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J. Phys. Chem. B 2008, 112, 12730–12739
Elucidation of the Structural Role of Fluorine in Potentially Bioactive Glasses by Experimental and Computational Investigation G. Lusvardi,‡ G. Malavasi,‡ M. Cortada,§ L. Menabue,*,‡ M. C. Menziani,‡ A. Pedone,‡ and U. Segre†,‡ Department of Chemistry and SCS center, UniVersity of Modena and Reggio Emilia, Via G. Campi 183, 41100 Modena, Italy, and Department of Physical, Chemical and Natural Systems, UniVersity “Pablo de OlaVide”, SeVilla, Spain ReceiVed: April 4, 2008; ReVised Manuscript ReceiVed: July 10, 2008
Glasses belonging to the Na2O-CaO-P2O5-SiO2 system and modified by CaF2 substitution for CaO and Na2O alternatively, were synthesized and characterized experimentally and computationally. The results of molecular dynamics simulations show that fluorine is almost exclusively bonded to modifier cations (Ca and Na) with coordination number close to 4. A similar mean coordination number value is found in the crystal phases obtained by means of thermal treatment at fixed temperature. Addition of fluorine increases the polymerization of silicate tetrahedra by removing modifiers from the siliceous matrix. No appreciable amount of Si-F bonds are detected. 1. Introduction Fluorine-containing glasses and glass-ceramics are currently used in dentistry and in steelmaking plants.1-5 Notwithstanding their interest from the applicative points of view, the structural role of fluorine and its relationships with thermal properties, diffusivity, viscosity, density, and chemical durability is not clearly defined. Moreover, controversial results have been obtained by several studies on fluoro-containing silicate and aluminosilicate systems.4-8 Wood and Hill4 showed that the increase of the fluorine content in aluminosilicate glasses of composition 2SiO2 · Al2O3 · (2 - x)CaO · xCaF2 causes a drastic reduction of the glass transition temperature Tg. On the basis of 27Al, 29Si, and 31P MAS NMR results, this effect was previously attributed by Stamboulis et al.5 to the replacement of bridging oxygens (BO) by nonbridging fluorine ions with a consequent reduction of the silicate-network connectivity which facilitates the motion of glass constituents at low temperature. 19F, 27Al and 29Si MAS NMR experiments performed on F-bearing silicate7 and aluminosilicate8 glasses (molar % of F ) 10%) showed that the formation of Si-F and Al-F nonbridging bonds can happen with a constant Si and Al coordination number of 4 or with an expansion to 5 or 6.7,8 In these cases the anionic repulsion between F and O results in a trigonal bipiramid and an octahedral coordination polyhedra for 5- and 6-fold coordination, respectively. Conversely, 19F, 27Al, 29Si, and 31P MAS NMR experiments performed by Stamboulis et al.6 on aluminosilicate glasses showed that the increase of fluorine content leads to the formation of F-Ca species and the replacement of bridging oxygens (BO) by nonbridging fluorine ions was not detected. Ambiguities in the interpretations of experimental results can be solved by molecular dynamics (MD) simulations which furnish a detailed description of the three-dimensional structure * Corresponding author. E-mail:
[email protected]. † Deceased. ‡ Department of Chemistry and SCS center, University of Modena and Reggio Emilia. § Department of Physical, Chemical and Natural Systems, University “Pablo de Olavide”.
of the glass from which detailed information of the environment and of the structural role of its components can be obtained, provided that reliable force fields are used. Several MD studies of the structure of bioactive glasses have been carried out so far9-14 but, to our knowledge, the computational study of fluorine-containing multicomponent glasses has been accomplished only by Hayakawa et al.,15 probably because of the lack of reliable interatomic potential models for such complex systems in which two anions are contemporaneously present (F and O in this case). In ref 15, MD simulations performed on glasses of composition MF2-MO-SiO2 (M ) Ca, Sr, and Ba) showed that an acidic environment (Ba ions) induces a greater amount of Si-F bonds while a basic environment (Ca and Sr) favors the formation of M-F aggregates (M ) Ca and Sr) and no Si-F bonds were detected. In the present work, potentially bioactive glasses based on the Bioglass 45S5,16,17 in which CaO and Na2O are alternatively substituted for CaF2, have been investigated by means of computational techniques with the aim of elucidating the structural role of fluorine in a multicomponent system. 2. Experimental Procedure 2.1. Glass Synthesis. Two new series of fluorine-containing glasses based on the composition of Bioglass 45S5 (46.2SiO2 · 24.3Na2O · 26.9CaO · 2.6P2O5 hereafter named H) have been synthesized. In the first series, named HNaCaF2, CaF2 substitutes for Na2O and the formula composition is 46.2SiO2 · (24.3 - x)Na2O · 26.9CaO · 2.6P2O5 · xCaF2 (where x ) 0, 5, 10, 15, 20, and 24.3). The second series, where CaF2 replaces CaO and the formula composition is 46.2SiO2 · 24.3Na2O · (26.9 - x)CaO · 2.6P2O5 · xCaF2 (where x ) 0, 5, 10, 15, 20, and 26.9), is named HCaCaF2. The nominal molar concentrations are reported in Table 1. The glass samples were prepared from a mixture of precursors: SiO2, Na2CO3, Na3PO4 · 12H2O, CaCO3 and CaF2. The powders were melted in a platinum crucible at 1350 °C by using two heating rates: 5 °C/min in the range 20-1000 and 15 °C/ min above 1000 °C. The melt was refined for 1 h at the melting temperature, then quenched on a graphite plate mold. The
