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To accurately predict grain boundary (GB) atomic structures and their energetics in CdTe, the present study constructs an artificial-neural-network (ANN) interatomic potential. To cover a wide range of atomic environments, large amounts of density functional theory (DFT) data are used as a training dataset including point defects, surfaces and GBs. Structural relaxation combined with the trained ANN potential is applied to symmetric tilt and twist GBs, many of which are not included in the training dataset. The relative stability of the relaxed structures and their GB energies are then evaluated with the DFT level. The ANN potential is found to accurately predict low-energy structures and their energetics with reasonable accuracy with respect to DFT results, while conventional empirical potentials critically fail to find low-energy structures. The present study also provides a way to further improve the transferability of the ANN potential to more complicated GBs, using only low

An artificial-neural-network (ANN) interatomic potential trained with data from density-functional-theory (DFT) calculations is developed to reveal favorable modes of large-sized vacancy clusters i...

It was recently found that extremely large plasticity is exhibited in bulk compression of single-crystal ZnS in complete darkness. Such effects are believed to be caused by the interactions between dislocations and photoexcited electrons and/or holes. However, methods for evaluating dislocation behavior in such semiconductors with small dimensions under a particular light condition had not been well established. Here, we propose the "photoindentation" technique to solve this issue by combining nanoscale indentation tests with a fully controlled lighting system. The quantitative data analyses based on this photoindentation approach successfully demonstrate that the first pop-in stress indicating dislocation nucleation near the surface of ZnS clearly increases by light irradiation. Additionally, the room-temperature indentation creep tests show a drastic reduction of the dislocation mobility under light. Our approach demonstrates great potential in understanding the light effects on dislocation nucleation and mobility at the nanoscale, as most advanced technology-related semiconductors are limited in dimensions.

Chromium germanides, namely, Nowotny chimney-ladder-phase CrGe1.77 and MoSi2-type CrGe2, were synthesized above 15 GPa or more via laser heating using a diamond anvil cell (LHDAC). MoSi2-type CrGe2, which is the most Ge-rich compound in the Cr-Ge system, crystallizes in the tetragonal structure with a space group of I4/mmm (no. 139) and lattice parameters of a = 3.24919(6) A and c = 8.0523(3) A and is isostructural with MoSi2. MoSi2-type CrGe2 has a deep pseudogap caused by the splitting of 3d orbitals with Cr, as evidenced by ab initio calculation. In this article, we have succeeded in synthesizing a binary compound between transition-metal and metalloid elements for the first time at high pressures above 10 GPa using the LHDAC. This pathway opens the possibility to explore more compounds in this system and may provide new insights into the fundamental interaction between these two elements.

A simulated annealing (SA) method based on molecular dynamics is employed to reveal atomic structures of asymmetric tilt grain boundaries (ATGBs) in MgO. Σ5 and Σ13 ATGBs with the [001] tilt axis are systematically investigated. The ATGBs after SA simulations dissociate into saw-toothed nanofacets composed of multiple structural units. These nanofacets are lower in GB energy than those obtained from a γ-surface method with structural optimization, demonstrating the importance of SA-based methods for obtaining low-energy structures of ATGBs. For most of the Σ5 ATGBs, the nanofacets consist of only structural units of Σ5 symmetric tilt GBs (STGBs). For the Σ13 ATGBs studied, their nanofacets do not consist of only Σ13 STGBs but always contain non-Σ13 structural units, which probably results from a large difference between the excess volume of Σ13(510) and Σ13(320) STGBs. It is also found that ATGBs have a larger number of metastable structures whose GB energies are close to the lowest energy structure than STGBs, due to the fact that ATGB nanofacets are more tolerant of variation in facet junction, structural units and their arrangement. Consequently, the lowest energy structures have low probabilities of being formed than metastable structures.

