AndreaBozzo/ceres-open-data-index huggingface dataset:
Large magnetoresistance and nonzero Berry phase in the nodal-line semimetal MoO2

We performed calculations of the electronic band structure and the Fermi surface as well as measured the longitudinal resistivity ρxx(T,H), Hall resistivity ρxy(T,H), and quantum oscillations of the magnetization as a function of temperature at various magnetic fields for MoO2 with a monoclinic crystal structure. The band structure calculations show that MoO2 is a nodal-line semimetal when the spin-orbit coupling is ignored. It was found that a large magnetoresistance reaching 5.03 × 10^4% at 2 K and 9 T, its nearly quadratic field dependence, and a field-induced up-turn behavior of ρxx(T), the characteristics common for many topologically nontrivial as well as trivial semimetals, emerge also in MoO2. The observed properties are attributed to a perfect charge-carrier compensation, evidenced by both calculations relying on the Fermi surface topology and the Hall resistivity measurements. Both the observation of negative magnetoresistance for the magnetic field along the current direction and the nonzero Berry phase in de Haas–van Alphen measurements indicate that pairs of Weyl points appear in MoO2, which may be due to the crystal symmetry breaking. These results highlight MoO2 as a platform for studying the topological properties of oxides.

Tags: berry-phase, cscs, epfl, magnetoresistance, marvel, moo2
Prediction of yield strength in refractory body-centered-cubic High Entropy Alloys

Energy efficiency is motivating the search for new high-temperature metals. Some new body-centered-cubic random multicomponent "high entropy alloys (HEAs)" based on refractory elements (Cr-Mo-Nb-Ta-V-W-Hf-Ti-Zr) possess exceptional strengths at high temperatures but the physical origins of this outstanding behavior are not known.
Here, by using a recent mechanistic theory, we have computed the high-temperature (T=1300K) yield strength based on solute strengthening of over 10 million alloys within the whole Al-Cr-Mo-Nb-Ta-V-W-Hf-Ti-Zr alloy family. Also the yield strength/density has been computed.
This database enables the efficient search of new alloys with exceptional high-temperature strength.

Tags: epfl, erc, high-entropy-alloys, high-temperature-strength, solute-strengthening
Polymer descriptor data set for machine learning prediction of specific heat

We have developed a polymer descriptor data set from the existing data. The data set has 188 descriptors which describe polymer atomic and molecular behavior. The descriptors are mapped to the specific heat of polymers using supervised and unsupervised machine learning approaches. The mapping helps predict the specific heat of polymers at room temperature. The descriptor data set is useful in synthesizing novel polymers with desired heat capacities.

Tags: descriptors, polymer, specific-heat
Smart local orbitals for efficient calculations within density functional theory and beyond

The record contains data to support our research findings regarding the development of a novel method for deriving localized basis sets in the projector augmented wave formalism, allowing to obtain a reduced basis set of atomic orbitals through the subdiagonalization of each atomic block of the Hamiltonian. The resulting local orbitals (LOs) inherit the information of the local crystal field. In the LO basis, it becomes apparent that the Hamiltonian is nearly block-diagonal, and we demonstrate that it is possible to keep only a subset of relevant LOs that provide an accurate description of the physics around the Fermi level. This reduces to some extent the redundancy of the original basis set, and at the same time, it allows one to perform post-processing of DFT calculations, ranging from the interpretation of electron transport to extracting effective tight-binding Hamiltonians, very efficiently and without sacrificing the accuracy of the results.

Tags: marveldd3, minimal-basis-sets, subdiagonalization, transport
Effects of interlayer confinement and hydration on capacitive charge storage in birnessite

