Comprehensive understanding of the crystal structure of perovskite-type Ba3Y4O9 with Zr substitution: a theoretical and experimental study.

We previously reported that Zr substitution improves the chemical stability of Ba3Y4O9 and nominally 20 mol% Zr-substituted Ba3Y4O9 is an oxide-ion conductor at intermediate temperatures (500-700 °C). However, the influence of Zr substitution on the structural properties of Ba3Y4O9 was poorly understood. This paper aims to comprehensively understand the crystal structure of Ba3Y4O9 with Zr substitution by powder X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) measurements, and first-principles calculations. From the results, firstly we found that the hexagonal unit cell of Ba3Y4O9 reported in the database should be revised as doubled along the c-axis in terms of the periodicity of oxide-ion positions. The revised unit cell of Ba3Y4O9 consists of 18 layers of BaO3 and 24 layers of Y which periodically stack along the c-axis. In this work, we focused on the cationic lattice and noticed that the periodical stacking of Ba and Y layers comprises a similar sequence to that in the body-centered cubic (BCC) structure. There are two regions in the Ba3Y4O9 structure: one is a hetero-stacking region of Ba and Y layers (Ba-Y-Ba-Y-Ba) and the other is a homo-stacking region (Ba-Y-Y-Ba). It is noteworthy that the former region is similar to a cubic perovskite. In Zr-substituted Ba3Y4O9, Zr ions preferentially substitute for Y ions in the hetero-stacking region, and therefore the local environment of Zr ions in Ba3Y4O9 is quite similar to that in BaZrO3. Besides, the Zr substitution for Y in Ba3Y4O9 increases the fraction of the cubic-perovskite-like region in the stacking sequences. The structural change in the long-range order strongly affects the other material properties such as chemical stability and the ionic-conduction mechanism. Our adopted description of perovskite-related compounds based on the stacking sequence of the BCC structure should help in understanding the complex structure and developing new perovskite-related materials.

High-Temperature Protonic Conduction in La2 NiO4+ δ -Based Ruddlesden-Popper Type Oxides: Correlation with Concentration of Interstitial Oxide Ions.

Oxygen-excess La2 NiO4+ δ (LNO) conducts oxide ions, electron holes, and hydroxide ions simultaneously on exposing to wet oxygen, exhibiting the potential as a cathode material in protonic ceramic fuel cells. Since the incorporation of protons in oxygen-excess LNO is via the hydration reaction assisted by interstitial oxide ions, in this work, the concentration of interstitial oxide ions is reduced and increased by substituting Ni with Cu and Co, respectively. A higher concentration of interstitial oxide ions leads to a high proton concentration, indicating the predominant role of interstitial oxide ions in the hydration reaction, different from that in the oxygen-deficient oxides, where protons are introduced by dissociative absorption of water molecules by oxygen vacancies. The theoretical calculation indicates that protons in Co-doped LNO prefer to locate between the interstitial oxide ions and unshared apical oxide ions. A trapping effect is found between protons and the oxide ions near Cu, leading to decreased proton mobility. Protonic conductivity at 400-575 °C is then directly measured by a Hebb-Wagner direct current polarization method with La0.99 Ca0.01 NbO4- δ as the blocking electrode, enabling the observation that Co-doped LNO has the highest protonic conductivity among the samples studied in this work.

Structure search method for atomic clusters based on the dividing rectangles algorithm.

The Dividing Rectangles (DIRECT) algorithm is a deterministic optimization method to explore optimal solutions by repeatedly dividing a given hyperrectangle search space into subhyperrectangles. Herein, we propose a structure search method for atomic clusters based on the DIRECT algorithm in combination with a gradient-based local optimizer to enable an efficient structure search in high-dimensional search spaces. We use the Z-matrix representation for defining the hyperrectangle search space, in which the bond lengths, bond angles, and dihedral angles specify a cluster structure. To evaluate its performance, we applied the proposed method to the Lennard-Jones clusters and two kinds of real atomic clusters with many metastable structures, i.e., phosphorus and sulfur clusters, and compared the results with those of conventional methods. The proposed method exhibits a higher efficiency than random search and a comparable efficiency to basin hopping.

Theoretical and Experimental Studies on the Ability of Intracrystalline Pores of ß-La2(SO4)3 To Accommodate Various Gas Species with a Special Focus on Ammonia Insertion Behaviors.

