Structure of Cerium Phosphate Glasses: Molecular Dynamics Simulation

Abstract

We performed molecular dynamics (MD) simulations of the structure and properties of cerium-containing phosphosilicate, aluminophosphate, and aluminophosphosilicate glasses based on recent spectroscopic data that revealed 95% of cerium ions in such glasses are Ce 31 . New Ce 31 -and Ce 41 -O 2À potentials were developed and used in the MD simulations of these ceriumcontaining glasses with mixed glass formers. The local environments around cerium ions and network-forming cations, the medium range structure including glass-forming network Q n distribution, clustering, and second coordination shell around cerium ions have been carefully characterized. The results showed a longer Ce-O bond length and larger coordination number for Ce 31 than Ce 41 (2.48 and 6.4 vs. 2.24Å and 5.8, respectively). Around 5% of Si 41 ions were found to be in fivefold and sixfold coordination states in cerium phosphosilicate glasses, rather than the usual fourfold in silicate glasses. At the same time, the silicon-oxygen polyhedra were highly polymerized (over 80% of Q 4 ) due to the presence of phosphorus oxide. Aluminum ions were found to be coordinated by four-, five-, and six oxygen ions, with an average coordination number of around 4.2. In both oxidation states, cerium ions were found to be preferentially surrounded by phosphorus-oxygen tetrahedra, which form a kind of solvation shell around them. The preference of network-forming cations around cerium ions in the second coordination shell was found to decrease in the sequence phosphorus, aluminum, silicon.

Effect of strontium inclusion on the bioactivity of phosphate-based glasses

“…Although the cooling rates for both classical and first-principles models are much faster than those used experimentally, models prepared using these methodologies have given structures in agreement with experiment for silicate [20,[43][44][45] and phosphate glasses [28,46], including for compositions intended for implantation. …”

Characterization of Y2O3 and CeO2 doped SiO2-SrO-Na2O glasses

“…It can also be seen that the Ce effect on Si-O environment is not affected by the presence of Y in the glass as the peak shifts and shoulders are similar in both YCe and LCe containing 4 mol% Ce each. This Q-species distribution is possibly due to the multivalent states and range in coordination environments adopted by Ce ions in glasses [9,45]. The compositional comparison as determined by XPS maintains a consistent reduction of Si and an increase in Na.…”

“…Y and Ce are similar in ionic size to Na, however, as trivalent and tetravalent ions their associated field strength is higher which may impart a covalent characteristic to the O atoms they associate with within the glass network. In addition, the coordination environment of both Y and Ce in glasses has been found to be between 6 and 8 where they preferentially associate with NBOs [45,52,53]. These characteristics are likely producing the increase seen in Tg where stronger Y-NBO and Ce-NBO association with multiple NBOs increase the stability, and likely has implications for their movement through the glass structure in vivo which has been observed through lack of release in SBF studies [9,34,54].…”

“…Brought to you by | MIT Libraries Authenticated Download Date | 5/12/18 6:20 AM is seen for Si [45], therefore is not expected to serve in typical Si tetrahedral positions, though may produce an analogous forming effect. NC calculations, assuming that the Y and Ce are incorporated into the glass network in a forming role, suggests Y will produce a greater increase as a result of the required charge compensation, and Ce will produce an increase in NC similar to that of Si.…”

Rationalizing the Biodegradation of Glasses for Biomedical Applications Through Classical and Ab-initio Simulations

“…Besides the bone-bonding properties, in many biomedical applications one is also interested in the rate of leaching of specific cations from the glass. For instance, active ions such as cerium, gallium, zinc, strontium and cobalt are incorporated in bioactive glasses with the purpose of delivering and gradually releasing them to a target tissue [42,[71][72][73][74][75][76][77][78]; also, yttrium and other ions are employed as radiation sources in in-situ radiotherapy, and silicate glasses are amongst the most common carriers for these radioisotopes [79,80]. In the latter applications, it is of vital importance that the radioactive species are confined within the glass carrier during the treatment, because their release in the bloodstream would have adverse effects [81].…”