Bulk Lattice Parameter Research Articles

Comparing transferability in neural network approaches and linear models for machine-learning interaction potentials

Atomic simulations using machine learning interatomic potential (MLIP) have gained a lot of popularity owing to their accuracy in comparison to conventional empirical potentials. However, the transferability of MLIP to systems outside the training set poses a significant challenge. Here, we compare the transferability of three MLIP approaches: (i) neural network potentials (NNP), (ii) physical LassoLars interactions potential (PLIP) and (iii) linear potentials with Belher-Parrinello descriptors, trained over a small but diverse configuration of zinc oxide polymorphs. We compared the obtained models with density functional theory reference results for physical properties including bulk lattice parameters, surface energies, and vibrational density of states and showed the superiority of both NNP and PLIP models. However, the NNP model performed poorly when compared to the other two linear models for the structural optimization of nanoparticles and molecular dynamics simulation of liquid phases, which are systems outside the training set. While providing less accurate prediction for solid Zinc Oxides phases, both linear models appear more transferable than NNP when testing for nanoscale systems and liquid phases. Our results are finally rationalized by a combination of different statistical analysis including spread in force evaluation, information imbalance, convex hull calculation, and density in descriptor space.

Uncertainty Propagation in UAM Time-Dependent Neutronics PWR Studies

A subset of pressurized water reactor (PWR) studies from Exercise II-2 of the Uncertainty Analysis in Modeling for Light Water Reactors (UAM-LWR) benchmark study has been performed to quantify the importance of both nuclear data uncertainties and manufacturing uncertainties in an assembly depletion calculation and a mini-core rod movement transient. The depletion study of the 15 × 15 PWR assembly using the SAMPLER and TRITON modules of the SCALE code system revealed a maximum uncertainty in keff of 0.49% for fresh fuel, decreasing to 0.38% at the midpoint of the fuel cycle and rising back to 0.48% at the end of the fuel cycle. Uncertainties and correlations of various homogenized cross sections and other group constant data, such as keff, have been determined, and the effect of randomly applied manufacturing uncertainties was found to be largely negligible relative to nuclear data uncertainties for bulk lattice parameters. However, for local parameters, such as the pin power factors, assembly discontinuity factors, and diffusion coefficients, the effects from manufacturing uncertainties were appreciable and sometimes dominant. Nuclear data uncertainties were found to be the dominant contributors to uncertainty in the isotopic composition of the overall assembly, with the exception of very early in the fuel cycle, where manufacturing uncertainties such as perturbations to the fuel density and pin radius made nonnegligible contributions to total uncertainty. The contribution of manufacturing uncertainties to isotopic uncertainties was nonnegligible at a pin-by-pin level, but still smaller than the contributions from nuclear data uncertainty. Studies of the PWR mini-core rod movement transient using homogenized data from the SCALE models in the PARCS diffusion code showed little difference between the tested modeling approaches and demonstrated that nuclear data uncertainties dominated the manufacturing uncertainties in the global figures of merit considered, such as the equilibrium core boron concentration, the maximum core power factor, and the maximum reactivity insertion. For local effects, such as maximum pin power during the transient, the randomly applied manufacturing uncertainties were dominant. It was found in general that for global system properties, nuclear data uncertainties made significantly larger contributions to total uncertainty, whereas for local parameters the impact of manufacturing uncertainties was at least nonnegligible, and for some parameters, dominant.

Effect of ‘in-plane’ contraction on the (0 0 1) surface of the model perovskite SrTiO3

Reflection high-energy electron diffraction in the temperature range 8–300 K revealed several surface structural transformations accompanied with a strong ‘in-plane’ lattice parameter growth on the (001) surface of the model perovskite SrTiO3 single crystal. Rather may occurring at sharp temperatures, these transformations are observed to extend over temperature intervals separated from each other by temperature zones where no structural changes occur and the crystal lattice is relaxed. In such relaxed zones the equilibrium lattice parameters were identified and analyzed as a function of penetration depth. A strong ‘in-plane’ contraction of the crystal lattice in the first surface layers was experimentally revealed at low temperatures and theoretically confirmed by hybrid DFT calculations. Along with such an ‘in-plane’ contraction of the (001) surface at low temperatures we have found that the thermal expansion on the surface is much higher than in the bulk which results in approaching of the surface ‘in-plane’ and the bulk lattice parameters at room temperature. This contradicts the general assumption that surface ‘in-plane’ and bulk lattice parameters are identical.