Room-temperature creep tests of ZnS single crystals were conducted along the [001] direction under controlled light conditions in order to investigate the effect of light irradiation on the dislocation motion in ZnS. ZnS crystals stably deform by creep even at room temperature if the tests are conducted in complete darkness. It was found that the strain rate during a creep test can be immediately reduced by irradiating the sample with light having wavelengths of 365 nm or 436 nm even for the case when a sufficient density of mobile dislocations has been introduced due to preceding creep deformation. Surprisingly, the drop in strain rate by the light irradiation was several orders of magnitude. This drop in strain rate is believed to be due to a drastic decrease in the density of mobile dislocations, as well as the mobility of such dislocations, which are derived from the interaction with light-induced electrons and holes. It's also important to note that a significant incubation time appears when creep deformation resumes in the dark (light off) followed by loading under light irradiation during which the creep deformation is very small. This indicates that the light-induced free electrons and holes have long lifetimes in ZnS and, therefore, the existing dislocations continue interacting with electronic defects for several minutes after the light has been turned off.

Inorganic semiconductors show interesting mechanical behavior in response to their surrounding lighting environments. In fact, we recently found out that zinc sulfide (ZnS) shows the light-illumination controlled plasticity at room temperature. However, its mechanism is poorly understood. Here we report systematic density functional theory results of electronic and atomic structures of glide partial dislocations in ZnS. We have revealed that partial dislocations in ZnS have excess electrostatic fields localized around their cores and can trap electrons or holes, depending on excess ionic species at the cores. Such carrier-dislocation interactions can induce energetically more stable bond reconstruction at their cores, as compared with the case before carrier trapping. Reconstructed bonds at the dislocation cores should be broken upon dislocation glide, and thus can restrict the dislocation motion significantly. Such reduced dislocation motion due to core reconstruction should give rise to increased plastic deformation stresses, namely, hardening of the materials under light illumination. The present results provide a critical understanding of the experimentally observed light-illumination controlled plasticity in ZnS at the electronic level, which is opposed to previously reported plasticity and dislocation motion in other elemental and III-V semiconductors.

Grain boundaries (GBs) modulate the macroscopic properties in polycrystalline materials because they have different atomic and electronic structures from the bulk. Despite the progress on the understanding of GB atomic structures, knowledge of the localized electronic band structures is still lacking. Here, we experimentally characterized the atomic structures and the band gaps of four typical GBs in α-Al2O3 by scanning transmission electron microscopy and valence electron energy-loss spectroscopy (EELS). It was found that the band gaps of the GBs are narrowed by 0.5-2.1 eV compared with that of 8.8 eV in the bulk. By combing core-loss EELS with first-principles calculations, we elucidated that the band gap reductions directly correlate with the decrease of the coordination numbers of Al and O ions at the GBs. These results provide in-depth understanding between the local atomic and electronic band structures for GBs and demonstrate a novel electronic-structure analysis for crystalline defects.

Thermodynamic phase stabilities of α, β, γ and δ polymorphs of RE2Si2O7 (RE = Yb, Gd), promising candidates for environmental barrier coating systems of gas turbine engines in next generations, were investigated using harmonic and quasi-harmonic lattice dynamics in conjunction with ab initio calculations. By quantifying their Gibbs free energies with dynamics of atoms taken into account, we showed the phase stability of β-Yb2Si2O7 up to melting temperature under an ideal condition, and also under thermal stresses, ensuring the reliability of β-Yb2Si2O7 in environmental barrier coating systems. It was also found that the difference of anharmonic phonons in these polymorphs, which was measured as mode Gruneisen parameters, significantly changed their phonon free energies and invoked the formation of δ-Gd2Si2O7 at high temperature. These results enable a quantitative comprehension of the phase transformation of RE2Si2O7 and future discussions of other factors which may affect the phase stability such as lattice defects.

