On the composition and atomic arrangement of calcium-deficient hydroxyapatite: An ab-initio analysis

Journal Article · Fri Aug 15 04:00:00 EDT 2008 · Journal of Solid State Chemistry

DOI:https://doi.org/10.1016/j.jssc.2008.03.035· OSTI ID:21128364

Zahn, Dirk ^[1]; Hochrein, Oliver ^[1]

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1. Max-Planck Institut, fuer Chemische Physik fester Stoffe, Noethnitzer Str. 40, Dresden 01187 (Germany)

A systematic study of defect constellations in calcium-deficient hydroxyapatite is reported. Along this line, we explore different arrangements for charge compensation, including cationic vacancies and substitutional defects. The overall defect constellation is governed by both the different proton affinity of the anions or energy costs related to vacancy formation and minimization of the Coulomb energy which implies small distances of the anionic and cationic defects. Depending on the type of the calcium-deficient site, this gives rise to two specific defect arrangements. Among these, the calcium ions forming triangles which embed the OH{sup -} ions of hydroxyapatite are most likely to be deficient. The resulting charge is compensated by protonation of the OH{sup -} ion within the deficient calcium-triangle and protonation of a PO{sub 4}{sup 3-} ion in the nearest neighbourhood of the vacant calcium site. The strong energetic favouring of such constellations indicates that the commonly used chemical formulae Ca{sub 10-x}(HPO{sub 4}){sub x}(PO{sub 4}){sub 6-x}(OH){sub 2-x}(H{sub 2}O){sub x} (0<x{<=}1) reflect a reasonable approximation of the composition of calcium-deficient hydroxyapatite. - Graphical abstract: Preferred constellation of a Ca vacancy in hydroxyapatite, accompanied by a H{sub 2}O defect and a HPO{sub 4}{sup 2-} defect adjacent to the deficient Ca site. The preferential defect arrangement reflects a compromise of local charge compensation, different proton affinities of the anions and hydrogen bonding.

• OSTI ID: 21128364
• Journal Information: Journal of Solid State Chemistry, Journal Name: Journal of Solid State Chemistry Journal Issue: 8 Vol. 181; ISSN 0022-4596; ISSN JSSCBI
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
AFFINITY
ANIONS
APATITES
CALCIUM COMPOUNDS
CALCIUM IONS
CALCIUM PHOSPHATES
COULOMB ENERGY
HYDROXIDES
PHOSPHORIC ACID
PROTONS
VACANCIES