Ab initio simulation of elastic and mechanical properties of Zn- and Mg-doped hydroxyapatite (HAP).

Hydroxyapatite (HAP) is an important bioceramic which constitutes the mineral components of bones and hard tissues in mammals. It is bioactive and used as bioceramic coatings for metallic implants and bone fillers. HAP readily absorbs a large amount of impurities. Knowledge on the elastic and mechanical properties of impurity-doped HAP is a subject of great importance to its potential for biomedical applications. Zn and Mg are the most common divalent cations HAP absorbs. Using density function theory based ab initio methods, we have carried out a large number of ab initio calculations to obtain the bulk elastic and mechanical properties of HAP with Zn or Mg doped in different concentration at the Ca1 and Ca2 sites using large 352-atom supercells. Detailed information on their dependece on the concetraion of the substitued impurity is obtained. Our results show that Mg enhances overall elastic and bulk mechanical properties whereas Zn tends to degrade except at low concentrations. At a higher concentration, the mechanical properties of Zn and Mg doped HAP also depend significantly on impurity distribution between the Ca1 and Ca2 sites. There is a strong evidence that Zn prefers Ca2 site for substituion whereas Mg has no such preference. These results imply that proper control of dopant concentration and their site preference must carefully considered in using doped HAP for specific biomedical applications.

Densification of a continuous random network model of amorphous SiO2 glass.

We have investigated the mechanism of densification of a nearly perfect continuous random network (CRN) model of amorphous SiO2 (a-SiO2) glass with 1296 atoms and periodic boundary conditions. The model has no under- or over-coordinated atoms and small bond length and bond angle distributions. This near-perfect model is systematically densified up to a pressure of 80 GPa using ab initio constant-pressure technique. By assessing a full spectrum of properties including atomic structure, bonding characteristics, effective charges, bond order values, electron density of states, localization of wave functions, elastic and mechanical properties, and interband optical absorption at each pressure, we reveal the pertinent details on the structural, mechanical and optical characteristics of the glass model under pressure. They all confirm the central theme that amorphous to amorphous phase transformation (AAPT) from a low-density state to a high-density state is at a pressure between 20 and 35 GPa in this nearly ideal a-SiO2 network. This pressure range represents an upper limit for such a transition in vitreous silica. The phase transformation roots from the change of Si-O bonding from a mixture of ionic and covalent nature at low pressure to a highly covalent bonding under high pressure. In addition, the calculated theoretical refractive index of the glass model as a function of the pressure is reported for the first time and in good agreement with the available experimental data.