by infra-red absorption, weight loss, and pyrophosphate analysis. Simple dehydration was continuous from 50 to about 400 °C. The major products between 325 and 600 °C were hydroxyapatite and β-Ca2P2O7; between 650 and 900 °C, the products were β-Ca2P2O7 and β-Ca3 (PO4)2. Formation of β-Ca2P2O7, instead of γ-Ca2P2O7, was detected at a lower temperature (325 °C) than has been reported. The maximum amount of pyrophosphate formed, about 40 per cent at 500 °C, was intermediate between the amounts predicted by reactions postulated in other studies. From 700 to 900 °C, the pyrophosphate content approximated the expected value.

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