The Structural, Electronic, Magnetic and Elastic Properties of Full-Heusler Co2CrAl and Cr2MnSb: An Ab Initio Study

In this paper, the full-potential, linearized augmented plane wave (FP-LAPW) method was employed in investigating full-Heusler Co2CrA1’s structural, elastic, magnetic and electronic properties. The FP-LAPW method was employed in computing the structural parameters (bulk modulus, lattice parameters, c/a and first pressure derivatives). The optimized structural parameters were determined by generalized gradient approximation (GGA) for the exchange-correlation potential, Vxc. Estimating the energy gaps for these compounds was accomplished through modified Becke–Johnson potential (mBJ). It was found that the conventional Heusler compound Co2CrA1 with mBJ and CGA approaches had a half-metallic character, and its spin-down configuration had an energy gap. It was also found that the conventional and inverse Heusler Cr2MnSb and tetragonal (139) (Co2CrA1, Cr2MnSb) compounds with a half-metallic character had direct energy gaps in the spin-down configuration. To a certain degree, the total magnetic moments for the two compounds were compatible with the theoretical and experimental results already attained. Mechanically, we found that the conventional and inverse full-Heusler compound Co2CrAl was stable, but the inverse Cr2MnSb was unstable in the ferromagnetic state. The conventional Heusler compound Cr2MnSb was mechanically stable in the ferromagnetic state.

Shedding Synchrotron Light on Catalyst Strain Dynamics in Electrochemical Environment

In the case of heterogeneous catalysis (where the reactions occur onto the catalyst surface), the most promising approach in material design follows the Sabatier principle: the ability of the surface to bind adsorbates and the strength of the bonds define the reaction thermodynamics and kinetics. In this respect, the catalyst surface chemistry and structure (crystallographic orientation of the facets and/or strain) are used as levers to optimize its performance. However, it is largely accepted that the structure of PEMWE anode and PEMFC cathode catalysts does not remain unaltered over the long term operation due to harsh (electro)chemical conditions and it is of a high interest to understand the structural changes during the electrocatalysis. In this contribution we use X-ray diffraction techniques to qualitatively and quantitatively describe those changes and shed light on the strain evolution in nanostructured catalysts during operation in the electrochemical environment.Using the newly upgraded 4th generation EBS source at European Synchrotron Radiation Facility, we study Pt and Pd catalysts in liquid half cells at various potentials. In the Pd case we focus on the phase and structural dynamics during H insertion, allowing to unveil the electrochemically-driven Pd hydrides phase transition, which was until now mostly investigated in the gas phase. The fine XRD patterns measured operando provided the first detection of theoretically predicted supersaturated and undersaturated metastable states, involved in a core-shell mechanism of the phase transition (Figure 1) [1].In the case of Pt nanocatalyst, monitoring the structural features of the catalyst during cyclic voltammetry permits studying adsorption and oxidation processes. Deconvoluting both effects (adsorption and oxidation) is possible by following different parameters of the analyzed XRD patterns with high temporal resolution. Such experiments reveal important dynamic trends linked to the surface oxidation and degradation, directly impacting the output of accelerated stress tests (AST) [2]. Carefully determining the charge passed through the circuit during the CV also allows to establish a near-linear correlation between the expansion of the bulk lattice parameter of Pt nanoparticles and their oxide surface coverage. Since the latter is known to be a descriptor of catalysts activity toward numerous reactions (oxygen reduction and fuel oxidation), the measurement of the lattice parameter according to this simple diffraction approach allows experimental access to this descriptor during operando measurements in device-relevant sample environments [2]. This constitutes a main advantage compared to spectroscopic techniques and is foreseen to find a wide range of applications, as demonstrated by measuring the structural behavior of the state-of-the-art PtNiIr octahedra catalyst in operating PEMFC. In this case, the results allow correlating the degradation of the catalysts cycled to different lower potentials with the oxidation and strain dynamics. Based on this data, the operating conditions for this catalyst can be determined, further defining the application of this material.[1] Chattot, R. et al., J. Am. Chem. Soc. 143(41), 17068–17078 (2021).[2] Martens, I. et al., ACS Appl. Energy Mater. 2 (11), 7772-7780 (2019). Figure 1

