Heavy Transition Metal Research Articles

Magnetism in tetragonal manganese-rich Heusler compounds

A comprehensive study of the total energy of manganese-rich Heusler compounds using density functional theory is presented. Starting from a large set of cubic parent systems, the response to tetragonal distortions is studied in detail. We single out the systems that remain cubic from those that most likely become tetragonal. The driving force of the tetragonal distortion and its effect on the magnetic properties, especially where they deviate from the Slater--Pauling rule, as well as the trends in the Curie temperatures, are highlighted. By means of partial densities of states, the electronic structural changes reveal the microscopic origin of the observed trends. We focus our attention on the magnetocrystalline anisotropy and find astonishingly high values for tetragonal Heusler compounds containing heavy transition metals accompanied by low magnetic moments, which indicates that these materials are promising candidates for spin-transfer torque magnetization-switching applications.

A Mild and General One-Pot Synthesis of Densely Functionalized Diaryliodonium Salts.

Diaryliodonium salts are powerful and widely used arylating agents in organic chemistry. Here we report a scalable, synthesis of densely functionalized diaryliodonium salts from aryl iodides under mild conditions. This two-step, one-pot process has remarkable functional group tolerance, is compatible with commonly employed acid-labile protective group strategies, avoids heavy metal and transition metal reagents, and provides a direct route to stable precursors to PET imaging agents.

Patterns of rare earth and other trace elements in Paleogene and Miocene clayey sediments from the Mondego platform (Central Portugal)

In the present study the origin of clay deposits occurring in an inland platform, in central Portugal, was investigated by their mineralogical and chemical composition. The clay deposits, exploited for ceramic industry are composed of silt-clay facies, the Monteira Member and the Arroça Member, which are assigned to the Coja Formation (Paleogene) and the Campelo Formation (Miocene), respectively. These clayey facies show almost compositional homogeneity, especially concerning texture. The mineralogical composition of the Monteira Member displays slightly higher content in smectite and interstratified clay minerals, which is supported by the chemical composition of samples analyzed. Both members present similar REE patterns, displaying an intense weathering record and little variation in the source area composition. Minor element geochemistry suggests low content in heavy minerals and transition metals. REE patterns and ratios of geochemical parameters support the dominant metasedimentary provenance, with a granite source contribution and also mature recycled sediments of continental origin. The study results’ suggest that the clays of these two members have the same source in terms of lithology and recycled sediments from the Hesperian massif. During the deposition of the Arroça Member, a major remobilization of the Monteira Member is suggested, explaining the geochemical similarity of both facies.

Theoretical Studies on Hexanuclear Oxometalates [M6L19](q-) (M = Cr, Mo, W, Sg, Nd, U). Electronic Structures, Oxidation States, Aromaticity, and Stability.

We here report a systematic theoretical study on geometries, electronic structures, and energetic stabilities of six hexanuclear polyoxometalates [M6O19](2-) of the six-valence-electron metals including the d-elements M = Cr, Mo, W, Sg from group 6 and the f-elements M = Nd, U. Scalar relativistic density functional theory was applied to these clusters in vacuum and in solution. It is shown that the Oh Lindqvist structure of the isolated [M6O19](2-) units with hexavalent M elements (M(+6)) is only stable for the three heavy transition metals M = Mo, W, and Sg. The rare Th symmetry is predicted for M = U both in vacuum and in solution, owing to pseudo-Jahn-Teller distortion of these closed-shell systems. The Oh and Th structures correspond to cyclic "aromatic" U-̇O-̇U and alternating U=O-U bonding of cross-linked U4O4 rings, respectively. The reduced [U6O19](8-) cluster with pentavalent U(+5) also shows Th symmetry in vacuum, but Oh symmetry in a dielectric environment. The occurrence of different structures for varying fractional oxidation states in different environments is rationalized. Theoretical investigation of the recently synthesized U(+5) complex [U6O13L6](0) (L6 = tetracyclopentadienyl dibipyridine) shows a distorted Th-type symmetry, too. The stabilities of these complexes of different metal oxidation states are consistent with the general periodic trends of oxidation states.

Topological phases in oxide heterostructures with light and heavy transition metal ions (invited)

Using a combination of density functional theory, tight-binding models, and Hartree-Fock theory, we predict topological phases with and without time-reversal symmetry breaking in oxide heterostructures. We consider both heterostructures containing light transition metal ions and those containing heavy transition metal ions. We find that the (111) growth direction naturally leads to favorable conditions for topological phases in both perovskite structures and pyrochlore structures. For the case of light transition metal elements, Hartree-Fock theory predicts the spin-orbit coupling is effectively enhanced by on-site multiple-orbital interactions and may drive the system through a topological phase transition, while heavy elements with intrinsically large spin-orbit coupling require much weaker or even vanishing electron interactions to bring about a topological phase.