Nanostructured birnessite (δ-MnO2) exhibits high specific capacitance and nearly ideal capacitive behavior in aqueous electrolytes, rendering it an important electrode material for low-cost, high power energy storage devices. The mechanism of electrochemical capacitance in birnessite has been described as both faradaic (involving redox) and non-faradaic (involving only electrostatic interactions). To clarify the capacitive mechanism, we characterized birnessite's response to applied potential using ex situ X-ray diffraction, electrochemical quartz crystal microbalance, in situ Raman spectroscopy, and operando atomic force microscopy dilatometry to provide a holistic understanding of its structural, gravimetric, and mechanical response. These observations are supported by atomic-scale simulations using density functional theory for the cation-intercalated structure of birnessite and ReaxFF-based molecular dynamics, as well as ReaxFF-based grand canonical Monte Carlo simulations on the dynamics at the birnessite/water/electrolyte interface. We show that capacitive charge storage in birnessite is governed by interlayer cation intercalation. We conclude that the intercalation appears capacitive due to the presence of nanoconfined interlayer structural water, which mediates the interaction between the intercalated cation and the birnessite host and leads to minimal structural changes.

Tags: electrochemistry, energy-storage, oxide
Low-frequency dielectric response of tetragonal perovskite CH3NH3PbI3

The dielectric properties of tetragonal hybrid perovskite CH3NH3PbI3 are studied through molecular dynamics at a temperature of 300 K in the presence of a finite electric field. The high-frequency dielectric constant ε∞ is found to be 4.5 along the a axis and 4.7 along the c axis. The values of the respective static dielectric constants ε0 are 43 ± 1 and 53 ± 3, much larger than the value of ∼25 pertaining to the orthorhombic phase, in which the organic cations cannot rotate. At frequencies below 3 cm–1, we observe a significant increase in ε0 by ∼23 (a axis) and ∼30 (c axis) compared to a vibrational approach, which does not account for the reorientation of the molecular units. The decomposition shows that the reorientation of the organic cations accounts for an increase of only ∼10. An increase of similar size results from the displacement of the cations within the cages of the lattice. The dominant contribution is found to arise from lattice vibrations coupled to the motion of the organic cations.

Tags: dielectric-properties, epfl, hybrid-perovskite, molecular-dynamics
In situ inorganic conductive network enables superior high-voltage operation of single-crystal Ni-rich cathode

High nickel content in LiNixCoyMnzO2 (NCM, x ≥ 0.8, x + y + z = 1) layered cathode material allows high energy density in lithium-ion batteries (LIBs). However, Ni-rich NCM cathodes suffer from performance degradation, mechanical and structural instability upon prolonged cell cycling. Although the use of single-crystal Ni-rich NCM can mitigate these drawbacks, the ion-diffusion in large single-crystal particles hamper its rate capability. Herein, we report a strategy to construct an in situ Li1.4Y0.4Ti1.6(PO4)3 (LYTP) ion/electron conductive network which interconnects single-crystal LiNi0.88Co0.09Mn0.03O2 (SC-NCM88) particles. The LYTP network facilitates the lithium-ion transport between SC-NCM88 particles, mitigates mechanical instability and prevents detrimental crystalline phase transformation. When used in combination with a Li metal anode, the LYTP-containing SC-NCM88-based cathode enables a coin cell capacity of 130 mAh g-1 after 500 cycles at 5 C rate in the 2.75-4.4 V range at 25 °C. Tests in Li-ion pouch cell configuration (i.e., graphite used as negative electrode active material) demonstrate capacity retention of 85% after 1000 cycles at 0.5 C in the 2.75-4.4 V range at 25 °C for the LYTP-containing SC-NCM88-based positive electrode.

Tags: conductive-network, crystalline-phase-transformation, experimental, in-situ, lithium-ion-transport, single-crystal-ni-rich-ncm
Efficient and low-scaling linear-response time-dependent density functional theory implementation for core-level spectroscopy of large and periodic systems

A new implementation of linear-response time-dependent density functional theory (LR-TDDFT) for core level near-edge absorption spectroscopy is discussed. The method is based on established LR-TDDFT approaches to X-ray absorption spectroscopy (XAS) with additional accurate approximations for increased efficiency. We validate our implementation by reproducing benchmark results at the K-edge and showing that spin–orbit coupling effects at the L2,3-edge are well described. We also demonstrate that the method is suitable for extended systems in periodic boundary conditions and measure a favorable sub-cubic scaling of the calculation cost with system size. We finally show that GPUs can be efficiently exploited and report speedups of up to a factor 2.

