Kaoru Nakamura

First-principles investigation of pressure-induced phase transition in LiNbO3

We explore the possible high-pressure phase transition of LiNbO3 using an evolutionary algorithm combined with first-principles calculations. A NaIO3-type structure with Pnma symmetry was predicted as the room temperature phase, and an apatite-like structure with P63/m symmetry was predicted as the high temperature, high-pressure phase. These predictions are consistent with the experimental findings of Mukaide et al. [J. Appl. Phys. 93, 3852 (2003)]. Interestingly, however, the thermodynamic stability of the Cmcm phase was found to be greater than that of the Pnma phase below 50 GPa. In order to explain this, we investigated the possible deformation paths between R3c and high-pressure phases and found that a high energy barrier hinders Cmcm formation, despite its greater thermodynamic stability. In sum, our results indicate that an understanding of the atomistic mechanisms behind phase transition is essential in order to correctly predict phase transition behavior.

Enhancement of piezoelectric constants induced by cation-substitution and two-dimensional strain effects on ZnO predicted by density functional perturbation theory

Using density functional perturbation theory, we investigated the effect of various substitutional dopant elements and in-plane strain on the piezoelectric properties of ZnO. The piezoelectric stress constant e33 of doped ZnO was found to depend on the formal charge of the substitutional dopant. By decomposing the piezoelectric stress constant e33 into the individual atomic contributions, the change in the piezoelectric properties was found to originate from a change in the coupling between the atomic displacement and the strain. Furthermore, we found that in-plane tensile strain along the a axis, which is specific to the thin film, can enhance the piezoelectric constant of ZnO. A phase transition from wurtzite to h-BN-type structure was found to occur with increasing in-plane tensile. The piezoelectric strain constant d33 was predicted to reach ∼200 pC/N for 2.78 at. % V-substituted ZnO at 5.5% in-plane strain, just before the phase transition. These theoretical results suggest that the piezoelectric con...

Theoretical and experimental investigation of defect formation / migration in Gd2Ti2O7: General rule of oxide-ion migration in A2B2O7 pyrochlore

We investigated the intrinsic defect formation energy and oxide-ion migration mechanism in Gd2Ti2O7 pyrochlore. It was found that the vacancy formation energy of Gd is lower than that of Ti. For the oxygen vacancy, O(48f) was found to show lower vacancy formation energy than O(8b). The formation energy of the vacancy complex showed that the Gd vacancy is accompanied with the O(48f) vacancy, which is consistent with our experiment. The migration energy of O(48f) along the direction, which is dominant migration path for ionic conduction, was calculated to be 0.43 eV. On the other hand, we found that Gd vacancy increases O(48f) migration energy. For example, the migration energy of O(48f) along the direction was increased to be 1.36 eV by the local compressive strain around Gd vacancy. This finding could explain our previous experimental result of decreasing conductivity with increasing Gd deficiency. Along with the oxide-ion migration mechanism in Gd2Ti2O7, O(48f) migration energies along both <...

Effects of guest atomic species on the lattice thermal conductivity of type-I silicon clathrate studied via classical molecular dynamics

The effects of guest atomic species in Si clathrates on the lattice thermal conductivity were studied using classical molecular dynamics calculations. The interaction between a host atom and a guest atom was described by the Morse potential function while that between host atoms was described by the Tersoff potential. The parameters of the potentials were newly determined for this study such that the potential curves obtained from first-principles calculations for the insertion of a guest atom into a Si cage were successfully reproduced. The lattice thermal conductivities were calculated by using the Green-Kubo method. The experimental lattice thermal conductivity of Ba8Ga16Si30 can be successfully reproduced using the method. As a result, the lattice thermal conductivities of type-I Si clathrates, M8Si46 (M = Na, Mg, K, Ca Rb, Sr, Cs, or Ba), were obtained. It is found that the lattice thermal conductivities of M8Si46, where M is IIA elements (i.e., M = Mg, Ca, Sr, or Ba) tend to be lower than those of M...

Atomic structure, energetics, and chemical bonding of Y doped Σ13 grain boundaries in α-Al2O3

The atomic structure, energetics and chemical bonding state of pristine and Y doped Σ13, (10 4) grain boundaries in α-Al2O3 were investigated by aberration-corrected Z-contrast scanning transmission electron microscopy combined with first-principles calculations. Combining observations from two orthogonal directions parallel to the grain boundary plane, we found that Y atoms segregate into specific atomic sites and form two-dimensionally ordered structure. We performed first-principles calculations to estimate stable atomic sites for Y segregation, and it was found that the calculation results are in good agreement with the experimental results. Local chemical bonding states at the core of the boundary were investigated by a first-principles orthogonalized linear combination of atomic orbitals (OLCAO) method, and Y atoms and neighboring O atoms were found to evince strong ionic character while O–Al back bonds evince strong covalent character.

First-principles sliding simulation of Al-terminated Σ13 pyramidal twin grain boundary in α-Al2O3

First principles plane wave basis calculations were performed to investigate the atomic-scale mechanism of grain boundary (GB) sliding of the Al-terminated Σ13 pyramidal twin GB in α-Al2O3 in order to investigate the difference in sliding properties from the previously reported result of O-terminated GB. Atomistic mechanism of sliding process was found to be similar to the previously reported O-terminated GB, that is, the sliding takes place with successive breaking/rebonding of Al–O bonds across the GB core [Nakamura et al., Phys. Rev. B 75 (2007) p.184109]. On the other hand, it was found that sliding resistance of the Al-terminated GB is higher than that of the O-terminated one, though they have the identical orientation relationship. Quantitative chemical bonding analyses show that the difference in the sliding resistance originates from the local Al–O bonding characteristics of individual Al–O pairs across the GB plane.

Simple bond-order-type interatomic potential for an intermixed Fe-Cr-C system of metallic and covalent bondings in heat-resistant ferritic steels

It is known that M23C6(M = Cr/Fe) behavior in heat-resistant ferritic steels affects the strength of the material at high temperature. The ability to garner direct information regarding the atomic motion using classical molecular dynamics simulations is useful for investigating the M23C6 behavior in heat-resistant ferritic steels. For such classical molecular dynamics calculations, a suitable interatomic potential is needed. To satisfy this requirement, an empirical bond-order-type interatomic potential for Fe-Cr-C systems was developed because the three main elements to simulate the M23C6 behavior in heat-resistant ferritic steels are Fe, Cr, and C. The angular-dependent term, which applies only in non-metallic systems, was determined based on the similarity between a Finnis-Sinclair-type embedded-atom-method interatomic potential and a Tersoff-type bond-order potential. The potential parameters were determined such that the material properties of Fe-Cr-C systems were reproduced. These properties include...

Theoretical Tensile Deformation of Σ13 Pyramidal Twin Grain Boundary in Alumina

First-principles grain boundary (GB) tensile deformation simulations were performed to investigate the atomic-scale mechanism of GB fracture of the Σ13 pyramidal twin GB in α-Al2O3. It was found that the specific Al-O bond broke at the GB core in the early stage of tensile deformation. From chemical bonding analyses, the first breaking bond was the weakest bond in the GB core. However, when the catastrophic GB fracture started, initially strong Al-O bond broke. This indicates that local atomic bonds should determine the microscopic GB fracture behavior.