Kyoto University · Materials Science
Professor Kazuaki Toyoura's research lab specializes in first-principles theoretical studies of ion diffusion and proton conduction in functional oxides, with a focus on understanding atomic-scale mechanisms in materials for energy applications. The lab investigates diffusion pathways, activation energies, and phase transitions in materials such as lithium-intercalated graphite (LiC₆), proton conductors (e.g., LaNbO₄, BaZrO₃), and ferroelectrics (e.g., LiNbO₃, LiTaO₃) using advanced computational methods including transition state theory, density functional theory, and machine learning-assisted sampling. A key emphasis is placed on accurately modeling lattice dynamics and vibrational effects using quantum statistical treatments, enabling quantitative predictions of ionic transport properties. The lab also develops innovative computational strategies, such as Gaussian process-based sampling, to efficiently explore complex potential energy surfaces relevant to ionic conductivity and structural transitions.
Figures are computed from collected data and may differ slightly.
We evaluate mean frequencies for atomic jumps in a crystal from first principles based on transition state theory, taking lithium diffusion by the interstitial and vacancy mechanisms in ${\text{LiC}}_{6}$ as a model case. The mean jump frequencies are quantitatively evaluated from the potential barriers and the phonon frequencies for both initial and saddle-point states of the jumps under the harmonic approximation. The lattice vibrations are treated within quantum statistics, not using the conv
Lithium diffusion in a stage-1 structure of LiC6 has been theoretically investigated from first principles on the basis of transition state theory. The calculated chemical diffusion coefficient of lithium atoms under the lithium excess conditions is much larger than that under the lithium deficient conditions, e.g., 2 × 10−7 cm2/s vs 1 × 10−10 cm2/s at room temperature. The calculated activation energies of the chemical diffusion coefficients under the lithium excess and deficient conditions are
The gradual change in the crystal structure of the high temperature proton conductor LaNbO(4) through a second order phase transition and its relation to the activation enthalpy of mobility of protons have been studied by means of first principles calculations and conductivity measurements. The computations have revealed that protons diffuse by an inter-tetrahedral mechanism where the activation enthalpies of mobility are 39 and 60 kJ mol(-1) in tetragonal and monoclinic LaNbO(4), respectively.
In this paper, we propose a selective sampling procedure to preferentially evaluate a potential energy surface (PES) in a part of the configuration space governing a physical property of interest. The proposed sampling procedure is based on a machine-learning method called the Gaussian process, which is used to construct a statistical model of the PES for identifying the region of interest in the configuration space. We demonstrate the efficacy of the proposed procedure for atomic diffusion and
The atomic-scale picture of proton conduction in highly doped barium zirconate has theoretically been clarified using first-principles calculations.
The phase transitions and ferroelectricity of LiNbO3 and LiTaO3 have been investigated theoretically from first principles. The phonon analyses and the molecular dynamics simulations revealed that the ferroelectric phase transition is not conventional displacive type but order-disorder type with strong correlation between cation displacements. According to the evaluated potential energy surfaces around the paraelectric structures, the large difference in ferroelectricity between the two oxides r
The proton-conducting network in lanthanum orthophosphate, LaPO4, has been theoretically clarified from first principles in the present study. It consists of as many as 20 kinds of migration paths with potential barriers below 1 eV, which are classified into three groups, i.e., rotations and intra- and intertetrahedral hoppings. As the results of the kinetic Monte Carlo simulations using the network of the migration paths, the calculated proton diffusion coefficients have anisotropy reflecting t
The effects of carrier–carrier interaction on the proton diffusivity in a proton-conducting perovskite, Y-doped BaZrO3, have theoretically been investigated in a first-principles manner. The proton diffusivity with the proton–proton interaction was estimated by solving the master equation under the single-particle approximation. The correlation effect between successive jumps was also taken into account to estimate the proton diffusivity with more accuracy. As a result, the proton–proton interac
The proton conduction behaviors in pyrochlore-structured lanthanum zirconate, La2Zr2O7, have been evaluated theoretically in a first-principles manner based on the transition state theory. The three-dimensional proton-conducting network consisting of two hopping paths was found, which lies along the corner-shared ZrO6 octahedral network in the crystal lattice. The calculated potential barrier of the proton-conducting network in the undoped system (0.39 eV) is lower than the apparent activation e
The interstitial oxide-ion mechanism in scheelite-type lanthanum niobate has theoretically been analyzed using first-principles calculations.
Phase stabilities in the La${}_{2}$O${}_{3}$-P${}_{2}$O${}_{5}$ pseudobinary system have been theoretically analyzed. Phonon modes of five crystals, i.e., La${}_{2}$O${}_{3}$, La${}_{3}$PO${}_{7}$, LaPO${}_{4}$, LaP${}_{3}$O${}_{9}$, and LaP${}_{5}$O${}_{14}$, and vibrational modes of gaseous P${}_{2}$O${}_{5}$(g) are computed from first principles in order to obtain the contribution of vibrations to the free energy. Additional dynamical contributions, i.e., rotations and translations are also t
Oxygen-ion conduction in apatite-type lanthanum silicate, La9.33+0.67x (SiO4)6O2+x (x = 1), has theoretically been analyzed in a first-principles manner followed by the nudged elastic band method and the kinetic Monte Carlo method. Unlike the conventional cooperative interstitialcy mechanism along the single O4 columns, diffusing interstitial oxygen ions are frequently blocked by adjacent interstitial oxygen ions (Oint ions), leading to the strongly-correlated diffusivity and conductivity of oxy
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