Intersystem-crossing and phosphorescence rates in fac-Ir(III)(ppy)3: a theoretical study involving multi-reference configuration interaction wavefunctions.

We have employed combined density functional theory and multi-reference configuration interaction methods including spin-orbit coupling (SOC) effects to investigate the photophysics of the green phosphorescent emitter fac-tris-(2-phenylpyridine)iridium (fac-Ir(ppy)3). A critical evaluation of our quantum chemical approaches shows that a perturbational treatment of SOC is the method of choice for computing the UV/Vis spectrum of this heavy transition metal complex while multi-reference spin-orbit configuration interaction is preferable for calculating the phosphorescence rates. The particular choice of the spin-orbit interaction operator is found to be of minor importance. Intersystem crossing (ISC) rates have been determined by Fourier transformation of the time correlation function of the transition including Dushinsky rotations. In the electronic ground state, fac-Ir(ppy)3 is C3 symmetric. The calculated UV/Vis spectrum is in excellent agreement with experiment. The effect of SOC is particularly pronounced for the metal-to-ligand charge transfer (MLCT) band in the visible region of the absorption spectrum which does not only extend its spectral onset towards longer wavelengths but also experiences a blue shift of its maximum. Pseudo-Jahn-Teller interaction leads to asymmetric coordinate displacements in the lowest MLCT states. Substantial electronic SOC and a small energy gap make ISC an ultrafast process in fac-Ir(ppy)3. For the S1↝T1 non-radiative transition, we compute a rate constant of kISC = 6.9 × 10(12) s(-1) which exceeds the rate constant of radiative decay to the electronic ground state by more than six orders of magnitude, in agreement with the experimental observation of a subpicosecond ISC process and a triplet quantum yield close to unity. As a consequence of the geometric distortion in the T1 state, the T1 → S0 transition densities are localized on one of the phenylpyridyl moieties. In our best quantum chemical model, we obtain phosphorescence decay times of 264 μs, 13 μs, and 0.9 μs, respectively, for the T1,I, T1,II, and T1,III fine-structure levels in dichloromethane (DCM) solution. In addition to reproducing the correct orders of magnitude for the individual phosphorescence emission probabilities, our theoretical study gives insight into the underlying mechanisms. In terms of intensity borrowing from spin-allowed transitions, the low emission probability of the T1,I substate is caused by the mutual cancellation of contributions from several singlet states to the total transition dipole moment. Their contributions do not cancel but add up in case of the much faster T1,III → S0 emission while the T1,II → S0 emission is dominated by intensity borrowing from a single spin-allowed process, i.e., the S2 → S0 transition.

In search for new bonding modes of the methylenedithiolato ligand: novel tri- and tetra-metallic clusters.

Building upon our earlier results on the chemistry of diruthenium analogue of pentaborane (9) with heterocumulenes, we continued to investigate the reactivity of arachno-[(Cp*Ru)2(B3H8)(CS2H)], 1, (Cp* = η(5)-C5Me5) towards group 7 and 8 transition metal carbonyl compounds under photolytic and thermolytic conditions. The metal carbonyl compounds show diverse reactivity pattern with arachno-1. For example, the photolysis of arachno-1 with [Re2(CO)10] yielded [(Cp*Ru)2B3H5(CH2S2){Re(CO)4}2], 2, [(Cp*RuCO)2(μ-H)2(CH2S2){Re(CO)4}{Re(CO)3}], 3 and [(Cp*Ru)2(μ-CO)(μ-H)(CH2S2){Re(CO)3}], 4. The geometry of 2 with a nearly planar eight-membered ring containing heavier transition metals rhenium, ruthenium is unprecedented. Compounds 3 and 4 can be considered as M4-quadrilateral and M3-triangle with a methylenedithiolato ligand attached to the metal centres, respectively. [Mn2(CO)10], on the other hand, reacts with arachno-1 to yield heterometallic binuclear [(Cp*RuCO){Mn(CO)4}(μ-H)(SCH3)], 5 and homocubane [(Cp*Ru)2{Mn(CO)3}-(CS2H2)B3H4], 6. In an attempt to generate group 8 analogues of 2-5, we performed the reaction of arachno-1 with [Fe2(CO)9] and [Ru3(CO)12]. Although, the objective of isolating analogous compounds was not achieved, the reaction with [Fe2(CO)9] led to novel tetrahedral cluster [(Cp*RuCO){(Fe(CO)3}2S(μ-H)], 7. [Ru3(CO)12], in contrast, yielded known compounds [{Cp*Ru(CO)}2B2H6], 9 and [Cp*Ru(CO)2]2, 10. All the cluster compounds have been characterized by mass spectrometry, IR, and (1)H, (11)B, and (13)C NMR spectroscopy, and the geometric structures were unequivocally established by crystallographic analysis of 2-5 and 7.