Electronic and Band Structure calculation of Wurtzite CdS Using GGA and GGA+U functionals

The wurtzite (wz) structure of CdS is analysed using density functional theory within the generalized gradient approximation (GGA) and Hubbard correction (GGA+U). The total energy convergence evaluation is carried out concerning energy cut-off (ecutwfc) and k-point sampling. The geometry optimization of wz-CdS is calculated using the total energy and force minimization process, which is based on the Broyden-Fletcher-Goldfarb-Shanno (BFGS) optimization algorithm. Bulk modulus and lattice parameters are estimated to ensure accuracy of the calculations. The electronic band structure, density of states (DOS), and projected density of states (PDOS) of wz-CdS are analysed. The band structure calculation shows CdS as direct band gap semiconductor. The electronic correlation in CdS is altered by varying U-parameters of valence orbitals of Cd and S. The alteration of electronic correlation results in convergence of the band gap to the experimental value 2.4 eV. The alteration of U-parameter affects substantially the density of states near the band edges.

Computational study of the energy landscape of water on the ThO2 {111} surface

The bulk and surface properties of ThO2 are studied computationally using density functional theory within the LSDA+U approach. The computational method is benchmarked against bulk lattice parameter and band gap, the best combined description of which is with the PBE functional and the Liechtenstein method with U = 7 and J = 1, and without spin-orbit coupling. The optimised computational settings are used to study the reaction of water with the {111} surface, which we find to be hydrophilic. Molecular adsorption of a single water molecule is energetically favourable, and dissociation of this water to form surface hydroxyl groups is barrierless. This wetted surface can aid further water adsorption, with an even lower free energy difference than the first adsorption, although dissociation of the second water is much harder than the first. Reaction of H+, H. radical or OH− groups with the wetted {111} surface has a substantial energy barrier, and reaction with H+ is very sensitive to temperature.

Surface and interface structures of epitaxial Sb2Se3 on mica

Sb2Se3 thin film is an emerging photon absorber used in solar cells. We report the study of surface and interface structures of Sb2Se3(120) film grown on mica substrate by a high-rate vapor transport method. The interface epitaxial relationship between Sb2Se3 and mica examined by the cross- sectional TEM images and diffraction patterns along the [001] and [10]2¯ directions of Sb2Se3 reveal a rectangular structure with lengths of 4.03 ± 0.1 Å and 5.29 ± 0.1 Å, consistent with the [120] out-of-plane direction of Sb2Se3 bulk lattice parameters. In contrast, the two-dimensional reciprocal space map (2D map) constructed from azimuthal reflection high-energy electron diffraction (ARHEED) patterns from the surface exhibits a decorated hexagonal structure. This surface structure emerges from six epitaxial orientation domains/rods and each domain has a rectangular unit mesh of 3.94 ± 0.09 Å and 26.95 ± 1.16 Å along the [001] and [10]2¯ directions. The 26.95 Å is consistent with the unit mesh of the outermost layer of the Sb2Se3(120) domains/rods. Our 2D map reveals surface information that are not easily observed by other diffraction techniques.

Theoretical investigation of structural, mechanical, electronic and thermal behavior of platinum group metals and their intermetallic alloys PtRhX (X = Pd, Ir, Os, Ru)

The first-principles study of platinum group metals (Pt, Pd, Rh, Ir, Os, Ru) and their intermetallic alloys PtRhX (X = Pd, Ir, Os and Ru) are presented. Total energy was calculated with two different correlation functionals i.e. the local density approximation (LDA) and the generalized gradient approximation (GGA). The ground state quantities such as bulk modulus and lattice parameter have been evaluated. The structure stability of the different alloys was investigated by the three independent elastic constants (C11, C12, and C44). We have also calculated the electronic band structures and densities of states. The electronic calculations revealed that the alloys show transitions of electrons from valence to conduction bands there by making these alloys metallic. To study the fundamental characteristics of these alloys, we have also analyzed the thermodynamic characteristics such as Debye temperatures, vibrational energy, entropy, and constant-volume specific heat at different temperatures range from 0 K to 1000 K.

