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Amorphous lithium lanthanum titanate (LLTO) thin films were successfully prepared by a sol-gel method with an all-alkoxide based route. The thin film resistance was determined from the complex spectra by fitting experimental data to the equivalent circuit. The ionic conductivities of amorphous LLTO thin films were 4.5 x 10-6, 6.9 x 10-6, 1.3 x 10-5, and 3.8 x 10-5 S/cm at 30°C, 50°C, 70°C, and 90°C, respectively. The activation energy of lithium ion conduction in the thin film was evaluated to be 0.36 eV in the temperature range of 30-90°C. The structure of amorphous LLTO was predicted by the ab-initio molecular dynamics (AIMD) simulations and analyzed by partial pair distribution functions, coordination numbers and Voronoi tessellation. It is observed that the local environment of Ti in amorphous LLTO is quite similar to that in the crystal state but the atomic packing is much less dense. The ionic diffusivities were derived from the mean square displacement curves and the conductivities were evaluated from the Nernst-Einstein's relation, showing good agreement with experimental data. The good ionic conductivity of amorphous LLTO is attributed to its open and disordered structure with large excessive volumes. [ABSTRACT FROM AUTHOR]

Thermodynamics of defects in the (LaxCa1-x)FeO3-δ perovskite is modeled by means of the CALPHAD approach. In this phase, the A-sites are occupied by La+3 and Ca+2, and Fe in the B-site is known to exist in +2, +3, and +4 oxidation states depending on the oxygen vacancy concentration. Therefore, the ionic sublattice model: (La+3, Ca+2)(Fe+2, Fe+3, Fe+4)(O-2, Va)3 is used to describe the phase, and the model parameters are evaluated from experimental oxygen nonstoichiometry and phase equilibria data. With the Fe+2 and Fe+4 treated as the major species in the B-site, the calculated phase diagrams are in good agreement with the experimentally reported phase equilibria data. The concentration of various defects in (LaxCa1-x)FeO3-δ as a function of oxygen partial pressure and temperature are calculated at different concentrations of Ca. At high oxygen partial pressures, Fe+4 is predicted to be dominant while Fe+2 is dominant at low oxygen partial pressures. [ABSTRACT FROM AUTHOR]

The Al2O3-Nd2O3-SiO2-Y2O3 pseudoquaternary system was thermodynamically modeled with a focus on the phase equilibria concerning the sintering of neodymium-doped yttrium aluminum garnet, Nd:YAG ((Y1-xNdx)3Al5O12). The Nd2O3-SiO2 and Nd2O3-Y2O3 systems were evaluated. Due to the lack of experimental thermochemical data, first-principles calculations were used to predict the enthalpy of formation of Nd2SiO5 and the enthalpy of mixing of Nd in YAG. Isothermal sections of the Al2O3-Nd2O3-SiO2, Al2O3-Nd2O3-Y2O3, and Nd2O3-SiO2-Y2O3 pseudoternary systems are presented. [ABSTRACT FROM AUTHOR]

The phase constitution and associated oxidation behavior were determined for an alloy of composition (in wt%) Ni–31.5Cr–11.5Al–0.61Y. The identity, relative amount, and approximate composition of each phase were determined after equilibration at 700, 900, and 1100°C using SEM/EDS, XRD, and image analysis. The CALPHAD method was used to predict the phase equilibria in this system, and the results showed good agreement with experiment. Cyclic oxidation behavior at 900, 1050, and 1100°C in air was determined and assessment of the results was aided by CALPHAD predictions in conjunction with dilatometric measurements. [ABSTRACT FROM AUTHOR]

In this article, a profile-fitting methodology was developed to measure the partition coefficients of solute elements during the solidification of Ni-base alloys. Better agreement with the theoretically calculated values is expected if the accuracy of the composition and the homogeneity of the model alloys are enhanced. Regular differential thermal analysis (DTA) measurements were consistently higher than the theoretical transition temperatures, and the differences were smaller when compared to the predictions performed with the thermodynamical database developed by Du et al. The better agreement between the experimental results and the theoretical predictions made with the newly developed database suggests that improvements in the accuracy of the theoretical predictions can still be obtained and are necessary for accurate freckling prediction. Quenching modified DTA (MDTA) experiments were proven to be appropriate for directly measuring the average partition coefficients of the solute elements. Regarding the cooling rate of the first stage of the quenching experiments, it was assumed successfully that the cooling rate prior to the quenching step of 0.083 Ks−1 was sufficiently slow to permit easy quenching, while being fast enough for the primary solidification reaction to depart from the equilibrium model and being closer to the Scheil model of segregation. The minimization of the error function defined from the Scheil equation was found to be an appropriate method for describing the segregation profiles of the quenched samples and permitted good estimations of the partition coefficients of the solute elements. The reliability of the methodology was found to be satisfactory given that the magnitudes calculated for the partition coefficients of the solutes in the multicomponent alloy 718 were found to be very close to the values reported in the literature. [ABSTRACT FROM AUTHOR]