Gluing together metallic and covalent layers to form Ru2C under ambient conditions.

Ru2C has recently been synthesised at high pressure and high temperature, and was assumed to have a structure with space group P3̅m1. However, subsequent theoretical work has revealed that this structure is unstable under ambient conditions, which motivated us to look for the stable structure. In this work, we explore the structures of Ru2C by using an unbiased swarm structure searching algorithm. The structures with R3m and R3̅m symmetries have been found to be lower in energy than the P3̅m1 structure, at the same time being dynamically stable under ambient conditions. These layered structures consist of alternating Ru bilayers and C monolayers in the R3m structure, and alternating Ru tetra-layers and C bilayers in the R3̅m structure. The C layers are more evenly distributed and more covalently bound to the Ru layers in the R3m structure than in the R3̅m structure. Instead, in the R3̅m structure there exists more Ru-Ru metallic bonding, which has a crucial role in diminishing the hardness of this material. Our findings should stimulate further explorations of the structures and properties of the heavy transition metal carbides and nitrides, potentially leading to industrial applications.

Topological insulators based on 2D shape-persistent organic ligand complexes.

Topological insulators (TIs) represent an exciting new class of materials with potential applications in spintronics and quantum computing. In this work, we present a theoretical study on a new family of two dimensional (2D) nanomaterials based on the coordination of shape persistent organic ligands (SPOLs) to heavy transition metal ions such as Pd(2+) and Pt(2+). These 2D structures may be readily fabricated and are expected to be stable under normal atmospheric conditions. From first principles calculations and tight-binding model simulations carried out to characterize the bulk band structures, edge states, spin Chern numbers, and the Z2 topological invariants, we were able to identify candidates with non-trivial topological properties that may serve as topological insulators in real world applications.

Structural and mechanical properties of platinum carbide.

Platinum carbide (PtC) was synthesized under extreme conditions and considered as a potential candidate for superhard materials. However, the unsettled issue concerning the structural identification has impeded the full understanding of its physical and chemical properties. Here, we examine by first-principles calculations the crystal structure under high pressure and ideal strength along several high-symmetry directions under large deformation. The current calculations reveal that the zinc blende structure is the thermodynamically stable phase, and the simulated X-ray diffraction data are in excellent agreement with the experimental pattern. Further strain-stress calculations indicate that anomalous fluctuating behaviors of ideal strength occur in PtC. These results are expected to broaden our understanding of the structural and mechanical properties for other potential superhard materials formed by heavy transition metals and light elements.

Topological Order in a Correlated Three-Dimensional Topological Insulator

Motivated by experimental progress in the growth of heavy transition metal oxides, we theoretically study a class of lattice models of interacting fermions with strong spin-orbit coupling. Focusing on interactions of intermediate strength, we derive a low-energy effective field theory for a fully gapped, topologically ordered, fractionalized state with an eightfold ground-state degeneracy. This state is a fermionic symmetry-enriched topological phase with particle-number conservation and time-reversal symmetry. The topological terms in the effective field theory describe a quantized magnetoelectric response and nontrivial mutual braiding statistics of dynamical extended vortex loops with emergent fermions in the bulk. We explicitly compute the expected mutual statistics in a specific model on the pyrochlore lattice within a slave-particle mean-field theory. We argue that our model also provides a possible condensed-matter realization of oblique confinement.

Stabilizing a hexagonal Ru2C via Lifshitz transition under pressure

A unique type of heavy transition metal carbide Ru2C was synthesized experimentally at high pressure-high temperature and consequently quenched to ambient condition. The dynamical stability study reveals the instability at ambient condition. We have found that it can be stabilized from 30 to 110 GPa. The stronger 4d-2p hybridization and the formation of a cage like Fermi surface do impact the stability. The mixed bands primarily 2p characteristic weighted crossing the Fermi level form a Fermi surface piece at Gamma point under pressure. The clear change of topology of Fermi surface verifies the Lifshitz transition from ambient condition to high pressure.

