Enhanced Piezoelectric Response at Nanoscale Vortex Structures in Ferroelectrics.

The piezoelectric response is a measure of the sensitivity of a material's polarization to stress or its strain to an applied field. Using in operando X-ray Bragg coherent diffraction imaging, we observe that topological vortices are the source of a 5-fold enhancement of the piezoelectric response near the vortex core. The vortices form where several low-symmetry ferroelectric phases and phase boundaries coalesce. Unlike bulk ferroelectric solid solutions in which a large piezoelectric response is associated with coexisting phases in the proximity of the triple point, the largest responses for pure BaTiO3 at the nanoscale are in spatial regions of extremely small spontaneous polarization at vortex cores. The response decays inversely with polarization away from the vortex, analogous to the behavior in bulk ceramics as the cation compositions are varied away from the triple point. We use first-principles-based molecular dynamics to augment our observations, and our results suggest that nanoscale piezoelectric materials with a large piezoelectric response can be designed within a parameter space governed by vortex cores. Our findings have implications for the development of next-generation nanoscale piezoelectric materials.

Enhanced carrier densities in two-dimensional electron gas formed at BaSnO3/SrTaO3and SrSnO3/SrTaO3interfaces.

Two-dimensional electron gases (2DEGs) realized at perovskite oxide interfaces offer great promise for high charge carrier concentrations and low-loss charge transport. BaSnO3(BSO) and SrSnO3(SSO) are well-known wide bandgap semiconductors for their high mobility due to the Sn-5s-dominated conduction band minimum (CBM). Ta4+with a 5d1valence configuration in SrTaO3(STaO) injects thed1electron across the interface into the unoccupied Sn-5sstates in BSO and SSO. The present study uses ACBN0 density functional theory computations to explore charge transfer and 2DEG formation at BSO/STaO and SSO/STaO interfaces. The results of the ACBN0 computations confirm the Ta-5dto Sn-5scharge transfer. Moreover, the Sn-5s-dominated CBM is located ∼1.4 eV below the Fermi level, corresponding to an excess electron density in BSO of ∼1.5 × 1021cm-3, a ∼50% increase in electron density compared to the previously studied BSO/SrNbO3(SNO) interface. Similarly, the SSO/STaO interface shows an improvement in interface electron density by ∼20% compared to the BSO/SNO interface. The improved carrier density in SSO/STaO and BSO/STaO is further supported by ∼13% and ∼15% increase in electrical conductivities compared to BSO/SNO. In summary, BSO/STaO and SSO/STaO interfaces provide novel material platforms for 2DEGs formation and ultra-low-loss electron transport.

High Mobility Two-Dimensional Electron Gas at the BaSnO3/SrNbO3 Interface.

Oxide two-dimensional electron gases (2DEGs) promise high charge carrier concentrations and low-loss electronic transport in semiconductors such as BaSnO3 (BSO). ACBN0 computations for BSO/SrNbO3 (SNO) interfaces show Nb-4d electron injection into extended Sn-5s electronic states. The conduction band minimum consists of Sn-5s states ∼1.2 eV below the Fermi level for intermediate thickness 6-unit cell BSO/6-unit cell SNO superlattices, corresponding to an electron density in BSO of ∼1021 cm-3. Experimental studies of analogous BSO/SNO interfaces grown by molecular beam epitaxy confirm significant charge transfer from SNO to BSO. In situ angle-resolved X-ray photoelectron spectroscopy studies show an electron density of ∼4 × 1021 cm-3. The consistency of theory and experiments show that BSO/SNO interfaces provide a novel materials platform for low loss electron transport in 2DEGs.

Molybdenum Disorder in Hydrated Sedovite, Ideally U(MoO4)2·nH2O, a Microporous Nanocrystalline Mineral Characterized by Three-Dimensional Electron Diffraction, Density Functional Theory Computations, and Complexity Analysis.

