The atomic level mechanism of incorporation of Zn(2+) into hydroxyapatite (HAp), which is a potential dopant to promote bone formation, was investigated, based on first principles total energy calculations and experimental X-ray absorption near edge structure (XANES) analyses. It was found that Zn(2+)-doped HAp tends to have a Ca-deficient chemical composition and substitutional Zn(2+) ions are associated with a defect complex with a Ca(2+) vacancy and two charge compensating protons. Moreover, first principles calculations demonstrated that Zn(2+) incorporation into HAp can take place by occupying the Ca(2+) vacancy of the defect complex. The Ca(2+) vacancy complex is not only the origin of the calcium deficiency in HAp, but also plays a key role in the uptake of trace elements during mineralization.

First-principles calculations are performed for hydroxyapatite ͑Hap͒ in order to investigate the relative ion exchange ability with divalent cations such as Mg 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Sr 2+ , Cd 2+ , Ba 2+ , and Pb 2+. Their ionic substitutional energies are calculated from total energies of supercells and chemical potentials for Ca 2+ and the foreign cations determined under chemical equilibrium between HAp and its saturated solution. It is found that, in most cases, the ion exchange ability is basically dependent on the ionic sizes and the larger or smaller-sized cations than Ca 2+ tend to exhibit more difficulty of substitution for Ca 2+ in HAp. However, Pb 2+ exhibits the extremely small substitutional energy, which originates from covalent bonds with the adjacent oxygen atoms. In particular, Pb 2+ at the Ca-1 site of HAp has covalency with the second-nearest-neighboring oxygen atoms as well as the first nearest-neighboring ones. The covalent bond formation plays an important role for the distinct ion exchange ability for Pb 2+ by HAp observed experimentally.

Novel ternary nitride semiconductor MgSnN 2 was synthesized using the metathesis reaction under high pressure (P = 5.5 GPa/T = 850°C/1 h). MgSnN 2 obtained in this study showed a rocksalt structure, although we have reported that ZnSnN 2 synthesized using a similar method has a wurtzite structure. The (111) plane of MgSnN 2 with a rocksalt structure is expected to match well with GaN (0001). The band gap of MgSnN 2 is estimated to be 2.3 eV and it shows a distinct [a] High pressure group,

Silicon dioxide has eight stable crystalline phases at conditions of the Earth's rocky parts. Many metastable phases including amorphous phases have been known, which indicates the presence of large kinetic barriers. As a consequence, some crystalline silica phases transform to amorphous phases by bypassing the liquid via two different pathways. Here we show a new pathway, a fracture-induced amorphization of stishovite that is a high-pressure polymorph. The amorphization accompanies a huge volume expansion of ~100% and occurs in a thin layer whose thickness from the fracture surface is several tens of nanometers. Amorphous silica materials that look like strings or worms were observed on the fracture surfaces. The amount of amorphous silica near the fracture surfaces is positively correlated with indentation fracture toughness. This result indicates that the fracture-induced amorphization causes toughening of stishovite polycrystals. The fracture-induced solid-state amorphization may provide a potential platform for toughening in ceramics.

First-principles calculations are performed to investigate atomic and electronic structures of Sr(2+) ions substituting for Ca(2+) in octacalcium phosphate (OCP). The defect formation energies are evaluated from total energies of supercells and ionic chemical potentials of Sr(2+) and Ca(2+) determined under the chemical equilibrium with aqueous solution saturated with hydroxyapatite (HAp). The defect formation energy depends on the solution pH and the substitutional Ca sites in OCP, and the estimated equilibrium concentrations of Sr(2+) in OCP and HAp are in reasonable agreement with previous experimental results obtained in physiological conditions. It is also found that Sr(2+) ions can be more favorably substituted in OCP than in HAp. It is thought, therefore, that Sr(2+) plays its role to promote bone formation by being incorporated into the metastable OCP phase occurring during HAp nucleation.