A first-principles calculation is a computational method predicting electronic states and physical properties of a target material based on the basic principles of quantum mechanics without using empirical parameters. In recent decades, a density-functional-theory calculation has often been used in many fields of physics, chemistry, and materials science. In this article, we introduce two case studies on theoretical analysis using first-principles calculations for (1) interstitial N and H defects in Si crystal growth, and (2) Ge₁-_xSn_x alloy (0.000 < x < 1.000) with informatics assistance.

  We developed a numerical simulation model of intrinsic point defect dynamics considering the effects of the dopant concentration and thermal stress in a silicon crystal growing by the Czochralski (Cz) method. The thermal equilibrium concentration of the intrinsic point defects (vacancy, V, and self-interstitial Si atom, I) was determined as a function of the incorporated dopant concentration and the thermal stress in a growing Si crystal, which was obtained through ab-initio calculations. Furthermore, point defect dynamics in the crystal were solved within a two-dimensional axisymmetric global heat and mass transport model by considering the thermal stress and the incorporation of a dopant using segregation for Si crystal growth by the Cz method. The numerical simulations showed that the formation and the distribution of intrinsic point defects significantly depend on the incorporated dopant concentration and the thermal stress in a growing Si crystal.

  Highly radiopure NaI(Tl) crystal has been developed to investigate cosmic dark matter. The dark matter problem is one of the essential subjects in particle physics and cosmology. Despite the many null results in the dark matter search, one group (DAMA/LIBRA) reported a significant annual modulation in the energy spectrum, which can be interpreted as the dark matter signal. This unknown signal must be a purer NaI(Tl) crystal. The PICOLON project has developed a highly radiopure NaI(Tl) crystal. They purified the NaI(Tl) crystal with combined methods; re-crystallization and resin. The ⁴⁰K were effectively removed from the NaI by the re-crystallization method. On the other hand, they successfully removed ²¹⁰Pb by selecting the appropriate resin. The concentration of radioactive impurities in their NaI(Tl) crystal reached below their goals.

  Physical properties of ferroelectric oxides are governed by point defects including oxygen vacancies (V_O^••). Defect chemistry has been well established in the paraelectric phase including high-temperature equilibrium states but also low^-temperature quenched ones. However, the interaction between spontaneous polarization and points defects is still not clear. Here, we describe how transition-metal acceptors and V_O^•• interact with each other in ferroelectric BaTiO₃ lattices. An attractive interaction occurs between Fe, Mn and Cu ions with V_O^••, whereas a repulsive interaction does between V (vanadium) ions and V_O^••. Moreover, defect design for controlling the formation and the alignment of defect dipoles composed of Cu³⁺ and V_O^•• in ferroelectric BaTiO₃ ceramics is capable of achieving an effective relative dielectric permittivity of 7,000, which is over twice as large as that of undoped ones.

  X-ray fluorescence holography (XFH), which can record three-dimensional atomic structures around specified elements, has a history of a quarter century, and it has investigated middle-range local structures around dopants in state-of-the-arts materials, so far. In this review, the background of XFH is introduced, and the theory and experimental setups for two different modes of XFH are described. Then, two examples of our recent applications, such as a significant displacement of Ca in ferroelectric Ba_0.9Ca_0.1TiO₃ single crystal and a peculiar local structure around Mn in high Tc ferromagnetic semiconductor Mn:ZnSnAs₂ thin film, are shown. Finally, ongoing researches and the perspective of XFH are stated.

  Performance of functional materials, such as semiconductors, can be controlled by foreign element doping. Thus, positions of dopants in lattices and effects of doping to lattices must be important to understand origins of properties. The atomic structures around dopants are called local structures. One of direct probes for local structures is atomic resolution holography. In this article, we report importance of atomic resolution holography for materials science, in particular, newly developed neutron holography using white neutrons, which is an effective probe for observation of local structures of light elements.

  Principle of three-dimensional atom probe (3DAP) is summarized, and an analysis of 3D distribution of oxygen atoms segregated at Σ9{114} grain boundaries (GBs) in silicon ingots, within a high spatial resolution of less than 0.5 nm by a correlative microscopy using 3DAP assisted by scanning transmission electron microscopy and a focused ion beam (FIB) processing operated at −150 °C, is demonstrated. The analysis reveals a segregation of oxygen atoms within a range of 2.5 nm across the GB plane, which is much narrower in comparison with the previous reports obtained using the conventional FIB processing operated at room temperature. The oxygen concentration profile accurately reflects the distribution of the segregation sites, which exist at bond-centered sites under tensile stresses above 2 GPa, as calculated by ab initio local stress calculations. The results strongly support the usefulness of this correlative technique to analyze the interactions between GBs and impurity atoms at an atomistic level.