Tags: low-scaling-algorithm, marveldd4, method-development, tddft, xas
Ferroelectricity promoted by cation/anion divacancies in SrMnO3

We investigate the effect of polar Sr-O vacancy pairs on the electric polarization of SrMnO3 (SMO) thin films using density functional theory (DFT) calculations. This is motivated by indications that ferroelectricity in complex oxides can be engineered by epitaxial strain but also via the defect chemistry. Our results suggest that intrinsic doping by cation and anion divacancies can induce a local polarization in unstrained non-polar SMO thin films and that a ferroelectric state can be stabilized below the critical strain of the stoichiometric material. This polarity is promoted by the electric dipole associated with the defect pair and its coupling to the atomic relaxations upon defect formation that polarize a region around the defect. This suggests that polar defect pairs affect the strain-dependent ferroelectricity in semiconducting antiferromagnetic SMO. For metallic ferromagnetic SMO we find a much weaker coupling between the defect dipole and the polarization due to much stronger electronic screening. Coupling of defect-pair dipoles at high enough concentrations along with their switchable orientation thus makes them a promising route to affect the ferroelectric transition in complex transition metal oxide thin films.

Tags: cationanion-divacancies, cscs, dftu, ferroelectricity, marveldd5, point-defects, polarization, snsf, srmno3, strain, thin-films
Yield strength and misfit volumes of NiCoCr and implications for short-range-order

The face-centered cubic medium-entropy alloy NiCoCr has received considerable attention for its good mechanical properties, uncertain stacking fault energy, etc, some of which have been attributed to chemical short-range order (SRO). Here, we examine the yield strength and misfit volumes of NiCoCr to determine whether SRO has measurably influenced mechanical properties. Polycrystalline strengths show no systematic trend with different processing conditions. Measured misfit volumes in NiCoCr are consistent with those in random binaries. Yield strength prediction of a random NiCoCr alloy matches well with experiments. Finally, we show that standard spin-polarized density functional theory (DFT) calculations of misfit volumes are not accurate for NiCoCr. This implies that DFT may be inaccurate for other subtle structural quantities such as atom-atom bond distance so that caution is required in drawing conclusions about NiCoCr based on DFT. These findings all lead to the conclusion that, under typical processing conditions, SRO in NiCoCr is either negligible or has no systematic measurable effect on strength.

Tags: epfl, high-entropy-alloy, marveldd2, short-range-order, snsf, yield-strength
Modeling the Ga/As binary system across temperatures and compositions from first principles

Materials composed of elements from the third and fifth columns of the periodic table display a very rich behavior, with the phase diagram usually containing a metallic liquid phase and a polar semiconducting solid. As a consequence, it is very hard to achieve transferable empirical models of interactions between the atoms that can reliably predict their behavior across the temperature and composition range that is relevant to the study of the synthesis and properties of III/V nanostructures and devices. We present a machine-learning potential trained on density functional theory reference data that provides a general-purpose model for the Ga/As system. We provide a series of stringent tests that showcase the accuracy of the potential, and its applicability across the whole binary phase space, computing with ab initio accuracy a large number of finite-temperature properties as well as the location of phase boundaries. We also show how a committee model can be used to reliably determine the uncertainty induced by the limitations of the machine-learning model on its predictions, to identify regions of phase space that are predicted with insufficient accuracy, and to iteratively refine the training set to achieve consistent, reliable modeling.

Tags: gaas, gallium-arsenide, marvel, molecular-dynamics-simulation, neural-network-potential, snsf
On-surface synthesis of non-benzenoid nanographenes by oxidative ring-closure and ring-rearrangement reactions