Sedovite, U4+(Mo6+O4)2·nH2O, is reported as being one of the earliest supergene minerals formed of the secondary zone. The difficulty of isolating enough pure material limits studies to techniques that can access the nanoscale combined with theoretical analyses. The crystal structure of sedovite has been solved and refined using the dynamical approach from three-dimensional electron diffraction data collected on natural nanocrystals found among iriginite. At 100 K, sedovite is monoclinic a ≈ 6.96 Å, b ≈ 9.07 Å, c ≈ 12.27 Å, and V ≈ 775 Å3 with space group C2/c. The microporous structure presents a characteristic framework built from uranium polyhedra and disordered Mo pyramids creating pore hosting water molecules. To confirm the formula U4+(Mo6+O4)2·nH2O, the possible presence of a hydroxyl group that would promote Mo5+ was tested with density functional theory (DFT) computations at the ambient temperature. DFT predicts that sedovite is a ferromagnetic insulator with a fundamental bandgap of Eg ∼ 1.7 eV with its chemical and physical properties dominated by U4+ rather than Mo6+. The structural complexity, IG,tot, of sedovite was evaluated in order to get indirect information about the missing formation conditions.

Layer dependent magnetism and topology in monolayer and bilayers ReX3(X=Br, I).

The realization of robust intrinsic ferromagnetism in two-dimensional materials with the possibility to support topologically non-trivial states has provided the fertile ground for novel physics and next-generation spintronics and quantum computing applications. In this contribution, we investigated the formation of topological states and magnetism in monolayer and bilayer systems of ReX3(X= Br, I), with PBE, ACBN0 (self-consistent Hubbard-U), excluding/including van der Waals (vdW) corrections and/or spin-orbit coupling. Bulk ReX3(X= Br, I) is predicted to crystallize in space groupR3¯(#148), similar to CrI3, with monolayer exfoliation energies that are comparable or less than that of graphite. The topological character of the monolayer and bilayer systems of ReX3(X= Br, I) is derived from anomalous Hall conductivity computations. Topologically non-trivial states in ReX3(X= Br, I) are absent in the Hubbard-Ucomputations if vdW interactions are included, a prediction that is attributed to the large Hubbard-Udifference between the chemical constituents, ΔU∼ 1.5-1.6 eV, and a significant ∼2.0%-3.6% compressive in-plane strain introduced by vdW interactions. In contrast to the fragile and likely absent topological states in ReX3(X= Br, I), magnetic properties are robust and independent of the level of theory: ferromagnetic monolayers are coupled antiferromagnetically to bilayers, with an energy separation between ferromagnetic and antiferromagnetic bilayer spin configurations that could be as low as 0.02 meV/Re (f= 4.8 GHz), well within the microwave range. This suggests that layer dependent magnetism in ReX3(X= Br, I) may support a microwave controllable magnetic qubit, consisting of a superposition of antiferromagnetic and ferromagnetic bilayer states.

Magnetocaloric Effect in a Frustrated Gd-Garnet with No Long-Range Magnetic Order.

In this paper, the hyperkagome lattice of Gd spins in a garnet compound, Gd3CrGa4O12, is studied using bulk measurements and density functional computations, and the observation of large magnetocaloric effect corresponding to an entropy change, ΔSm = 45 J kg-1K-1 (≈ 45 J mol-1K-1) at 2 K, 8 T is reported. Though the compound defies long-range magnetic order down to 0.4 K, a broad feature below 10 K is observed in the specific heat with two low temperature anomalies at T* ≈ 0.7 K and TS ≈ 2.45 K. The anomaly at T* is reminiscent of one in Gd3Ga5O12, where it is related to the development of a complex magnetic phase, whereas the TS-peak is accounted for by a multilevel Schottky-like model. The spin-lattice relaxation times studied by nuclear magnetic resonance experiments show that the relaxation is dominated by the magnetic fluctuations in Cr which has a longer relaxation time compared to that of the garnet, Lu3CrGa4O12 containing a nonmagnetic rare earth. Our first-principles density functional theory calculations agree well with the experimental results and support short-range magnetic order in the Gd-sublattice and antiferromagnetism in the Cr-sublattice. The importance of spin fluctuations and short-range order in the rare earth and transition metal lattices in garnets resulting in large magnetocaloric effect is brought out through this work.

Hydrogen bonding in the crystal structure of phurcalite, Ca2[(UO2)3O2(PO4)2]·7H2O: single-crystal X-ray study and TORQUE calculations.