BiFeO3 thin films have been deposited on (001) SrTiO3, (101) DyScO3, (011) DyScO3, (0001) AlGaN/GaN, and (0001) 6H-SiC sungle crystal substrates by reactive molecular beam epitaxy in an absorption-controlled growth regime. This is achieved by supplying a bismuth over-pressure and utilizing the differential vapor pressures between bismuth oxides and BiFeO3 to control stoichiometry in accordance with thermodynamic calculations. Four-circle x-ray diffraction and transmission electron microscopy reveal phase-pure, epitaxial films with rocking curve full width at half maximum values as narrow as 7.2 arc seconds (0.002°). Epitaxial growth of (0001)-oriented BiFeO3 thin films on (0001) GaN, including AlGaN HEMT structures, and (0001) SiC has been realized using intervening epitaxial (111) SrTiO3 / (100) TiO2 buffer layers. The epitaxial BiFeO3 thin films have 2 in-plane orientations: [11¯20] BiFeO3 ∣∣ [11¯20] GaN (SiC) plus a twin variant related by a 180° in-plane rotation. This epitaxial integration of the ferroelectric with the highest known polarization, BiFeO3, with high bandgap semiconductors is an important step toward novel field-effect devices. [ABSTRACT FROM AUTHOR]

Using first-principles calculations, the enthalpies of formation of NdAlO3 and Nd4Al2O9 from neodymia and alumina are predicted to be −41.1 and −104.5 kJ/mol-form, respectively, and Nd4Al2O9 is predicted to be a stable compound at all temperatures up to its peritectic decomposition temperature. In the case of NdAlO3, where reported experimental enthalpies of formation differ, our predicted value is used to determine which experimental value is more accurate. The enthalpies of formation calculated by the first-principles method are combined with experimental phase equilibria to evaluate the Gibbs energies of phases in the Al2O3–Nd2O3 system. The resulting Gibbs energy functions are compared with previous modelings of the system and are shown to be an improvement as more thermochemical data have been included. The combined first-principles/thermodynamic modeling approach employed in the current work shows promise in enabling the evaluation of important ceramic systems in which experimental thermochemical data are scarce. [ABSTRACT FROM AUTHOR]

Defect and phase equilibria have been investigated via the ferroelectric phase transition behavior of pure and equilibrated nonstoichiometric BaTiO3 powder samples. Through fabricating the BaTiO3 materials under highly controlled conditions to preserve the equilibrium conditions with respect to Ba/Ti ratio, annealing temperature ( T), and oxygen partial pressure ( PO2), systematic variations in the phase transition temperature can be noted with respect to Ba/Ti ratio and T. From the data extracted, we can then determine solubility limits. Equilibrating the defect reactions at the solubility limits provides a direct approach to identify and calculate the defect energetics. The phase transition temperature decreased with increasing concentration of the TiO2 partial-Schottky defects (BaTi1−δO3−2δ) and the BaO partial-Schottky defects (Ba1−δTiO3−δ), and showed discontinuous changes in the two-phase region. The formation enthalpy and entropy for the partial-Schottky defect reactions was evaluated to be 2.32±0.1 eV and 10.15±0.7 kB for the BaO partial-Schottky defect, and 2.89±0.1 eV and 8.0±1.5 kB for the TiO2 partial-Schottky defects equilibrated under air annealing conditions. [ABSTRACT FROM AUTHOR]