In this record we provide data supporting our recent results discussed in the fabrication of non-benzenoid nanographenes. Nanographenes (NGs) have gained increasing attention due to their immense potential as tailor-made organic materials for nanoelectronics and spintronics. They exhibit a rich spectrum of physicochemical properties that can be tuned by controlling the size or the edge structure or by introducing structural defects in the honeycomb lattice. In the published manuscript we report the design and on-surface synthesis of NGs containing several odd-membered polycycles induced by a thermal procedure on Au(111). Our scanning tunneling microscopy, noncontact atomic force microscopy, and scanning tunneling spectroscopy measurements, complemented by computational investigations, describe the formation of two nonbenzenoid NGs (2A,B) containing four embedded azulene units in the polycyclic framework, via on-surface oxidative ring-closure reactions. Interestingly, we observe surface-catalyzed skeletal ring rearrangement reactions in the NGs, which lead to the formation of additional heptagonal rings as well as pentalene and as-indacene units in 2A,B, respectively. 2A,B on Au(111) both exhibit narrow experimental frontier electronic gaps of 0.96 and 0.85 eV, respectively, and Fermi level pinning of their HOMOs together with considerable electron transfer to the substrate. Ab initio calculations estimate moderate open-shell biradical characters for the NGs in the gas phase.

Tags: ab-initio-simulations, cscs, dfg-nsfc, horizon-2020, marveldd3, nanographenes, open-shell, snsf
Understanding thermal quenching of photoluminescence in oxynitride phosphors from first principles

Understanding the physical mechanisms behind thermal effects in phosphors is crucial for white light-emitting device (WLEDs) applications, as thermal quenching of their photoluminescence might render them useless. We analyze from first-principles, before and after absorption/emission of light, two chemically close Eu-doped Ba₃Si₆O₁₂N₂ and Ba₃Si₆O₉N₄ crystals for WLEDs. The first one has an almost constant emission intensity with increasing temperature whereas the other one does not. Our results, in which the Eu-5d levels are obtained inside the band gap thanks to the removal of an electron from the 4f⁷ shell, and the atomic neighborhood properly relaxed in the excited state, attributes the above-mentioned experimental difference to an autoionization model of the thermal quenching, based on the energy difference between Eu 5d and the conduction band minimum. Our depleted-shifted 4f method can identify luminescent centers and therefore allows for effective crystal site engineering of luminescent centers in phosphors from first principles.

Tags: ab-initio, bson, ceci, first-principles, frs-fnrs, oxynitride-phosphors, photoluminescence, rare-earth, thermal-quenching
Plasmon energy changes in FeMo14C15B6Erx (x=0-2) bulk metallic glass during in-situ heating

Variations of volume plasmon energy of both ribbon and bulk FeMo14C15B6Erx (x=0-2) metallic glasses were measured as a function of the temperature in an analytical transmission electron microscope using valence electron energy loss spectroscopy (VEELS). The plasmon energy was found to decrease with increasing temperature, due not only to thermal expansion but also to chemical reordering in the glasses. The chemical reordering stimulates a specific solute cluster formation; M23(C, B)6 solute clusters began to form above about 200°C in both ribbon and bulk FeMo14C15B6Erx (x=0, 0.5, 1) metallic glasses. The formation of the M23(C, B)6 solute clusters was only found above 400°C in the ribbon FeMo14C15B6Er2 metallic glass, indicating inhibition of the M23(C, B)6 solute clusters occurred owing to the formation of Er-(C, B) complexes/clusters. The Er-(C, B) complexes/clusters were formed in the cooling process of the sample fabrication. In contrast to the ribbon sample, the formation of the M23(C, B)6 solute clusters did not occur in the bulk FeMo14C15B6Er2 metallic glass during heating. The amount of the Er-(C, B) complexes/clusters in the bulk sample was larger than that in the ribbon sample because of the slow cooling rate in the bulk sample fabrication. Because the solute carbon and boron atoms were mostly consumed by the formation of Er-(C, B) complexes/clusters in the bulk FeMo14C15B6Er2 metallic glass, the formation of M23(C, B)6 solute clusters was suppressed. This study demonstrates that chemical reordering during the relaxation of metallic glasses can be detected by the VEELS, providing valuable insight into the relaxation stage of metallic glass during heating.

