Hydrogen Bonding in the Calcium Phosphates

A comparative study of the metastable equilibrium solubility behavior of high-crystallinity and low-crystallinity carbonated apatites using pH and solution strontium as independent variables

“…An example is the established correlation [4,13] between the mean pIAP of CAP and CAP crystallinity (crystallite microstrain) where crystallinity is a (mean) bulk property. Another example is that based on the outcomes of the present research: the MES of CAPs of low crystallinity may be governed by a surface complex with the calcium/hydroxide deficient hydroxyapatite stoichiometry-which is consistent with literature that poorly crystallized hydroxyapatite has a propensity toward having a bulk stoichiometry of Ca 9 (HPO 4 )(PO 4 ) 5 OH [29][30][31][32][33][34][35][36][37][38].…”

Theoretical investigation of the defect formation mechanism relevant to nonstoichiometry in hydroxyapatite

“…Winand et al pointed out formation of OH − vacancies, and took into account charge compensation among Ca 2+ vacancies, OH − vacancies, and protons, which results in the chemical formula of Ca 10−x ͑HPO 4 ͒ x ͑PO 4 ͒ 6−x ͑OH͒ 2−x . 5,6 This mechanism was also supported by Bery. 7,8 The chemical formula of Ca-deficient HAp proposed by Kuhl and Nebergall 9 is similar to the one of Winand et al, 5,6 but arises from two independent charge-compensation mechanisms: ͑a͒ a pair of Ca 2+ and OH − vacancies with one charge-compensating proton and ͑b͒ two OH − vacancies with one Ca 2+ vacancy.…”

“…5,6 This mechanism was also supported by Bery. 7,8 The chemical formula of Ca-deficient HAp proposed by Kuhl and Nebergall 9 is similar to the one of Winand et al, 5,6 but arises from two independent charge-compensation mechanisms: ͑a͒ a pair of Ca 2+ and OH − vacancies with one charge-compensating proton and ͑b͒ two OH − vacancies with one Ca 2+ vacancy. The defect-formation mechanisms at a low temperature would also be important for HAp in human bodies, because successive dissolution and reprecipitation processes of HAp always takes place in body fluids, leading to the observed nonstoichiometry.…”

“…A number of mechanisms for point-defect formation in Ca-deficient HAp were proposed, which were based on experimental chemical analyses for HAp crystals prepared by low-temperature processes such as solution precipitation. [2][3][4][5][6][7][8][9] In the mechanisms, incorporation of protons that occurs from aqueous solutions surrounding HAp crystals is responsible for the nonstoichiometry. Posner et al and their co-workers suggested that the Ca deficiency is due to Ca 2+ vacancies charge compensated by two protons, where the chemical formula is represented as Ca 10−x H 2x ͑PO 4 ͒ 6 ͑OH͒ 2 .…”

Nature and transformation of crystalline phosphates produced by interaction of phosphate fertilizers with slightly acid and alkaline soils

“…Many investigators have observed that precipitated calcium phosphates which give apatitic X-ray patterns often have mole ratios of calcium to phosphorus less than that in Ca^Q(0H)2(P0i^^)g (Arnold, 1950;Carlstrom, 1955;Eisenberger et a]^., 1940;Neuman and Neuman, 1958;Posner et al, 1960;Stutman et al, 1962;Winand and Dallemagne, 1962). Suggestions by these investigators as to the reason for this apparent anomaly include (a) solid solutions among different phosphates with varying ratios of calcium to phosphorus, (b) defect-apatite structures, (c) isomorphous replacement of calcium ions of apatite by hydronium ions, and (d) solids with compo sitions that have been altered by adsorption or by other changes in the calcium to phosphorus ratio on the surface.…”