Sub-molecular structural relaxation at a physisorbed interface with monolayer organic single-crystal semiconductors

Arranging molecules into highly symmetric, topological crystal structures has been recognized as the best approach to functionalize electronic properties in molecular crystals, where the constituent molecules have been assumed to be rigid in shape. Here, in striking contrast, we demonstrate that the molecules in a monolayer organic crystal can undergo a significant deformation in proximity to the substrate, which is reflected by an asymmetry in the electron density profile. X-ray reflectivity and X-ray absorption spectroscopies in conjunction with density-functional theory calculations reveal that the highly planarized π-core are deformed into a bent shape, while the bulk lattice parameters are maintained. The molecular shape change is found to be perfectly suppressed in a bilayer single crystal, which leads to a 40% increase in mobility in the bilayer crystal. Our finding of a unique, sub-molecular scale shape change in monolayer single crystals can offer possibilities for functionalizing electrical properties via nano-scale physisorption.

Lattice Parameter Hysteresis in Li- and Mn-Rich Layered Oxides and Its Dependence on State of Charge and Open Circuit Voltage

Li- and Mn-rich layered oxides, also known as high energy NCM (HE-NCM, Li1+x[NiCoMn]1-xO2 with typically 0.1 < x < 0.2), are promising next-generation cathode active materials (CAMs) for Li-ion batteries, since they contain intrinsically more lithium ions than regular NCM materials (x < 0.05). The gain in capacity is accompanied by the drawbacks of voltage hysteresis (between charge and discharge) and voltage fade (during long-term cycling).1 The voltage hysteresis is a unique property of over-lithiated layered oxides, which does not only manifest itself in the voltage profile, but also in the cathode resistance,2 the redox activity of transition metals (TM) and oxygen,2 and the bond distances.3 Figure 1 shows our results of an in-situ X-ray diffraction (XRD) experiment during the initial cycles of a Li-rich NCM material. As seen in Figure 1a and b, the state of charge (SOC, equivalent to total xLi) dependence of the bulk lattice parameters a and c after the first activation cycle is characterized by a pronounced hysteresis between charge and discharge. Here, the hysteresis loop of the lattice parameter a is clearly different from that of c. Furthermore, the hysteresis of the lattice parameter a vanishes when it is correlated to the open circuit voltage (OCV) of the respective SOC (see Figure 1c), but c maintains the shifted maxima during charge and discharge (see Figure 1d). Based on the literature, these observations could be either explained by a path dependency of the reversible TM migration between TM and Li layer,4 the oxygen redox,2 or a combination of both.5 In addition to in-situ XRD data, we will show co-refined X-ray and neutron diffraction data of selected ex-situ samples, which have either the same SOC or OCV. By means of diffraction techniques, it is our goal to elucidate the lattice parameter variations in Li-rich CAMs and to clarify the often described contribution of TM migration to the hysteresis effects. Figure 1: In-situ XRD measurement of HE-NCM during the first 3 cycles. The pouch cell was cycled at a rate of C/10 between 2.0-4.8 V against metallic lithium with intermediate OCV steps to collect XRD patterns (at a laboratory diffractometer with Mo-Kα1 radiation). Using the trigonal model (space group R-3m), the lattice parameters a and c are shown as a function of SOC (a,b) and OCV (c,d), respectively. Acknowledgements: We want to acknowledge BASF SE for the support within the frame of its scientific network on electrochemistry and batteries and the BMBF (Federal Ministry of Education and Research, Germany) for its financial support within the ExZellTUM II project (grant number 03XP0081). We also thank the research neutron source FRM II for access to the instrument SPODI.

