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First‐principles calculations were performed to reveal an effect of Ca vacancies on the stability of substitutional divalent cations M2+ (M = Mg, Zn, Sr) in Ca‐deficient hydroxyapatite (dHAp). M2+ concentrations up to 20 mol% in dHAp were considered, and the most stable substitutional sites and their lowest energy configurations in the dHAp lattice were examined with the aid of a generic algorithm method. It was found that defect formation energies of substitutional M2+ are lower in dHAp than in stoichiometric HAp (sHAp) at all M2+ concentrations. This indicates that these M2+ ions are more favorably involved in dHAp than in sHAp, which is in reasonable agreement with experiment. Detailed analyses on atomic structures in dHAp show that the presence of a Ca vacancy varies its surrounding Ca–O bond lengths over a wide area so that Ca–O polyhedrons with various sizes are produced. As a result, M2+ ions can predominantly occupy Ca sites at which M2+ fits better, depending on the ionic radii of M2+. For Zn2+ substitution in dHAp, its defect formation energy decreases more with the increasing concentrations and has the minimum value at 15 mol%. Such a trend can be understood from changes in effective coordination numbers of Zn in dHAp. [ABSTRACT FROM AUTHOR]

Electronic and atomic structures of interfaces between TiN coatings and Al2O3–WC composites were investigated by Density functional theory (DFT) calculations. As typical examples, interfacial orientation relationships of TiN(001)/Al2O3(0001), TiN(001)/WC(0001), TiN(111)/Al2O3(0001), and TiN(111)/WC(0001) were chosen. It was found that the TiN(111)/Al2O3(0001) and TiN(111)/WC(0001) interfaces have interfacial structural units of six‐fold coordinated triangular prisms centered on Ti atoms. In contrast, those of the interfaces with TiN(001) tend to have more distorted structure units due to their larger lattice misfits. Theoretical works of separation showed that interfacial strength is much more increased at the TiN(111) interfaces, as compared to those at the TiN(001) interfaces. Accordingly, experimental controls of TiN‐coating orientations on Al2O3–WC composites were attempted by using the cathodic arc ion plating method. It was found that orientations of (111) in the TiN coatings can be more enhanced and then interfacial mechanical strengths and hardnesses of the TiN coatings can increase more with rising bias voltages. [ABSTRACT FROM AUTHOR]

The phase stability of γ‐P3N5 and the possible formation of new phosphorus nitrides were investigated via high‐pressure in situ Raman spectroscopy, X‐ray diffraction measurements, and first‐principles calculations up to approximately 80 GPa. In this study, γ‐P3N5 was synthesized via the direct nitridation of black phosphorus at a pressure approximately above 12 GPa. The Raman spectrum, bulk modulus (K0 = 130.27(43) GPa), and compression behaviors (order of axial compressibility: βc > βa > βb) were experimentally measured for the first time. These experimental results were in good agreement with those of first‐principles calculations. Our high‐pressure in situ measurements and first‐principles calculations revealed that γ‐P3N5 persisted up to 80 GPa at room temperature. The compression of γ‐P3N5 proceeded with the folding of the layer consisting of the corner‐ and edge‐sharing PN4 and PN5. The present findings indicate that the P–N bonding with a low coordination number (PN4 and PN5) is preferable and stabilized for phosphorus nitride over a wide pressure range. However, laser heating between 67 and 70 GPa in the presence of nitrogen resulted in the formation of new PxNy, which included the possibility of a new high‐pressure P3N5 phase. The Raman scattering measurements along with the decompression demonstrated that the local structure of the newly synthesized PxNy metastably persisted at atmospheric pressure. The present experimental and theoretical studies on phosphorus nitrides offer new insights into the high‐pressure behaviors of covalent compounds consisting of highly coordinated polyhedra. [ABSTRACT FROM AUTHOR]