Grain boundaries often have a decisive effect on the macroscopic properties of polycrystalline materials, but the wide variety of their atomic structures, interfacial lengths, and compositions makes them difficult to characterize all-encompassingly. An indispensable example in which an understanding of the relationship between grain boundary structures and properties would greatly facilitate development of superior materials is thermal transport, especially with respect to microstructure evolution, thermoelectrics and thermal barrier coatings. To contribute to a more comprehensive understanding, we performed a systematic study of lattice thermal conduction across a wide range of symmetric tilt grain boundaries in MgO using perturbed molecular dynamics. It was found that thermal conductivities vary significantly with grain boundary structure but are strongly correlated with excess volume, which is a measure of the number density of atoms in the vicinity of the grain boundary planes. Real-space analysis revealed that dislocation densities determine the phonon transport paths and thermal conductivity in low-angle boundaries whereas it is the amount of open volume rather than the shape of structural units in high-angle boundaries that determine thermal conductivity. We also found that low-angle boundaries mainly reduce phonon transports at low frequencies whereas high-angle boundaries also reduce it at intermediate and high frequencies effectively, regardless of the shape of structural units. These insights are expected to be applicable to other close-packed oxide systems, and should aid the design of next-generation thermal materials through tailoring of grain boundaries.

Surface charge states of biomaterials are often important for the adsorption of cells, proteins, and foreign ions on their surfaces, which should be clarified at the atomic and electronic levels. First-principles calculations were performed to reveal thermodynamically stable surface atomic structures and their charge states in hydroxyapatite (HAp). Effects of aqueous environments on the surface stability were considered using an implicit solvation model. It was found that in an air atmosphere, stoichiometric {0001} and P-rich {100} surfaces are energetically favorable, whereas in an aqueous solution, a Ca-rich {100} surface is the most stable. This difference suggests that preferential surface structures strongly depend on chemical environments with and without aqueous solutions. Their surface potentials at zero charge were calculated to obtain the isoelectric points (pHPZC). pHPZC values for the {0001} surface and the Ca-rich {100} surface were obtained to be 4.8 and 8.7, respectively. This indicates that in an aqueous solution at neutral pH, the {0001} and Ca-rich {100} surfaces are negatively and positively charged, respectively. This trend agrees with experimental data from chromatography and zeta potential measurements. Our methodology based on first-principles calculations enables determining macroscopic charge states of HAp surfaces from atomic and electronic levels.

First-principles lattice dynamics is applied to symmetric tilt grain boundaries (GBs) in Al, Si and MgO, with the goal of revealing critical factors in determining excess vibrational entropies at the atomic level. Excess vibrational entropies at GBs are found to vary depending on the substances. Al GBs tend to show larger excess entropies and hence larger temperature dependence of the GB free energies than those in Si and MgO. Most of the Si GBs show small excess entropies. For Al and MgO, atom-projected vibrational entropies are well correlated with bond-length changes at GB cores, and have large positive values as bond lengths increase for GB atoms. This demonstrates that a similar mechanism likely dominates excess vibrational entropies of GBs for both substances, despite their dissimilar bonding nature. For Si GBs, atoms with threefold coordination do not simply follow such a correlation, implying the importance of other factors that are different from bond-length changes. These systematic comparisons will be a foothold for understanding a physical origin of excess entropies at GBs even in more complex substances.

Grain boundary (GB) migration plays an important role in modifying the microstructures and the related properties of polycrystalline materials, and is governed by the atomistic mechanism by which the atoms are displaced from one grain to another. Although such an atomistic mechanism has been intensively investigated, it is still experimentally unclear as to how the GB migration proceeds at the atomic scale. With the aid of high-energy electron-beam irradiation in atomic-resolution scanning transmission electron microscopy, we controllably triggered the GB migration in α-Al

First-principles calculations were performed to investigate electronic and atomic structures of edge and screw dislocations in NaCl and AgCl. 1/2 edge and 1/2 screw dislocations, which are the ones due to the easiest slip system of these crystals, were treated in a first-principles manner. It was found that the edge dislocation has extended core structures over several atomic planes in both NaCl and AgCl. In contrast, the screw dislocation in AgCl showed the different shear strain around the dislocation core from that in NaCl although the one has a similar atomic structure in the two crystals. Local electronic structures around the dislocation cores in AgCl exhibit larger bonding interactions between atomic orbitals than those in NaCl, indicating that covalent interactions are more significant at the dislocation cores of AgCl. From excess dislocation energies, it was found that the dislocations in AgCl are much more stable than those in NaCl. Especially, the screw dislocation of AgCl has...