Influence of composition and oxygen-vacancy ordering on lattice parameter and elastic moduli of Ce1-xGdxO2-x/2: A theoretical study

We study the behaviour of the lattice parameter and elastic moduli of Ce1-xGdxO2-x/2 for the random (fluorite-like) and C-type ordered oxygen-vacancy configurations [Žguns et al., PCCP 20 (2018) 11805-11818]. For the fluorite phase, elastic moduli decrease linearly with Gd concentration. For the C-type phase, the bulk, shear and Young moduli are found to be systematically larger and the lattice parameter smaller than those for disordered fluorite phase. Essentially the linear behaviour of the bulk modulus and lattice parameter depending on the degree of the C-type order is found. Our findings explain the experimentally observed elastic moduli of Ce1-xGdxO2-x/2.

(Energy Technology Division Supramaniam Srinivasan Young Investigator Award Address) Enhanced Oxygen Electrocatalysis By Means of Electronic and Geometric Effects

The slow kinetics of the oxygen reduction and evolution reactions (ORR and OER) impede the widespread uptake of polymer electrolyte membrane (PEM) fuel cells and electrolysers. In order to improve the kinetics of the ORR and reduce the Pt loading at the fuel-cell cathode, we need to enhance the activity, stability and selectivity of the catalyst. This can be achieved by alteration of the electronic properties of the Pt surface atoms (electronic effects [1,2]) and/or modification of the geometric structure (atomic ensemble effects [3]). First, I will address atomic ensemble effects in electrocatalysis. We used a self-ordered molecular pattern of cyanide on Pt(111) surfaces to study the role of atomic ensembles for the ORR [3]. Cyanide groups form an ordered structure that blocks all the three-fold hollow sites, effectively blocking the adsorption of spectator anions in the electrolyte. Nonetheless, the holes in this structure are sufficiently large to allow the adsorption of reaction oxygen molecules. As a consequence, cyanide-modified Pt(111) presents a 25-fold enhancement over Pt(111) [3]. Secondly, I will focus on electronic effects by alloying Pt with other metals. In order to reduce the Pt loading, researchers have intensively studied alloys of Pt with late transition metals such as Ni and Co. However, these compounds typically degrade under fuel-cell conditions, due to dealloying. In contrast, Pt-lanthanide alloys present a very negative alloying energy, which should increase their resistance to degradation. We have studied novel Pt-lanthanide and Pt-alkaline earth electrocatalysts for the ORR. Pt-lanthanide and Pt-alkaline earth alloys are amongst the most active polycrystalline Pt-based catalysts ever reported [1,2], presenting up to a 6-fold increase in ORR activity, relative to Pt. The active phase consists of a compressed Pt overlayer of few Pt layers formed onto the bulk alloys by acid leaching. Notably, the oxygen reduction activity versus the bulk lattice parameter follows a volcano relation [1]. We use the lanthanide contraction to control strain effects and tailor the activity, stability and reactivity of Pt alloys. Finally, I will present new approaches to design and develop more efficient and stable OER electrocatalysts. We have explored the OER activity and stability of RuO2 nanoparticulate catalysts. Sub-monolayer amounts of IrOx on top of RuO2 thin films minimise Ru dissolution [4]. The strategy of tailoring the surface of the catalyst with sub-monolayer amounts of a stable oxide may allow us to tune the stability of active OER catalysts for PEM electrolysers.

Combined strain and composition-induced effects in the metal-insulator transition of epitaxial VO2 films

The role of epitaxial strain, thermal strain, and bulk (strain-free) lattice parameters in the metal-insulator transition (MIT) and the structural phase transition (SPT) of VO2 is investigated for the case of epitaxial films grown on (001)-oriented TiO2 substrates. Temperature-resolved X-ray reciprocal space mapping has been used to determine the absolute state of strain as well as the bulk lattice parameters of VO2 at 100 °C. For the thinnest film (15 nm), the state of strain is dominated by the film/substrate lattice mismatch yielding an in-plane tensile strain which, in turn, shifts both the MIT and the SPT towards lower temperatures. Conversely, for the thickest film (100 nm), the epitaxial strain is relaxed, so that the state of strain is dominated by the VO2/TiO2 thermal expansion mismatch which is responsible for a compressive in-plane strain. In all cases, a swelling of the strain-free VO2 unit-cell is observed which indicates the presence of interfacial oxygen vacancies and/or Ti diffusion into the VO2 films. The presence of oxygen vacancies stabilizes the metallic rutile phase and counterbalances the action of thermal strain on the MIT and the SPT and degrades the electric properties for the thinnest film. For the thickest film, the resistivity ratio is 6.4 × 104.