The crystal structure of phurcalite, Ca2[(UO2)3O2(PO4)2]·7H2O, orthorhombic, a = 17.3785 (9) Å, b = 15.9864 (8) Å, c = 13.5477 (10) Å, V = 3763.8 (4) Å3, space group Pbca, Z = 8 has been refined from single-crystal XRD data to R = 0.042 for 3182 unique [I > 3σ(I)] reflections and the hydrogen-bonding scheme has been refined by theoretical calculations based on the TORQUE method. The phurcalite structure is layered, with uranyl phosphate sheets of the phosphuranylite topology which are linked by extensive hydrogen bonds across the interlayer occupied by Ca2+ cations and H2O groups. In contrast to previous studies the approach here reveals five transformer H2O groups (compared to three expected by a previous study) and two non-transformer H2O groups. One of the transformer H2O groups is, nevertheless, not linked to any metal cation, which is a less frequent type of H2O bonding in solid state compounds and minerals. The structural formula of phurcalite has been therefore redefined as {Ca2(H2[3]O)5(H2[4]O)2}[(UO2)3O2(PO4)2], Z = 8.

Does ß-PbO2 harbor topological states?

The electronic properties of ß-PbO2, have been controversial for several decades. Experiments find behavior ranging from metallic, attributed to oxygen vacancies, to indirect semiconducting for stoichiometric samples with a gap of 0.61 eV. Theory leads to similar ambiguities, and predicts this phase to be metallic (PBE, HSE06) or to possess a small bandgap (HSE06). An area where this inconsistency is amplified, is when a material behavior depends on the electronic structure in the vicinity of the Fermi energy, such as topological states. In our work, we use a self-consistent DFT + U approach and find that stoichiometric ß-PbO2 to be an indirect semiconductor with a band gap of ∼0.8 eV, similar to experiment. The larger bandgap requires at least ∼4% strain, to drive ß-PbO2 into a nodal line semimetallic state, significantly larger strains than reported previously. Moreover, we find that the nodal line semimetallic state is not protected against spin-orbit-coupling. Also, the surface computations do not show any evidence for topologically protected states near the Fermi energy. Therefore, our results strongly suggest that ß-PbO2 is a topologically trivial material, consistent with experiment, but in stark contrast to previous computations. Previously reported topologically protected states in ß-PbO2 are attributed to an inaccurate description of the (bulk) optical properties.

Crystal structure of vyacheslavite, U(PO4)(OH), solved from natural nanocrystal: a precession electron diffraction tomography (PEDT) study and DFT calculations.

The crystal structure of the U(iv)-phosphate mineral vyacheslavite has been solved from precession electron diffraction tomography (PEDT) data from the natural nano-crystal and further refined using density-functional theory (DFT) calculations. Vyacheslavite is orthorhombic, with the space group Cmca, with a ≈ 6.96 Å, b ≈ 9.07 Å and c ≈ 12.27 Å, V ≈ 775 Å3 (obtained from PEDT data at 100 K), Z = 8. Its structure is a complex heteropolyhedral framework consisting of sheets of UO7(OH) and PO4 polyhedra, running parallel to (001), interconnected by additional PO4 polyhedra. There is an (OH) group associated with the U(iv) polyhedron. The question of H2O presence within the small cavities of the framework has been addressed by the DFT calculations, which have proved that vyacheslavite does not contain any significant amount of H2O at room temperature.

Revealing hydrogen atoms in a highly-absorbing material: an X-ray diffraction study and Torque method calculations for lead-uranyl-oxide mineral curite.