First‐principles calculations were performed to examine defect formation energies and site preference of substitutional divalent cations M2+ (M = Mg, Cu, Zn, Cd, Sr, Pb, and Ba) in hydroxyapatite (HAp, Ca10(PO4)6(OH)2) and carbonated apatite (CAp). All inequivalent substitutional sites of CO32- and M2+ were investigated to determine their most preferential sites. For all M2+ studied, their defect formation energies for the most stable substitutional sites were lower in CAp than in hydroxyapatite (HAp), demonstrating that M2+ are preferentially substituted into CAp over HAp. For Ca sites in CAp, correlations between the defect formation energies and Ca‐O bond lengths showed that bigger and smaller M2+ than Ca2+ are preferentially substituted for Ca sites with longer and shorter bond lengths than those in HAp, respectively. In addition, Ca sites with lower coordination numbers than 6 are preferentially substituted by Zn2+ and Cu2+ that originally tend to form 4‐ or 5‐fold coordination in their phosphate crystals. CO32- substitution is therefore likely to effectively stabilize substitutional foreign ions by modifying bond lengths and coordination numbers of Ca sites from those in pure HAp. These effects may play an important role in enhancing the M2+ solubility into CAp. [ABSTRACT FROM AUTHOR]

Vacancies, impurities, and foreign ions dissolving in calcium phosphate bioceramics play an important role in the biological properties of the materials. However, little is known about the thermodynamic stability of the defects. In this regard, point defects in hydroxyapatite (HAp) and octacalcium phosphate (OCP) were calculated in a first-principles manner, and the chemical-potential dependence of the defect formation energies was revealed. In particular, because calcium phosphates are usually subjected to an aqueous solution, a methodology to evaluate ionic chemical potentials under chemical equilibrium of the solid–aqueous solution was introduced. In the present article, recent results based on such a methodology (the solution pH dependence of Ca/P molar ratio of HAp and the ion-exchange ability with foreign cations in HAp and OCP) were reviewed. [ABSTRACT FROM AUTHOR]

Compression tests were conducted at 1400°C in air for undoped and Si-doped yttria-stabilized zirconia (YSZ) bicrystals with the same orientation relationship (Σ = 5, [001]/‘210’ grain boundary). It was found that the macroscopic grain-boundary slidings are observed during the deformation in both undoped and doped bicrystals, and the sliding displacements increase almost linearly with increasing total displacements. It is distinctly demonstrated that the Si-doped bicrystal exhibits a sliding displacement that is a few times larger than the undoped bicrystal. In addition, the total strain of the Si-doped bicrystal at a failure is much larger than that of the undoped bicrystal. It can be stated that Si doping in YSZ not only enhances the grain-boundary sliding but also suppresses its failure. [ABSTRACT FROM AUTHOR]

Symmetrical Σ7 tilt grain boundaries of alumina (Al[sub 2]O[sub 3]) were studied using bicrystals. Three types of Σ7 boundaries were successfully fabricated, that is, rhombohedral twin (Σ7{1&1mac;02}) and two types of [0001] symmetrical tilt grain boundaries with grain-boundary planes {4&5macr;10} and {2&3macr;10} (Σ7{4&5macr;10} and Σ7{2&3macr;10}). Their atomic structures and grain-boundary energies were investigated using high-resolution transmission electron microscopy (HRTEM) and a thermal grooving technique, respectively. HRTEM observations showed that the Σ7{1&1macr;02} boundary had a completely symmetrical atomic arrangement with respect to the grain-boundary plane. In contrast, Σ7{2&3macr;10} and Σ7{4&5macr;10} boundaries exhibited asymmetrical atomic structures, which were confirmed by analyzing the atomic configurations using static lattice calculations. Thermal grooving experiments showed that the grain-boundary energies strongly depended on the properties of the grainboundary planes. [ABSTRACT FROM AUTHOR]

Performs molecular dynamics simulations of amorphous silicon nitride containing boron. Examination of short-range atomic arrangements and of amorphous Si-B-N systems; Self-diffusion constant of nitrogen in Si-B-N; Importance of boron in decreasing the mobility of atoms in amorphous Si-B-N.

Investigates the crystallization behavior of organometallic-precursor-derived amorphous silicon carbonitride ceramics. Use of x-ray diffractometry in investigating the crystallization behavior; Preparation of amorphous Si-C-N ceramics; Factors affecting crystallization.