Defect and phase equilibria have been investigated through the ferroelectric phase transition behavior of pure and equilibrated nonstoichiometric BaTiO3 powders. The paraelectric–ferroelectric phase transition temperature ( T C– T) was found to vary systematically with materials fabricated with different Ba/Ti ratio ( g*) and under various oxygen partial pressure ( ) conditions. 1 The solubility regime, as determined through the T C– T variation, decreased with decreasing . 2 Determining the solubility limits and equilibrating the defect reactions at the solubility limits provide a direct approach to calculate the defect formation energies and provide data to test a new defect model for concurrent defect reactions of partial Schottky and reduction defects. A refined approach introduces a balanced equilibrium between the oxygen vacancy concentrations controlled by the partial Schottky and reduction reactions. In the limiting ambient cases the approach gives the expected results, and also fully explains the solubility trends under low 's. Universally, the theory supports all the experimental data over different temperatures and 's. [ABSTRACT FROM AUTHOR]

The Magnesium Powertrain Cast Components (MPCC) Project is an effort, jointly sponsored by the U.S. Department of Energy and the U.S. Automotive Materials Partnership (USAMP), to demonstrate the readiness of magnesium for use in powertrain applications by testing a set the magnesium-intensive engines which were designed, cast, and assembled. A second MPCC goal is to promote new and strengthen existing magnesium scientific research in North America. The project investigated several of the newly developed high-temperature (creep-resistant) magnesium alloys, which will potentially experience service conditions in the temperature range of 150-200°C and about 50-110 MPa in stresses (typical powertrain). However, the mechanical and physical behaviors of these new alloys are not fully understood. This article outlines MPCC-supported fundamental scientific research into the workings of these new alloys. The areas of research are: phase equilibrium and computational thermodynamics, creep deformation mechanisms, corrosion, hot tearing, and alloy recycling. [ABSTRACT FROM AUTHOR]

The ferroelectric phase transition behavior in BaTiO3 was investigated for various annealing times, temperatures, and Ba/Ti ratios by means of a differential scanning calorimeter. Coupling these observations with powder X-ray diffraction and transmission electron microscopy allowed new insights into the barium oxide (BaO)–titanium dioxide (TiO2) phase diagram. The transition temperature was varied systematically with the Ba/Ti ratio at annealing temperatures from 1200° to 1400°C in air. The transition temperature decreased with increasing concentrations of BaO and TiO2 partial Schottky defects, and showed a discontinuous change at the phase boundaries. Beyond the solubility region, two peritectoid reactions were confirmed and revised; first around 1150°C for Ba1.054Ti0.946O2.946→Ba2TiO4+BaTiO3 and second 1250°C for BaTi2O5→Ba6Ti17O40+BaTiO3, respectively. All other regimes of the BaO–TiO2 were found to be consistent with the reported diagrams in the literature. [ABSTRACT FROM AUTHOR]

Seeking structure-property relationships is an accepted paradigm in materials science, yet these relationships are often not linear. The challenge is to be able to link materials behavior and seek patterns among multiple length and time scales. Materials informatics and statistical learning techniques permit one to survey complex, multiscale information in an accelerated and statistically robust and yet meaningful manner. When this is coupled with advanced tools for computational thermodynamics and kinetic simulations, the result is a powerful computational infrastructure for materials design. This paper provides an overview of the value of this integrated approach to materials modeling in addressing the challenges in linking length scales. [ABSTRACT FROM AUTHOR]

The phase equilibria and thermodynamic properties of the binary Ba–O system were analyzed. The Gibbs energy functions of individual phases were modeled, and the model parameters were obtained by means of the CALculation of PHAse Diagram (CALPHAD) technique, based on available experimental data in the literature. The liquid phase was described with a two-sublattice ionic model. For the non-stoichiometric BaO and BaO2 phases, a two-sublattice ionic model was applied, and two sets of Gibbs energy functions were obtained by treating them either as separate phases or as a common solution phase with a miscibility gap. The ionic sublattice models used in the present work provide an approach to better describe the physical behavior of the Ba–O binary system such as defect chemistry and doping effects, which will be studied in multicomponent systems. [ABSTRACT FROM AUTHOR]

First-principles (FP) energetics of both the constituent elements and the compounds in the Al-Ca binary system are used in the CALPHAD (CALculation of PHase Diagrams) approach of thermodynamic modeling. First-principles calculations are performed using both an all-electron full-potential linearized augmented plane-wave method, as well as an ultrasoft pseudopotential/plane wave method. We perform calculations of T = 0 ground state total energies of the pure Al and Ca in fcc, bcc, and hcp structures, and the binary compounds in their observed crystal structures. Al4Ca, Al14Ca13, and Al3Ca8 are modeled in CALPHAD as simple stoichiometric compounds; however, the Laves C15 compound, Al2Ca, is modeled using two sublattices (Al,Ca)2(Al,Ca)1, necessitating first-principles energies of both the stable Al2Ca compound as well as the three nonstable Al2Al2, AlCa2, and Ca2Ca compounds. From these total energies, we obtain the formation enthalpies of all the binary compounds that are then used to assist in evaluating the Gibbs energy functions for the individual phases. The entropy contribution in the Gibbs energy function for each individual compound is obtained via the observed equilibria with the liquid phase. We provide a complete thermodynamic description of the Al-Ca binary system, evaluated by this combined CALPHAD-FP approach. [ABSTRACT FROM AUTHOR]