The crystal structure of lead uranyl-oxide hydroxy-hydrate mineral curite, ideally Pb3(H2O)2[(UO2)4O4(OH)3]2, was studied by means of single-crystal X-ray diffraction and theoretical calculations in order to localize positions of hydrogen atoms in the structure. This study has demonstrated that hydrogen atoms can be localized successfully also in materials for which the conventional approach of structure analysis failed, here due to very high absorption of X-rays by the mineral matrix. The theoretical calculations, based on the Torque method, provide a robust, fast real-space method for determining H2O orientations from their rotational equilibrium condition. In line with previous results we found that curite is orthorhombic, with space group Pnma, unit-cell parameters a = 12.5510(10), b = 8.3760(4), c = 13.0107(9) Å, V = 1367.78(16) Å3, and two formula units per unit cell. The structure (R 1 = 3.58% for 1374 reflections with I > 3σI) contains uranyl-hydroxo-oxide sheets of the unique topology among uranyl oxide minerals and compounds and an interlayer space with Pb2+ cations and a single H2O molecule, which is coordinated to the Pb-site. Current results show that curite is slightly non-stoichiometric in Pb content (∼3.02 Pb per unit cell, Z = 2); the charge-balance mechanism is via (OH) ↔ O2 substitution within the sheets of uranyl polyhedra. Disproving earlier predictions, the current study shows that curite contains only one H2O group, with [4]-coordinated oxygen. The hydrogen bonding network maintains the bonding between the sheets in addition to Pb-O bonds; among them, a H-bond is crucial between the OH group on an apical OUranyl atom of an adjacent sheet that stabilizes the entire structure. The results show that the combination of experimental X-ray data and the Torque method can successfully reveal hydrogen bonding especially for complex crystal structures and materials where X-rays fail to provide unambiguous hydrogen positions.

Computational and experimental evidence for a new TM-N3/C moiety family in non-PGM electrocatalysts.

In the present work, we demonstrate that the formation energies of previously unexplored single and double carbon vacancy based TM-N3/C defect moieties are favourable. This prediction suggests that these defect motifs, in particular DV-Fe-N3/C can form during high-temperature catalyst synthesis. Defect specific N 1s core-level shifts were computed from first-principles for the deconvolution of XPS observations.

Low-temperature carbon monoxide oxidation catalysed by regenerable atomically dispersed palladium on alumina.

Catalysis by single isolated atoms of precious metals has attracted much recent interest, as it promises the ultimate in atom efficiency. Most previous reports are on reducible oxide supports. Here we show that isolated palladium atoms can be catalytically active on industrially relevant γ-alumina supports. The addition of lanthanum oxide to the alumina, long known for its ability to improve alumina stability, is found to also help in the stabilization of isolated palladium atoms. Aberration-corrected scanning transmission electron microscopy and operando X-ray absorption spectroscopy confirm the presence of intermingled palladium and lanthanum on the γ-alumina surface. Carbon monoxide oxidation reactivity measurements show onset of catalytic activity at 40 °C. The catalyst activity can be regenerated by oxidation at 700 °C in air. The high-temperature stability and regenerability of these ionic palladium species make this catalyst system of potential interest for low-temperature exhaust treatment catalysts.

Applicability of density functional theory in reproducing accurate vibrational spectra of surface bound species.

The structural equilibrium parameters, the adsorption energies, and the vibrational frequencies of the nitrogen molecule and the hydrogen atom adsorbed on the (111) surface of rhodium have been investigated using different generalized-gradient approximation (GGA), nonlocal correlation, meta-GGA, and hybrid functionals, namely, Perdew, Burke, and Ernzerhof (PBE), Revised-RPBE, vdW-DF, Tao, Perdew, Staroverov, and Scuseria functional (TPSS), and Heyd, Scuseria, and Ernzerhof (HSE06) functional in the plane wave formalism. Among the five tested functionals, nonlocal vdW-DF and meta-GGA TPSS functionals are most successful in describing energetics of dinitrogen physisorption to the Rh(111) surface, while the PBE functional provides the correct chemisorption energy for the hydrogen atom. It was also found that TPSS functional produces the best vibrational spectra of the nitrogen molecule and the hydrogen atom on rhodium within the harmonic formalism with the error of -2.62 and -1.1% for the N-N stretching and Rh-H stretching frequency. Thus, TPSS functional was proposed as a method of choice for obtaining vibrational spectra of low weight adsorbates on metallic surfaces within the harmonic approximation. At the anharmonic level, by decoupling the Rh-H and N-N stretching modes from the bulk phonons and by solving one- and two-dimensional Schrödinger equation associated with the Rh-H, Rh-N, and N-N potential energy we calculated the anharmonic correction for N-N and Rh-H stretching modes as -31 cm(-1) and -77 cm(-1) at PBE level. Anharmonic vibrational frequencies calculated with the use of the hybrid HSE06 function are in best agreement with available experiments.