This study focuses on transformation of grain boundary (GB) structures under high pressures up to 60 GPa by using a simulated annealing technique with molecular dynamics and lattice statics calculations for various symmetric tilt GBs (STGBs) of MgO. It is found that except for the Σ 3 ( 111 ) / [ 1 1 ¯ 0 ] that is a rather stable GB, all the STGBs studied transform into a metastable structure more than once at threshold pressures. In addition, the GBs with an open-core structure and small tilt angle are found to be more “flexible” to transform into different structures than the GBs with a dense structure. For polycrystalline MgO, therefore, GBs may also exhibit GB transformation under high pressures and flexible GBs may govern overall transformation and deformation. These findings also suggest that polycrystals sintered at high pressures consist of more pressure-resistant GBs than those at normal pressures.

La-excess compositions of lanthanum silicate apatites (La9.33+0.67xSi6O26+x or La9.33+0.67xSi6–0.5xO26; x > 0) exhibit higher oxide-ion conductivity than basic composition (La9.33Si6O26) due to smaller activation energies. On the contrary, the activation energies for oxide-ion conduction tend to increase with more La vacancies, less La content, at the La 4f site. However, the effect of La vacancies on the activation energy of oxide-ion conduction is not fully understood. In the present work, first-principles calculations are performed for La9.33+0.67xSi6O26+x (x = 0.0, 0.5, 1.0) to clarify the relationship between the amount of La vacancy at the La 4f site and the activation energy of oxide-ion migration in the O4 channel. It is found that the potential barrier of oxide-ion migration is extremely low in La10Si6O26 (x = 1.0) whereas that becomes higher with increasing the amount of La 4f vacancies. This is closely related to sizes of La 6h triangles that surround the O4 channel. In the presence of more La 4f vacancies, La 6h triangles tend to be more expanded than those in La10Si6O26. Since migrating oxide ions have attractive interactions with their surrounding La 6h ions, more expanded La 6h triangles lead to more energy costs for atomic displacements of the La 6h ions during the oxide-ion migration, resulting in their higher potential energy barriers. The effect of La 4f vacancies on the oxide-ion migration can be explained by such local atomic arrangements of La 6h and their interactions with migrating oxide ions.

Interstitial oxide-ion conduction in tetragonal scheelite-structured oxides has theoretically been analyzed using first-principles calculations, taking lanthanum niobate (t-LaNbO4) as a model system. In the crystal, two interstitial sites of oxide ions were identified by exhaustive structural optimizations, which are located between two NbO4 tetragonal units to form an Nb2O9 unit. Among the four elementary paths found by the nudged elastic band method, the three common paths enable interstitial oxide ions to migrate over a long range both in the ab-plane and along the c-axis, leading to the almost isotropic oxide-ion conductivity. However, the oxide-ion conductivities estimated on the basis of the kinetic Monte Carlo method were much higher than the reported experimental values. The apparent activation energies were also different, ∼0.4 eV in the estimated conductivities vs. ∼0.9 eV in the experimental conductivities. As a result of additional evaluation of the trapping effect by dopants, interstitial oxide ions were found to be strongly trapped by W dopants on La sites. The corrected oxide-ion conductivity was drastically reduced by the trapping effect, finally to be in reasonable agreement with the experimental conductivities.

To find out growing rules for huge clusters of self-interstitials detected in as-grown Si crystals, i.e., Frank partial dislocation loops (FDLs), approach processes forming a di-interstitial from two individual self-interstitials are investigated by ab initio calculations. Interactions between two self-interstitials in a Si crystal are analyzed by changes of both an energy and a stress around the defects in their relative configurations. Interestingly, relations of energy and stress of two self-interstitials that are adjacent to each other show systematic changes and are categorized into three groups. Characteristic structures of di-interstitials are found as rules for constructing FDLs by collecting self-interstitials one by one.

