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Periodic B3LYP study of hydroxyapatite (001) surface modelled by thin layer slabs

Marta Corno, Roberto Orlando, Bartolomeo Civalleri and Piero Ugliengo
Periodic B3LYP study of hydroxyapatite (001) surface modelled by thin layer slabs
European Journal of Mineralogy (October 2007) 19 (5): 757-767

Abstract

The (001) surface of the hexagonal hydroxyapatite HA [Ca (sub 10) (PO (sub 4) ) (sub 6) (OH) (sub 2) , layer group P3] is simulated with the slab approach by fully optimizing (cell size and internal coordinates) two models, respectively 6 Aa and 14 Aa thick, in a fully ab initio periodic approach. The B3LYP hybrid functional and a Gaussian basis set of polarized double zeta quality and pseudo potentials for Ca ions only have been adopted, as encoded in the CRYSTAL03 computer program. Both slab models are cut out of the optimized structure of the hexagonal HA bulk phase (P6 (sub 3) space group). Because the (001) surface derived from the hexagonal HA shows ferroelectricity due to the orientation of the OH groups, the convergence of the E (sub surf) with the slab thickness (until a thickness of about 60 Aa) has been studied at B3LYP level on slabs whose geometry has been optimized using the GULP program with a recently developed shell-ion model potential. Structural and electronic features are addressed and a comparison between results for the considered slabs is carried out with respect to the: i) surface energy, E (sub surf) ; ii) geometrical relaxation; iii) band gap, field across the slab and Mulliken analysis; iv) electrostatic features in close proximity of the surface; v) harmonic/anharmonic OH vibrational features. The same procedure has been adopted for non-ferroelectric slabs derived from the HA monoclinic phase (bulk belonging to the P2 (sub 1) /b space group). E (sub surf) for hexagonal HA increases slightly, as a function of the slab thickness, from 1.080 J/m (super 2) (doublelayer) to 1.107 J/m (super 2) (nonalayer), showing that the OH ferroelectricity imparts an instability of nearly equal 0.003 J/m (super 2) for each added layer (7 Aa thick). For the non-ferroelectric HA monoclinic phase, E (sub surf) converges to 1.337 J/m (super 2) within 1.0 (super -4) J/m (super 2) already for the doublelayer. It is shown that the OH ferroelectricity does not prevent the formation of a (001) slab of thickness of at least 10 nm, a fact relevant for technological applications. The HA doublelayer slab is suggested as a proper model of the HA (001) face to study adsorption processes relevant to understand the role of hydroxyapatite surface in biological processes.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 19
Serial Issue: 5
Title: Periodic B3LYP study of hydroxyapatite (001) surface modelled by thin layer slabs
Affiliation: Universita di Torino, Dipartimento di Chimica IFM, Turin, Italy
Pages: 757-767
Published: 200710
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 55
Accession Number: 2008-002157
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Includes appendices
Illustration Description: illus. incl. 4 tables
Secondary Affiliation: Universita del Piemonte Orientale, ITA, Italy
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200802
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