The YO1.5-ZrO2 system consists of five solid solutions, one liquid solution, and one intermediate compound. A thermodynamic description of this system is developed, which allows calculation of the phase diagram and thermodynamic properties. Two different solution models are used—a neutral species model with YO1.5 and ZrO2 as the components and a charged species model with Y3+, Zr4+, O2-, and vacancies as components. For each model, regular and subregular solution parameters are derived from selected phase equilibrium and thermodynamic data. The neutral species and charged species modeling results are compared. [ABSTRACT FROM AUTHOR]

Considering the effects of temperature and composition, a phenomenological description of lattice parameters in solid states was developed. The lattice parameter of the pure element is modeled under the assumption of a linear temperature dependence of thermal expansion, while the lattice parameters of substitutional solid-solution phases are treated similar to the Gibbs-energy modeling in the CAL-PHAD (CALculation of PHAse Diagram) approach. Using this model, the lattice parameters of the γ and γ′ phases in the Ni-Al system were analyzed and the model parameters were evaluated. The calculated lattice parameters and mismatches show good agreement with existing experimental data. [ABSTRACT FROM AUTHOR]

We have carried out thermodynamics studies of the Mg-B system with the calculation of phase diagrams (CALPHAD) modeling technique and found that the superconductor MgB[sub 2] phase is thermodynamically stable only under fairly high Mg pressures at elevated temperatures. This has lead us to the investigation of chemical vapor deposition in which the pressure during the film deposition can be high. Although the initial effort on metal-organic chemical vapor deposition (MOCVD) was not successful due to carbon contamination, a unique hybrid physical-chemical vapor deposition (HPCVD) technique has successfully produced high quality in situ MgB[sub 2] films. The epitaxially-grown MgB[sub 2] films show high transition temperature and low resistivity comparable to the best bulk samples, and their surfaces are smooth. In this paper, the details of the technique and the results of the HPCVD films are presented. [ABSTRACT FROM AUTHOR]

We perform first-principles calculations to investigate neutral and charged H vacancies dynamics involving vacancy formation and diffusion in Na3AlH6. We find that the activation barrier for local diffusion (diffusion within AlH5 unit) is smaller than that of nonlocal diffusion (diffusion between AlH6 and AlH5 units) for all types of H vacancies; H diffusion in Na3AlH6 is dominated by mobility of positively charged H vacancies. Furthermore, our results confirm that the observed highly mobile species by anelastic spectroscopy measurements is probably the positively charged H vacancies in the form of local diffusion. [ABSTRACT FROM AUTHOR]

The available thermodynamic databases for magnesium alloys are discussed in this paper. Of particular interest are the features of a magnesium database developed by the authors with 19 elements: Mg-Al-Ca-Ce-Cu-Fe-KLa-Li-Mn-Na-Nd-Pr-Si-Sn-Sr-Y-Zn-Zr. Using this database, two applications are presented. One is the phase evolution in AZ61 magnesium alloy including the variations of phase fractions, alloying compositions, and partition coefficients of alloying elements as a function of temperature (or solid fraction). The other is to understand sodium-induced high-temperature embrittlement in the Al-Mg alloy, which is ascribed to the formation of a liquid phase due to the presence of sodium traces. [ABSTRACT FROM AUTHOR]

The article discusses that the kinetics and processing involve the change of materials with respect to time, such as the time-temperature- transformation (TTT) or continuous cooling transformation (CCT) diagrams. It mentions that the aiming to bridge the gap between the strategic plan and operations of ASM International and engage more discussions among members of the Board of Trustees (BoT) and with volunteers and operations management team (OMT).