Tags: bulk-metallic-glass, chemical-reordering, electron-energy-loss-spectroscopy, experimental, in-situ-heating, volume-plasmon
On-surface synthesis of singly and doubly porphyrin-capped graphene nanoribbon segments

In this record we provide data to support our recent findings on the synthesis of porphyrin-capped graphene nanoribbons. On-surface synthesis has emerged as a powerful tool for the construction of large, planar, π-conjugated structures that are not accessible through standard solution chemistry. Among such solid-supported architectures, graphene nanoribbons (GNRs) hold a prime position for their implementation in nanoelectronics due to their manifold outstanding properties. Moreover, using appropriately designed molecular precursors, this approach allows the synthesis of functionalized GNRs, leading to nanostructured hybrids with superior physicochemical properties. Among the potential "partners" for GNRs, porphyrins (Pors) outstand due to their rich chemistry, robustness, and electronic richness, among others. However, the use of such π-conjugated macrocycles for the construction of GNR hybrids is challenging and examples are scarce. In a recent publication we report singly and doubly Por-capped GNR segments presenting a commensurate and triply-fused GNR–Por heterojunction. The study of the electronic properties of such hybrid structures by high-resolution scanning tunneling microscopy, scanning tunneling spectroscopy, and DFT calculations reveals a weak hybridization of the electronic states of the GNR segment and the Por moieties despite their high degree of conjugation.

Tags: cscs, dft, graphene-nanoribbons, marveldd3, on-surface-synthesis, porphyrin, snf
DFT investigation of Ca mobility in reduced-perovskite and oxidized-marokite oxides

Progress in the development of rechargeable Ca-ion batteries demands the discovery of potential cathode materials. Transition metal oxides are interesting candidates due to their theoretical high energy densities, but with the drawback of a low Ca mobility. Previous computational/experimental investigations associate the electrochemical inactivity of various oxides (CaMO3-perovskite, CaMn2O4-post-spinel and CaV2O5) to high energy barriers for Ca migration. The introduction of oxygen and/or Ca vacancies in ternary transition metal oxides is a likely way to reshape the local topology and hence improve the Ca diffusivity. In this work, the energy barriers for Ca migration are calculated and discussed for (i) oxygen-deficient perovskites within the related Ca2Fe2O5-brownmillerite and Ca2Mn2O5 structures, and (ii) tunnel CaMn4O8, a derivative of the CaMn2O4-marokite with Ca vacancies.

Tags: ca2mn2o5, calcium-batteries, marokite, perovskite, post-spinel
High-performance NiOOH/FeOOH electrode for OER catalysis

The outstanding performance of NiOOH/FeOOH-based oxygen evolution reaction (OER) catalysts is rationalized in terms of a bifunctional mechanism involving two distinct active sites. In this mechanism, the OOH_ads reaction intermediate, which unfavorably affects the overall OER activity due to the linear scaling relationship, is replaced by O2 adsorbed at the active site on FeOOH, and H_ads adsorbed at the NiOOH substrate. Here, we use the computational hydrogen electrode method to assess promising models of both the FeOOH catalyst and the NiOOH hydrogen acceptor. These two materials are interfaced in various ways to evaluate their performance as bifunctional OER catalysts. In some cases, overpotentials as low as 0.16 V are found, supporting the bifunctional mechanism as a means to overcome the limitations imposed by linear scaling relationships.

Tags: bifunctional-mechanism, density-functional-theory, epfl, free-energy-steps, linear-scaling-relationships, marveldd3, oxygen-evolution-reaction
Chemistry of oxygen ionosorption on SnO2 surfaces

Ionosorbed oxygen is the key player in reactions on metal-oxide surfaces. This is particularly evident for chemiresistive gas sensors, which operate by modulating the conductivity of active materials through the formation/removal of surface O-related acceptors. Herein, we carried out a detailed study of various charged oxygen species on three naturally occurring surfaces of SnO2. We employed first-principles calculations and revealed that two types of surface acceptors can form spontaneously upon the adsorption of atmospheric oxygen: (i) superoxide O2 (in 1- charged state) on the (110) and the (101) surfaces and (ii) doubly ionized O (in 2- charged state) on the (100) facet, with the experimental evidence pointing to the latter as the source of sensing response. In this dataset, we present the optimized geometries (in CIF format) of different O and O_2 adsorption configurations in the most relevant charged states.

Tags: charged-oxygen-species, chemiresistive-sensing, ionosorption-model, surface-chemistry