ß-La2(SO4)3 is a microporous inorganic crystal with one-dimensional perforated pores where H2O molecules can be inserted. To evaluate the nature of the pores and extend the application range, we investigate the ability to accommodate various hydrogen compound molecules XHn (CH4, NH3, HF, H2S, HCl, and HI) by insertion. The stable structures of the XHn molecules in the pores of ß-La2(SO4)3 and the change in the Gibbs energy for XHn insertion ΔinsertG (T) are estimated by first-principles calculations. The guest XHn molecules are stabilized by forming H-O and X-La bonds with the ß-La2(SO4)3 host structure. Based on the values of ΔinsertG (T), NH3, H2O, and HF are energetically stable in the crystal even above 0 °C. Correspondingly, thermogravimetry (TG) of ß-La2(SO4)3 in NH3, CH4, and CO2 gases revealed that NH3 can be inserted into ß-La2(SO4)3 below 360 °C, but CH4 and CO2 cannot. Unlike the case of H2O insertion, NH3 insertion proceeds via two steps. The first step is a single-solid-phase reaction of ß-La2(SO4)3·yNH3, where NH3 molecules are inserted into the host structure with a continuously changing nonstoichiometric y value between 0 and 0.1. The second step is a two-solid-phase reaction between ß-La2(SO4)3·0.1NH3 and ß'-La2(SO4)3·0.3NH3, which is a phase formed after further NH3 insertion into ß-La2(SO4)3·0.1NH3 with a minor change in the host structure. The fact that both NH3 and H2O can be inserted confirms that the pores of ß-La2(SO4)3 allow for the insertion of molecules with a strong polarity. This nature is similar to zeolites and metal-organic frameworks (MOFs) with polar surfaces in the pores.

Theoretical study on proton diffusivity in Y-doped BaZrO3 with realistic dopant configurations.

We theoretically revisit the proton diffusivity in yttrium-doped barium zirconate (Y-doped BaZrO3) with realistic dopant configurations under processing conditions. In a recent study employing the replica exchange Monte Carlo method, the equilibrium Y configurations at typical sintering temperatures were shown to deviate from the random configuration assumed in earlier theoretical studies. In the present study, we took this observation into account and evaluated the effect of the Y configuration on the proton diffusivity. Using the master equation approach based on local diffusion barriers calculated from first principles, the proton diffusivities under realistic Y configurations were estimated to be higher than those in the random configuration. This is explained by the fact that realistic Y configurations have fewer trap sites with deep potential wells compared to the random configuration due to the isolation trend of Y dopants. In addition, the effects of proton-proton interaction and the abundance of preferential conduction pathways are discussed; it is found that both are relatively minor factors compared to the trap site effect in determining the dependence of the proton diffusivity on the Y configurations.

Active Learning for Enumerating Local Minima Based on Gaussian Process Derivatives.

We study active learning (AL) based on gaussian processes (GPs) for efficiently enumerating all of the local minimum solutions of a black-box function. This problem is challenging because local solutions are characterized by their zero gradient and positive-definite Hessian properties, but those derivatives cannot be directly observed. We propose a new AL method in which the input points are sequentially selected such that the confidence intervals of the GP derivatives are effectively updated for enumerating local minimum solutions. We theoretically analyze the proposed method and demonstrate its usefulness through numerical experiments.

Adsorption sites of single noble metal atoms on the rutile TiO2 (1 1 0) surface influenced by different surface oxygen vacancies.

Atomic adsorption of Au and Pt on the rutile (1 1 0) surface was investigated by atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM) measurements combined with density functional theory calculations. Au single atoms were deposited on the surface in a vacuum condition, and the observed results were compared with Pt single atoms on the same surface prepared by the same experimental manner. It was found that Au single atoms are stably adsorbed only at the bridging oxygen vacancy sites, which is quite different from Pt single atoms exhibiting the most frequently observed adsorption at the basal oxygen vacancy sites. Such a difference in oxygen-vacancy effect between Au and Pt can be explained by electronic structures of the surface vacancies as well as characters of outermost atomic orbitals of Au and Pt.

Strong correlation in 1D oxygen-ion conduction of apatite-type lanthanum silicate.

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 oxygen ions in the case of chemical compositions with large x values. The getting-out mechanism from the O4 column is of importance in the long-range conduction, which temporarily transfers a part of Oint ions out of the columns to relax the blocking effect. The getting-out mechanism plays a key role also in the conduction perpendicular to the c axis (in the ab plane).

First-principles calculations of divalent substitution of Ca(2+) in tricalcium phosphates.