KTaO3 heterostructures have recently attracted attention as model systems to study the interplay of quantum paraelectricity, spin-orbit coupling, and superconductivity. However, the high and low vapor pressures of potassium and tantalum present processing challenges to creating heterostructure interfaces clean enough to reveal the intrinsic quantum properties. Here, we report superconducting heterostructures based on high-quality epitaxial (111) KTaO3 thin films using an adsorption-controlled hybrid PLD to overcome the vapor pressure mismatch. Electrical and structural characterizations reveal that the higher-quality heterostructure interface between amorphous LaAlO3 and KTaO3 thin films supports a two-dimensional electron gas with substantially higher electron mobility, superconducting transition temperature, and critical current density than that in bulk single-crystal KTaO3-based heterostructures. Our hybrid approach may enable epitaxial growth of other alkali metal-based oxides that lie beyond the capabilities of conventional methods. [ABSTRACT FROM AUTHOR]

Anisotropy and temperature dependence of structural, thermodynamic and elastic properties of crystalline cellulose Iβ were computed with first-principles density functional theory (DFT) and a semi-empirical correction for van der Waals interactions. Specifically, we report the computed temperature variation (up to 500 K) of the monoclinic cellulose Iβ lattice parameters, constant pressure heat capacity, Cp, entropy, S, enthalpy, H, the linear thermal expansion components, ξi, and components of the isentropic and isothermal (single crystal) elastic stiffness matrices, and , respectively. Thermodynamic quantities from phonon calculations computed with DFT and the supercell method provided necessary inputs to compute the temperature dependence of cellulose Iβ properties via the quasi-harmonic approach. The notable exceptions were the thermal conductivity components, λi (the prediction of which has proven to be problematic for insulators using DFT) for which the reverse, non-equilibrium molecular dynamics approach with a force field was applied. The extent to which anisotropy of Young's modulus and Poisson's ratio is temperature-dependent was explored in terms of the variations of each with respect to crystallographic directions and preferred planes containing specific bonding characteristics (as revealed quantitatively from phonon force constants for each atomic pair, and qualitatively from charge density difference contours). Comparisons of the predicted quantities with available experimental data revealed reasonable agreement up to 500 K. Computed properties were interpreted in terms of the cellulose Iβ structure and bonding interactions. [ABSTRACT FROM AUTHOR]

Thermodynamic modeling of the Sn-Sr binary system and the Mg-Sn-Sr ternary system is carried out by means of the CALPHAD approach in combination with available experimental data in the literature and first-principles calculations in the present work. The finite temperature thermodynamic properties of the compounds in the Sn-Sr system and the MgSnSr ternary compound are predicted using the quasi-harmonic phonon calculations and the Debye model with inputs from first-principles calculations. The associate solution model is used to describe the short-range ordering behavior in the liquid phase. Combined with the previously modeled Mg-Sn and Mg-Sr binary systems in the literature, the thermodynamic modeling of the Mg-Sn-Sr system is performed. [ABSTRACT FROM AUTHOR]

Unprecedented interest has been spurred recently in two-dimensional (2D) layered transition metal dichalcogenides (TMDs) that possess tunable electronic and optical properties. However, synthesis of a wafer-scale TMD thin film with controlled layers and homogeneity remains highly challenging due mainly to the lack of thermodynamic and diffusion knowledge, which can be used to understand and design process conditions, but falls far behind the rapidly growing TMD field. Here, an integrated density functional theory (DFT) and calculation of phase diagram (CALPHAD) modeling approach is employed to provide thermodynamic insight into lateral versus vertical growth of the prototypical 2D material MoS2. Various DFT energies are predicted from the layer-dependent MoS2, 2D flake-size related mono- and bilayer MoS2, to Mo and S migrations with and without graphene and sapphire substrates, thus shedding light on the factors that control lateral versus vertical growth of 2D islands. For example, the monolayer MoS2 flake in a small 2D lateral size is thermodynamically favorable with respect to the bilayer counterpart, indicating the monolayer preference during the initial stage of nucleation; while the bilayer MoS2 flake becomes stable with increasing 2D lateral size. The critical 2D flake-size of phase stability between mono- and bilayer MoS2 is adjustable via the choice of substrate. In terms of DFT energies and CALPHAD modeling, the size dependent pressure-temperature-composition (P-T-x) growth windows are predicted for MoS2, indicating that the formation of MoS2 flake with reduced size appears in the middle but close to the lower T and higher P "Gas + MoS2" phase region. It further suggests that Mo diffusion is a controlling factor for MoS2 growth owing to its extremely low diffusivity compared to that of sulfur. Calculated MoS2 energies, Mo and S diffusivities, and size-dependent P-T-x growth windows are in good accord with available experiments, and the present data provide quantitative insight into the controlled growth of 2D layered MoS2. [ABSTRACT FROM AUTHOR]