A density functional theory study of oxygen reduction reaction on non-PGM Fe-Nx-C electrocatalysts.

First-principles density functional theory (DFT) calculations were performed to explain the stability of catalytically active sites in Fe-Nx-C electrocatalysts, their ORR activity and ORR mechanism. The results show that the formation of graphitic in-plane Fe-N4 sites in a carbon matrix is energetically favorable over the formation of Fe-N2 sites. Chemisorption of ORR species O2, O, OH, OOH, and H2O and O-O bond breaking in peroxide occur on both Fe-N2 and Fe-N4 sites. In addition to the favorable interaction of ORR species, the computed free energy diagrams show that elementary ORR reaction steps on Fe-Nx sites are downhill. Thus, a complete ORR is predicted to occur via a single site 4e(-) mechanism on graphitic Fe-Nx (x = 2, 4) sites. Because of their higher stability and working potential for ORR, Fe-N4 sites are predicted to be prime candidate sites for ORR in pyrolyzed Fe-Nx-C electrocatalysts.

Catalytic activity of Co-N(x)/C electrocatalysts for oxygen reduction reaction: a density functional theory study.

First-principles DFT computations are performed to explain the origin and the mechanism of oxygen reduction reaction (ORR) on Co-N(x) (x = 2, 4) based self-assembled carbon supported electrocatalysts in alkaline and acidic media. The results show that the formation of graphitic Co-N(4) defect is energetically more favorable than the formation of graphitic Co-N(2) defect. Furthermore graphitic Co-N(4) defects are predicted to be stable at all potentials (U = 0-1.23 V) in the present study while Co-N(2) defects are predicted to be unstable at high potentials. Therefore the Co-N(4) defect is predicted to be the dominant in-plane graphitic defect in Co-N(x)/C electrocatalysts. O(2) chemisorbs to Co-N(4) and Co-N(2) defects indicating that both defect motifs are active for the reduction of O(2) to peroxide. However, the weak interaction between peroxide and Co-N(4) defect shows that this defect does not promote complete ORR and a second site for the reduction of peroxide is required, supporting a 2 × 2e(-) dual site ORR mechanism independent of pH of the electrolyte. In contrast, the much stronger interaction between peroxide and Co-N(2) defect supports a 2 × 2e(-) single site ORR mechanism in alkaline and acidic media.

Crystal structure of graphite under room-temperature compression and decompression.

Recently, sophisticated theoretical computational studies have proposed several new crystal structures of carbon (e.g., bct-C(4), H-, M-, R-, S-, W-, and Z-carbon). However, until now, there lacked experimental evidence to verify the predicted high-pressure structures for cold-compressed elemental carbon at least up to 50 GPa. Here we present direct experimental evidence that this enigmatic high-pressure structure is currently only consistent with M-carbon, one of the proposed carbon structures. Furthermore, we show that this phase transition is extremely sluggish, which led to the observed broad x-ray diffraction peaks in previous studies and hindered the proper identification of the post-graphite phase in cold-compressed carbon.

Aerosol-derived Ni(1-x)Zn(x) electrocatalysts for direct hydrazine fuel cells.

This article reports the synthesis and performance of unsupported Ni(1-x)Zn(x) electrocatalysts for the oxidation of hydrazine in alkaline media. Characterization of these catalysts was achieved using XRD, SEM, and TEM to confirm phase compositions, crystal structures, and morphologies. High performance was observed for the α-Ni(0.87)Zn(0.13) and ß(1)-Ni(0.50)Zn(0.50) electrocatalysts with an onset potential of -0.15 V (vs. RHE) and a mass activity of 4000-3800 A g(cat)(-1) at 0.4 V (vs. RHE), respectively. Additionally, in situ IRRAS studies were conducted to understand the mechanism of oxidation. These results demonstrate the feasibility of Ni(1-x)Zn(x) catalysts for direct hydrazine anionic fuel cells.