First-principles calculations were carried out to reveal local atomic arrangements and thermodynamic stability of substitutional divalent cations of Mg(2+), Zn(2+), Sr(2+) and Ba(2+) in tricalcium phosphates (TCPs). There are two modifications of α-TCP and ß-TCP, and a number of inequivalent Ca sites are present in the crystal structures. It was found that each divalent cation has energetically preferential Ca sites for substitution. For instance, Mg(2+) and Zn(2+) favor the substitution at the Ca-5 site of ß-TCP while Sr(2+) and Ba(2+) tend to occupy Ca-3 and Ca-4 in the ß-type crystal structure. The calculated site preference of these cations was in reasonable agreement with available experimental data. Moreover, it was found that these cations have negative formation energies at specific Ca sites especially in ß-TCP, indicating the stabilization of the ß phase.

The effect of chemical potential on the thermodynamic stability of carbonate ions in hydroxyapatite.

First-principles calculations were performed for CO3(2-) ions in hydroxyapatite in order to investigate the atomic structures and thermodynamic stability of CO3(2-) and its related defects. Two different chemical equilibrium conditions in high-temperature and aqueous-solution environments were considered, and atomic and ionic chemical potentials for the individual chemical equilibrium conditions were evaluated to calculate defect formation energies. It was found that A-type CO3(2-) (substituting OH(-)) is energetically more favorable than B-type CO3(2-) (substituting PO4(3-)) in the high-temperature environment, whereas B-type is preferred to A-type in the aqueous solution environment. This result successfully reproduces experimentally observed trends. In the formation of A-type and B-type CO3(2-), OH(-) vacancies or protons (interstitial or substitutional) act as charge-compensating defects.

Direct imaging of Pt single atoms adsorbed on TiO2 (110) surfaces.

Noble metal nanoparticles (e.g., gold and platinum) supported on TiO2 surfaces are utilized in many technological applications such as heterogeneous catalysts. To fully understand their enhanced catalytic activity, it is essential to unravel the interfacial interaction between the metal atoms and TiO2 surfaces at the level of atomic dimensions. However, it has been extremely difficult to directly characterize the atomic-scale structures that result when individual metal atoms are adsorbed on the TiO2 surfaces. Here, we show direct atomic-resolution images of individual Pt atoms adsorbed on TiO2 (110) surfaces using aberration-corrected scanning transmission electron microscopy. Subangstrom spatial resolution enables us to identify five different Pt atom adsorption sites on the TiO2 (110) surface. Combining this with systematic density functional theory calculations reveals that the most favorable Pt adsorption sites are on vacancy sites of basal oxygen atoms that are located in subsurface positions relative to the top surface bridging oxygen atoms.

Atomic sites and stability of Cs+ captured within zeolitic nanocavities.

Zeolites have potential application as ion-exchangers, catalysts and molecular sieves. Zeolites are once again drawing attention in Japan as stable adsorbents and solidification materials of fission products, such as (137)Cs(+) from damaged nuclear-power plants. Although there is a long history of scientific studies on the crystal structures and ion-exchange properties of zeolites for practical application, there are still open questions, at the atomic-level, on the physical and chemical origins of selective ion-exchange abilities of different cations and detailed atomic structures of exchanged cations inside the nanoscale cavities of zeolites. Here, the precise locations of Cs(+) ions captured within A-type zeolite were analyzed using high-resolution electron microscopy. Together with theoretical calculations, the stable positions of absorbed Cs(+) ions in the nanocavities are identified, and the bonding environment within the zeolitic framework is revealed to be a key factor that influences the locations of absorbed cations.

Defect chemistry of a BaZrO3 Σ3 (111) grain boundary by first principles calculations and space-charge theory.

Defect calculations from density functional theory are implemented with space-charge theory models to describe the equilibrium defect chemistry of a Σ3 (111) symmetric tilt boundary in BaZrO(3). As such, the space-charge potential and the concentrations of , , , NH and in the bulk, core and space-charge regions of the interface are calculated as a function of temperature and atmospheric conditions. Our results show that the core will be predominated by under hydrating conditions and that the space-charge potential increases with water vapor pressure. Under nitriding conditions, , NH and will predominate the core in different temperature regimes and effects of these defects on the space-charge properties are discussed.

Proton mobility through a second order phase transition: theoretical and experimental study of LaNbO4.

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. The activation enthalpy of mobility of protons for tetragonal LaNbO(4), determined from the conductivity curve, is 35 kJ mol(-1). Below the transition temperature the conductivity curve bends; initially dropping off steeply, followed by a less steep decrease towards lower temperatures. The bend in the conductivity curve at the onset of the phase transition in LaNbO(4) should not be given the traditional interpretation as an abrupt change in the activation enthalpy of mobility. After application of the proper analysis of the conductivity data, which takes the second order transition into account, the activation enthalpy of mobility of protons is found to continuously increase with increasing monoclinic angle at decreasing temperature, reaching approximately 57 kJ mol(-1) at 205 degrees C for the end monoclinic phase.