Ti-48Al-6Nb porous alloys were synthesized by the powder metallurgy (PM) method, and the associated phase transformation and pore parameter were investigated in order to reveal the pore-formation mechanism. The present results indicate that the Nb-Al and Ti-Al phase transformations contribute to the pore-formation. It was found that the five-step phase transformations for the Ti-48Al-6Nb porous alloys occur as follows: (1) Ti + Al → TiAl3 at 600-700 °C; (2) Nb + Al → NbAl3 at 700-900 °C; (3) TiAl3 + Ti → TiAl at 900-1100 °C; (4) TiAl + Ti → Ti3Al/TiAl at 1100-1350 °C; (5) NbAl3 + Nb → Nb2Al and the Ti3Al turns to the major phase at 1350 °C. These phase transformations made the pore-diameter increasing continuously from 1.71 μm to 12.10 μm and also made the pore volume distributing widely. At the second step of 700-900 °C, the Nb-Al phase transformation leads to 5% more volume expansion compared to the Ti-Al based porous alloys. Meanwhile, the porosity and total pore area initially increase and then decrease at this step, but they increase intensely at the final step, which is needed as a catalytic carrier. [ABSTRACT FROM AUTHOR]

Ferroelectric-paraelectric (FE-PE) phase transitions have been primarily explained by the phenomenological Landau-Devonshire theory and a soft-zone-center mode of vibration in the literature. In this work, we study the atomic structure and polarization evolution of PbTiO3 as a function of temperature using ah initio molecular dynamics simulations. In contrast to conventional molecular dynamics analyses where results are averaged over time, we categorize the atomic configurations as a function of time in terms of Ti-0 bond lengths in the nearest-neighboring shell. We show that an appreciable amount of cubic configuration exists at temperatures about 300 K below its FE-PE phase transition temperature of 763 K, even though the time-averaged overall atomic configuration is tetragonal. The quantitative results depict that as the temperature increases the population of the cubic configuration increases and that of the tetragonal configuration decreases, signifying that the FE-PE phase transition is intrinsically second order. It reveals that the thermal fluctuation of the cubic configurations in the tetragonal matrix makes a significant contribution to the negative thermal expansion in the FE phase region because the cubic configuration has smaller volume and higher entropy than the tetragonal matrix. [ABSTRACT FROM AUTHOR]

We report the lattice dynamic properties of CaP2 and CeO2 obtained using a direct method in combination of a mixed-space approach accounting for the vibration-induced dipole-dipole interactions. We overcame the overestimation of T1u TO mode shown in prior calculations using the linear response theory. For CaP2, we employed the Perdew-Burke-Emzerhof functional and achieved a significant improvement over previous linear response calculations by comparing the results with experimentally measured phonon dispersion curves. For CeO2, we adopted the Heyd-Scuseria-Ernzerhof hybrid functional and an elongated supercell and obtained results in excellent agreement with experimental measurements. We attributed the improved calculation results to the convenience of the direct method in implementing new exchange-correlation functionals and use of the mixed-space approach for treating the long-range dipole-dipole interactions. [ABSTRACT FROM AUTHOR]

We determined the atomic structures and energies of 109°, 180°, and 71° domain walls in BiFeO3, combining density functional theory+U calculations and aberration-corrected transmission electron microscopy images. We find a substantial Bi sublattice shift and a rather uniform Fe sublattice across the walls. The calculated wall energies (γ) follow the sequence γ109<γ180<γ71 for the 109°, 180°, and 71° walls. We attribute the high 71° wall energy to an opposite tilting rotation of the oxygen octahedra and the low 109° wall energy to the opposite twisting rotation of the oxygen octahedra across the domain walls. [ABSTRACT FROM AUTHOR]

We perform first-principles calculations to investigate neutral and charged H vacancies dynamics involving vacancy formation and diffusion in Na3AlH6. We find that the activation barrier for local diffusion (diffusion within AlH5 unit) is smaller than that of nonlocal diffusion (diffusion between AlH6 and AlH5 units) for all types of H vacancies; H diffusion in Na3AlH6 is dominated by mobility of positively charged H vacancies. Furthermore, our results confirm that the observed highly mobile species by anelastic spectroscopy measurements is probably the positively charged H vacancies in the form of local diffusion. [ABSTRACT FROM AUTHOR]