10.1021/jp803031z CCC: $40.75 2008 American Chemical Society Published on Web 09/11/2008
Structural Role of Fluorine
J. Phys. Chem. B, Vol. 112, No. 40, 2008 12731
TABLE 1: Molar Composition and Density of Studied Systems H HNaCaF2 5% HNaCaF2 10% HNaCaF2 15% HNaCaF2 20% HNaCaF2 24.3% HCaCaF2 5% HCaCaF2 10% HCaCaF2 15% HCaCaF2 20% HCaCaF2 26.9%
% SiO2
% Na2O
% CaO
% P2O5
% CaF2
density [g/cm3]
46.2 46.2 46.2 46.2 46.2 46.2 46.2 46.2 46.2 46.2 46.2
24.3 19.3 14.3 9.3 4.3
26.9 26.9 26.9 26.9 26.9 26.9 21.9 16.9 11.9 6.9 -
2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6
5.0 10.0 15.0 20.0 24.3 5.0 10.0 15.0 20.0 26.9
2.719 2.737 2.761 2.774 2.788 2.800 2.703 2.679 2.641 2.599 2.536
24.3 24.3 24.3 24.3 24.3
samples were then grinded in agate mill jars and sieved to produce a particle size o.d. < 26 µm for XRD and density analysis. During the melting process P- and F-compounds can volatilize leading to a composition different with respect to the nominal one. Therefore, the powder compositions were determined with semiquantitative SEM-EDS (scanning electron microscopy with energy dispersive spectrometer) analysis in order to confirm the theoretical molar composition reported in Table 1 with an error of less than 2%. All samples were obtained as transparent homogeneous glasses, only HNaCaF2 24.3% showed an opaque effect due to partial crystallization. 2.2. Glass Characterization. Experimental density was determined with a picnometer (Micromeritics, Accupyc 1330) at room temperature with an accuracy of 0.002 g/cm3. Each value is an average of three independent measurements. XRD diffraction analysis was performed on powders of the as-quenched glasses and after their thermal treatment (2 h) with a PANalytical X’Pert Pro Bragg-Brentano diffractometer, using Ni-filtered Cu KR radiation (λ ) 1.54060 Å) with X’Celerator detector. Data were collected in the range 3 < 2θ < 70°, with a time step of 50 s and a step size of 0.03°. 3. Computational Procedure The structures of the glasses experimentally characterized in this work were modeled by means of NVT molecular dynamics (MD) simulations. For each composition 2800 atoms were placed randomly in a cubic box with dimensions imposed by the experimental density (Table 1). The pairwise interatomic potentials developed by Teter and reported in a previous work by Cormack et al., were used.18 The potential is based on a rigid ionic model, with partial charges to handle the partial covalency of silicate systems and is given by the sum of two terms: the long-range Coulombic potential and the short-range forces, which are represented by a Buckingham function. The expression for the model potential is:
U(r) )
Cij zizje2 + Aije-r/Fij - 6 r r
(1)
where zi, zj, Aij, Fij, and Cij are parameters whose values were derived by a systematically fitting on a large number of crystal structures of oxides and fluorides. The indices i and j refer to the different atomic species. The values of the atomic charges zi and of the other parameters in eq 1 are listed in Table 2. These interatomic potentials were tested on simple silicate glasses (binary alkali-silicate and ternary soda-lime-silicate glasses) by Cormack et al.18 In order to verify the extendibility to reproduce the structure and properties of glasses containing atoms (in particular F) of interest for this study we have
TABLE 2: Potential Parameters used for Molecular Dynamics Simulations Si+2.4-O-1.2 P+3.0-O-1.2 Na+0.6-O-1.2 Ca+1.2-O-1.2 O-1.2-O-1.2 Si+2.4-F-0.6 Na+0.6-F-0.6 Ca+1.2-F-0.6 F-0.6-F-0.6 O-1.2-F-0.6
Aij [eV]
Fij [Å]
Cij [eV · Å6]
13702.905 26655.472 4383.75555 7747.1834 1844.7458 53193.487 58286.140 976421.09 11510.594 1863.6049
0.193817 0.181968 0.243838 0.252623 0.343645 0.146851 0.169113 0.147304 0.225005 0.328812
54.681 86.856 30.700 93.109 192.58 5.0196 4.1555 12.163 29.257 141.27
performed geometry optimization by using the GULP code19 on the crystals of mallandrite, Na2SiF6,20 fluorapatite, Ca5(PO4)3F,21 cuspidine, Ca4Si2O7F2,22 and nachaphite, Na2CaPO4F.23 The structural parameters are accurately reproduced (see Table 3), with errors below 5.5%. Also the bond distances are generally accurate (
