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

¹ Dipartimento di Chimica IFM, Università degli Studi di Torino and NIS – Nanostructured Interfaces and Surfaces –Centre of Excellence and INSTM (Materials Science and Technology) National Consortium, UdR Torino, Via P. Giuria 7, 10125 Torino, Italy
² Dipartimento di Scienze e Tecnologie Avanzate, Università del Piemonte Orientale, Via Bellini 25/G, 15100 Alessandria, Italy

^* Corresponding author, e-mail: piero.ugliengo{at}unito.it

The (001) surface of the hexagonal hydroxyapatite HA [Ca₁₀(PO₄)₆(OH)₂, layer group P3] is simulated with the slab approach by fully optimizing (cell size and internal coordinates) two models, respectively 6 Å and 14 Å 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 (P6₃ 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_surf with the slab thickness (until a thickness of about 60 Å) 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_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 P2₁/ b space group). E_surf for hexagonal HA increases slightly, as a function of the slab thickness, from 1.080 J/m² (doublelayer) to 1.107 J/m² (nonalayer), showing that the OH ferroelectricity imparts an instability of 0.003 J/m² for each added layer (7 Å thick). For the non-ferroelectric HA monoclinic phase, E_surf converges to 1.337 J/m² within 1.0^–4 J/m² 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.

Key-words: hydroxyapatite, surfaces, B3LYP, biomaterials, ab initio, vibrational frequency.