Understanding the impacts of Al+3-substitutions on the enhancement of magnetic, dielectric and electrical behaviors of ceramic processed nickel-zinc mixed ferrites: FTIR assisted studies

Al-incorporated Ni-Zn ferrites (Ni0.65Zn0.35AlxFe2−xO4, where x=0, 0.03, 0.06, 0.09, 0.12) synthesized via standard ceramic technology were investigated for their structural, magnetic, dielectric, and electrical properties. Al substitution plays a remarkable role in refining the structural features and other properties of the ferrite particles. The bulk density, porosity and lattice parameters of Ni-Zn ferrites were decreased with increased Al-content while the X-ray density was reduced. XRD patterns confirmed the single-phase cubic spinel structure of the ferrites particles. The room temperature infra-red spectra shows the features of higher and lower energy bands detected at ν2∼400–405cm−1 and ν1∼590–594cm−1, respectively corresponded to tetrahedral (T-band) and octahedral vibration (O-band) complexes that also confirm the formation of Ni-Zn inverse spinels. The real part of the complex initial permeability, Curie temperature, ac resistivity and loss tangent of the ferrite powders were reduced while the quality factor was enhanced with the subsequent Al-substitution. The observed bi-layer space charge polarization feature was believed to correlate the normal dielectric behavior of Al-substituted Ni-Zn matrix.

Structural and electronic properties of molecular beam epitaxially grown Ni1+xTiSn films

This paper demonstrates the molecular beam epitaxial growth of metallic full-Heusler Ni2TiSn precipitates within semiconducting NiTiSn host matrix, and discusses the structural and chemical stability of this biphasic epitaxial composite and its influence on the electronic properties. Structural properties of the epitaxial films were characterized by in-situ reflection high-energy electron diffraction and ex-situ by X-ray diffraction and transmission electron microscopy. The results indicate the presence of a secondary phase with full Heusler ordering within half-Heusler host matrix. Both the parent half-Heusler and the secondary full-Heusler phases in the epitaxial films are strained from their bulk lattice parameters. Electronic properties show an increase in resistivity at low Ni excess.

Exploring the Lanthanide Contraction to Tune the Activity and Stability of Pt

The high platinum loadings required to compensate for the slow kinetics of the oxygen reduction reaction (ORR) impede the widespread uptake of polymer electrolyte membrane fuel cells. In order to improve the ORR kinetics and reduce the Pt loading, we can tailor the electronic properties of the Pt surface atoms by means of alloying Pt with other metals. Researchers have intensively studied alloys of Pt with late transition metals such as Ni and Co during the last decades. However, these compounds typically degrade under fuel cell reaction conditions, due to dealloying. In contrast, alloys of Pt and lanthanides present very negative enthalpy of formation [1,2], which should increase their resistance to degradation. Herein we present eight novel Pt-lanthanide and Pt-alkaline earth ORR electrocatalysts: Pt5La, Pt5Ce, Pt5Sm, Pt5Gd, Pt5Tb, Pt5Dy, Pt5Tm and Pt5Ca [3]. All the materials are highly active, presenting a 3 to 6-fold activity enhancement over Pt. Pt5Tb is the most active polycrystalline Pt-based catalyst reported in the literature. A Pt overlayer with a thickness of few Pt layers is formed onto the bulk alloys by acid leaching [1-3]. Notably, the experimental ORR activity as a function of the bulk lattice parameter and the Pt-Pt distance follows a “volcano” relation [3], with Pt5Tb presenting the highest initial activity while Pt5Gd is the most active after 10 000 cycles stability test between 0.6 and 1.0 V versus the reversible hydrogen electrode. We use the lanthanide contraction to control strain effects and tune the electrocatalytic activity, stability and reactivity of Pt [3].

