Hajime Kimizuka

The adiabatic elastic constants ( C(ij)) of cristobalite have been evaluated successfully over the temperature range of 300-1800 K using the molecular-dynamics method with a fluctuation formula. Cristobalite shows a negative Poisson ratio over this temperature range. However, the mechanisms differ between the alpha and beta phases. In the cubic beta phase, C44 exhibits a value extremely close to C11 rather than C12, in contrast to the Cauchy relation. This predicts a remarkable property that the

Here we explicitly present the diffusion coefficients ($D$) and activation energies (${E}_{a}$) of interstitial H in $\ensuremath{\alpha}$-Fe over a temperature range of 100 to 1000 K. These values were predicted by applying path-integral molecular dynamics modeling based on first principles. The obtained $D$ and ${E}_{a}$ values exhibit clear non-Arrhenius temperature dependence and a transition from quantum to classical behavior at around 500 K. Our results show that the quantum effects not on

We calculated all the independent elastic constants of $\ensuremath{\alpha}$-quartz under hydrostatic pressure up to $20\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ using density functional theory. The predicted pressure-dependent elastic behavior differs significantly from a recent Brillouin spectroscopy measurement [E. Gregoryanz et al., Phys. Rev. Lett. 84, 3117 (2000)], but is consistent with x-ray data in the literature.

Here we demonstrate and characterize the H-diffusion behavior around a screw dislocation in body-centered cubic (bcc) $\ensuremath{\alpha}$-Fe by performing path-integral molecular dynamics modeling and adopting an ab initio--based potential. Counterintuitively, our results indicate that the H diffusivity along the dislocation line is significantly lower than lattice diffusion. Thus, the ``fast'' pipe diffusion does not occur for H in $\ensuremath{\alpha}$-Fe.

We have presented the molecular-dynamics (MD) results for the temperature dependence of the adiabatic elastic constants ${C}_{\mathrm{ij}}$ of $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ quartz, using a statistical fluctuation formula. It is noteworthy that the calculated ${C}_{\mathrm{ij}}$ values are in a good agreement with the experimental values in the entire temperature range of 300--1100 K, including the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\beta}$ phase-transition region. We

Theoretical prediction of the heterogeneous atomic structures in multicomponent alloys is one of the most challenging issues in materials science. Here we present a first-principles demonstration of this by constructing an on-lattice effective multibody potential model to describe the energetics of hexagonal close-packed Mg crystals containing stacking faults (SFs) with Al and Gd substitutions. Remarkably, we showed that our intuitive model can describe the segregation of solute atoms to SFs and

Understanding the underlying mechanism of the nanostructure-mediated high diffusivity of H in Pd is of recent scientific interest and also crucial for industrial applications. Here, we present a decisive scenario explaining the emergence of the fast lattice-diffusion mode of interstitial H in face-centered cubic Pd, based on the quantum mechanical natures of both electrons and nuclei under finite strains. Ab initio path-integral molecular dynamics was applied to predict the temperature- and stra

Predicting the equilibrium ordered structures at internal interfaces, especially in the case of nanometer-scale chemical heterogeneities, is an ongoing challenge in materials science. In this study, we established an ab-initio coarse-grained modeling technique for describing the phase-like behavior of a close-packed stacking-fault-type interface containing solute nanoclusters, which undergo a two-dimensional disorder-order transition, depending on the temperature and composition. Notably, this a

Abstract The structural and elastic changes with volume expansion in α ‐quartz and α ‐cristobalite, the typical polymorphs of silicon dioxide (SiO 2 ), have been studied through first‐principles calculations using the projector‐augmented‐wave (PAW) method, with particular emphasis on their negative Poisson's ratio behavior under negative pressure. The dominant mechanism of expansion is the increase of the Si–O–Si angles within their crystalline structures, whereas the SiO 4 tetrahedron undergoes

The behavior of H isotopes in crystals is a fundamental and recurrent theme in materials physics. Especially, the information on H diffusion over a wide temperature range provides a critical insight into the quantum mechanical nature of the subject; however, this is not yet fully explored. From state-of-the-art ab initio calculations to treat both electrons and nuclei quantum mechanically, we found that the temperature dependence of H isotope diffusivities in face-centered-cubic (fcc) Pd has an

Abstract The elastic properties of cristobalite and quartz, the typical polymorphs of SiO 2 , are studied with particular emphasis on the structural phase transition and the high‐temperature phase. Using the equilibrium molecular dynamics (MD) method with a stress‐fluctuation formula, we have successfully evaluated the adiabatic elastic constants ( C ij ) of both cristobalite and quartz in a wide temperature range, including the α – β phase‐transition region. In the results for cristobalite, the

Using density-functional theory, we computed all the independent elastic constants of coesite, a high-pressure polymorph of silica, as functions of pressure up to 15 GPa. The results are in good agreement with experimental measurements under ambient conditions. Also, the predicted pressure-dependent elastic properties are consistent with x-ray data in the literature concerning lattice strains at high pressures. We find that coesite, like quartz, exhibits a gradual softening of a shear modulus B4