Sitaram Aryal

Density functional calculations of the electronic structure and optical properties of aluminosilicate polymorphs (Al2SiO5)

Abstract The density functional theory (DFT) based orthogonalized linear combination of atomic orbitals (OLCAO) method is used to study the electronic structure and spectroscopic properties of three alumino silicate polymorphs (Al2SiO5), andalusite, sillimanite, and kyanite. These polymorphs are precursors to mullite, which is an excellent refractory material. The electronic structure results include the band structure, total and partial density of states, bond order, and Mullikan effective charge (Q*), whereas the spectroscopic properties include X-ray absorption near-edge-structure (XANES), and the complex optical dielectric functions (ε1 + iε2) for each polymorph. For the XANES calculations, we use a supercell approach and account for the electron-core hole interaction. Our calculations show that the polymorphs are insulators with direct band gaps of 5.05, 5.21, and 5.80 eV for andalusite, sillimanite, and kyanite, respectively. The calculated refractive indices (n) for each material are in agreement with experimental values from the literature. Results on the XANES spectral calculations (K-edge and L-edge) for all crystallographically nonequivalent ions in the polymorphs are presented. It is shown that to achieve excellent agreement with the experimentally measured spectrum, a weighted sum of the spectra from crystallographically inequivalent sites must be used. The analysis of the XANES spectra based on differences in the local bonding environments are provided.

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.

Atomic picture of elastic deformation in a metallic glass

The tensile behavior of a Ni60Nb40 metallic glass (MG) has been studied by using ab initio density functional theory (DFT) calculation with a large cell containing 1024 atoms (614 Ni and 410 Nb). We provide insight into how a super elastic limit can be achieved in a MG. Spatially inhomogeneous responses of single atoms and also major polyhedra are found to change greatly with increasing external stress when the strain is over 2%, causing the intrinsically viscoelastic behavior. We uncover the origin of the observed super elastic strain limit under tension (including linear and viscoelastic strains) in small-sized MG samples, mainly caused by inhomogeneous distribution of excess volumes in the form of newly formed subatomic cavities.