First-principles lattice dynamics calculations were performed to reveal an atomic-level origin of excess vibrational entropies at grain boundaries (GBs) in MgO. Fourteen symmetric tilt GBs with various structural units were systematically examined. The excess vibrational entropies were found to vary depending on the individual GBs, and as a result, the relative thermodynamic stability of the GBs studied changed with temperature. The excess GB volumes were less correlated with the excess vibrational entropies. By contrast, classifying ions in terms of their coordination numbers, bond-length changes at GBs were well correlated with atom-projected excess vibrational entropies for both Mg and O ions. Bond-length changes at GBs, which originate from changes in bond strength, are therefore a critical, well-defined descriptor for evaluating excess vibrational entropies and thereby excess free energies in MgO.

Artificial neural-network (ANN) interatomic potentials for simulating atomic structures and energetics of grain boundaries (GBs) in silicon were constructed and integrated into structural optimization and molecular dynamics (MD) algorithms. A training dataset including various atomic environments of symmetric tilt GBs was generated by performing density-functional-theory (DFT) calculations. The ANN potential after training was found to be capable of approximating the potential-energy surface at GBs even with dangling bonds and large atomic displacements at high temperatures, which cannot be well reproduced with empirical interatomic potentials. Additionally, reliability of the ANN potential for molecular simulations was evaluated. GB structures optimized or equilibrated by the ANN molecular simulations were also energetically lower for DFT calculations, without significant errors. The ANN potential is therefore expected to greatly reduce structural-optimization iterations and required time steps to acquire stable or equilibrium GB structures in MD simulations, enabling us to address even large-scale systems of general GBs in silicon, with high accuracy and low computational cost.

In order to reveal the conduction mechanisms of interstitial oxide ions (Oint ions) in neodymium silicate apatite, the low-energy sites and conduction pathways have been determined using first-principles calculations. Two inequivalent Oint sites are found close to the c axis and at the periphery of the O4 channel. The potential barriers of Oint ions are then evaluated by the nudged elastic band method along three types of conduction pathways connecting the Oint sites, i.e., two pathways along the c axis and one pathway in the ab plane. Along the c axis, the interstitialcy mechanism shows the lowest potential barrier of 0.57 eV, which dominantly contributes to fast long-range conduction along the c axis. The Oint conduction in the ab plane is also governed by the interstitialcy mechanism, whose potential barrier largely depends on the configuration of Nd vacancies in the range of 0.38–0.75 eV.

Basal dislocations having a Burgers vector of 1/3 in zinc oxide (ZnO) with the wurtzite structure are known to strongly affect physical properties in bulk. However, the core structure of the basal dislocation remains unclear. In the present study, ZnO bicrystals with a {2 1 ¯ 1 ¯ 0}/ 2° low-angle tilt grain boundary were fabricated by diffusion bonding. The resultant dislocation core structure was observed by using scanning transmission electron microscopy (STEM) at an atomic resolution. It was found that a basal edge dislocation in α-type is dissociated into two partial dislocations on the (0001) plane with a separation distance of 1.5 nm, indicating the glide dissociation. The Burgers vectors of the two partial dislocations were 1/3 and 1/3, and the stacking fault between the two partials on the (0001) plane has a formation energy of 0.14 J/m2. Although the bicrystals have a boundary plane of {2 1 ¯ 1 ¯ 0}, the boundary basal dislocations do not exhibit dissociation along the boundary plane, but along the (0001) plane perpendicular to the boundary plane. From DFT calculations, the stacking fault on the (0001) plane was found to be much more stable than that on {2 1 ¯ 1 ¯ 0}. Such an extremely low energy of the (0001) stacking fault can realize transverse dissociation of the basal dislocation of ZnO.

The effects of solute and vacancy segregation on APB migration in Ti3Al, and their dependence on composition, have been investigated by using a phase-field simulation in which vacancy distribution is taken into account. Al-atoms are depleted and vacancies segregate at APB in stoichiometric Ti3Al (Ti-25Al), whereas Al-atoms segregate and vacancies are depleted in Alrich one (Ti-28Al). The simulation indicates that APB in Ti3Al migrates much faster in Ti-25Al than in Ti-28Al with the effect of vacancy segregation whereas it migrates slightly faster in Ti-28Al than in Ti-25Al in the absence of the effect of vacancy segregation.