Asynchronous Stress and Strain Measurements of LiMn2O4 Composite Electrodes

LiMn2O4 (LMO) is a promising cathode material for Li ion batteries due to its high voltage, environmental benignity, and natural abundance.1,2 Currently, LMO suffers from capacity fade due to Mn ion dissolution and a Jahn-Teller distortion which reduces the stability of the cathode structure.3,4 Mechanical thin film measurements can provide information on the mechanical deformations of LMO during cycling and how these deformations affect the stability and longevity of LMO. The surface stress and bulk strain associated with the mechanical deformation of LiMn2O4 as a result of lithiation and delithiation is measured using an in situ bending cantilever technique and an in situ digital image correlation technique, respectively. The combination of these techniques allows for a comparison between bulk and surface changes in the cathode material in a slurry cast electrode. The composite electrodes consist of ball-milled LMO active material, carboxymethylcellulose sodium salt binder, and conductive Super P carbon black and were tested in 1 M LiPF6 electrolyte in 1:1 ethylene carbonate/dimethyl carbonate. Strain measurements show volume contraction upon delithiation and volume expansion upon lithiation of the LMO electrode. The derivative of strain with respect to potential exhibits four distinct features, two upon delithiation (see figure) and two upon lithiation. The main features are aligned with the peaks in the cyclic voltammogram (CV) and correlate to changes in the LMO spinel lattice parameters.5 Tensile stress (surface contraction) is observed upon LMO delithiation, and compressive stress (surface expansion) is observed upon lithiation. The derivative of stress with respect to potential also exhibits four distinct features. The stress derivative feature at lower potentials during delithiation precedes the current peak and therefore indicates that a significant surface contraction occurs before delithiation (see figure). Stress and strain occur asynchronously during delithiation of LMO suggesting that different processes control bulk versus surface expansion and contraction. Strain is governed by changes in the bulk lattice parameters, and stress is controlled by numerous processes such as surface film growth and lattice distortions at the electrode/electrolyte interface. References Thackeray, M. M.; Johnson, P. J.; Depicciotto, L. A.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1984, 19, 179.Yi, T. F.; Zhu, Y. R.; Zhu, X. D.; Shu, J.; Yue, C. B.; Zhou, A. N. Ionics 2009, 15, 779.Thackeray, M. M.; David, W. I. F.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1983, 18, 461.Amatucci, G. G.; Schmutz, C .N.; Blyr, A.; Sigala, C.; Gozdz, A. S.; Larcher, D.; Tarascon, J. M. J. Power Sources 1997, 69, 11.Sun, X.; Yang, X. Q.; Balasubramanian, M.; McBreen, J.; Xia, Y.; Sakai, T. J. Electrochem. Soc. 2002, 149, A842. Figure 1

Tuning the activity of Pt alloy electrocatalysts by means of the lanthanide contraction.

The high platinum loadings required to compensate for the slow kinetics of the oxygen reduction reaction (ORR) impede the widespread uptake of low-temperature fuel cells in automotive vehicles. We have studied the ORR on eight platinum (Pt)-lanthanide and Pt-alkaline earth electrodes, Pt5M, where M is lanthanum, cerium, samarium, gadolinium, terbium, dysprosium, thulium, or calcium. The materials are among the most active polycrystalline Pt-based catalysts reported, presenting activity enhancement by a factor of 3 to 6 over Pt. The active phase consists of a Pt overlayer formed by acid leaching. The ORR activity versus the bulk lattice parameter follows a high peaked "volcano" relation. We demonstrate how the lanthanide contraction can be used to control strain effects and tune the activity, stability, and reactivity of these materials.

Pressure-induced phase transition in CrO2.

The ab initio constant pressure molecular dynamics technique and density functional theory with generalized gradient approximation (GGA) was used to study the pressure-induced phase transition of CrO2. The phase transition of the rutile (P42/mnm) to the orthorhombic CaCl2 (Pnnm) structure at 30 GPa was determined successfully in a constant pressure simulation. This phase transition was analyzed from total energy calculations and, from the enthalpy calculation, occurred at around 17 GPa. Structural properties such as bulk modules, lattice parameters and phase transition were compared with experimental results. The phase transition at 12 ± 3 GPa was in good agreement with experimental results, as was the phase transition from the orthorhombic CaCl2 (Pnnm) to the monoclinic (P21/c) structure also found at 35 GPa.