5d iridium oxide as a material for spin-current detection

Devices based on pure spin currents have been attracting increasing attention as key ingredients for low-dissipation electronics. To integrate such spintronics devices into charge-based technologies, electric detection of spin currents is essential. The inverse spin Hall effect converts a spin current into an electric voltage through spin-orbit coupling. Noble metals such as Pt and Pd, and also Cu-based alloys, have been regarded as potential materials for a spin-current injector, owing to the large direct spin Hall effect. Their spin Hall resistivity ρSH, representing the performance as a detector, is not large enough, however, due mainly because of their low charge resistivity. Here we report that a binary 5d transition metal oxide, iridium oxide, overcomes the limitations encountered in noble metals and Cu-based alloys and shows a very large ρSH~38 μΩ cm at room temperature.

Systematic analysis of spectroscopic characteristics of heavy transition metal ions with 4dN and 5dN (N=1…10) electronic configurations in a free state

Systematic consideration of the spectroscopic properties of the di-, tri- and tetravalent 4d and 5d ions in a free state is presented. Variations of the Racah parameters A, B, C, spin-orbit interaction constant ζ and averaged values of the 4d and 5d electrons' radial coordinate across the considered series were considered; functional dependencies between the mentioned quantities were obtained. As a result, knowledge of one out of four parameters (A, B, C, and ζ) allows to estimate the remaining three. The established relations between A, B, C, and ζ reduce the number of independent parameters in the free ion Hamiltonian and can be further applied to the case of crystal-field fitting calculations as the empirical constraints. The third, fourth and fifth ionization energies and barycenters of the electronic 4dN/5dN configurations were calculated; systematic trends across the considered groups of ions were revealed and discussed.

Light-induced catalytic and cytotoxic properties of phosphorescent transition metal compounds with a d 8 electronic configuration

Transition metal compounds are well documented to have diverse applications such as in catalysis, light-emitting materials and therapeutics. In the areas of photocatalysis and photodynamic therapy, metal compounds of heavy transition metals are highly sought after because they can give rise to triplet excited states upon photoexcitation. The long lifetimes (more than 1 μs) of the triplet states of transition metal compounds allow for bimolecular reactions/processes such as energy transfer and/or electron transfer to occur. Reactions of triplet excited states of luminescent metal compounds with oxygen in cells may generate reactive oxygen species and/or induce damage to DNA, leading to cell death. This article recaps the recent findings on photochemical and phototoxic properties of luminescent platinum(II) and gold(III) compounds both from the literature and experimental results from our group.

Effects of annealing on the structure of ion beam sputtered amorphous tantalum oxide: Oxygen-17 NMR spectra and relaxation times

Unlike well-studied network-structured oxide glasses such as silicates and borates, the structures of amorphous films of heavy transition metal oxides such as tantalum oxide (Ta2O5) are poorly known, in spite of their importance in applications such as the multilayer mirror coatings in optical interferometers, such as the Laser Interferometer Gravitational Wave Observatory (LIGO). 17O MAS NMR can provide new and unique structural information about these materials, but requires isotopic enrichment. For optical coatings formed by ion-beam sputtering (IBS), annealing at temperature up to 600°C is known to improve properties such as optical absorption and thermal noise. In this paper we report NMR results on tantala films deposited by IBS in an 17O-enriched environment, which allows the structure to be examined by NMR for the first time without an added step of isotope exchange at elevated temperature. Subsequent annealing at temperatures from 300 to 600°C shows only subtle changes in the proportions of two- and three-coordinated oxygen anions and in bond distances, which may be related to minor structural relaxation and property changes. In contrast, the nuclear spin-relaxation rates increase dramatically on annealing in oxygen gas, suggesting the progressive removal of defects in the as-grown films that contain paramagnetic, unpaired electron spins. EPR spectra qualitatively confirm this observation. The most likely candidate for this effect is the oxidation of lower-valent tantalum cations (e.g. Ta4+) and thus the removal of a slight oxygen deficit present in the as-deposited films. This may lead to reduction in optical absorption and possibly to improvements in other physical properties during annealing.

Correlated Quantum Phenomena in the Strong Spin-Orbit Regime

We discuss phenomena arising from the combined influence of electron correlation and spin-orbit coupling (SOC), with an emphasis on emergent quantum phases and transitions in heavy transition metal compounds with 4d and 5d elements. A common theme is the influence of spin-orbital entanglement produced by SOC, which influences the electronic and magnetic structure. In the weak-to-intermediate correlation regime, we show how nontrivial band-like topology leads to a plethora of phases related to topological insulators (TIs). We expound these ideas using the example of pyrochlore iridates, showing how many novel phases, such as the Weyl semimetal, axion insulator, topological Mott insulator, and TIs, may arise in this context. In the strong correlation regime, we argue that spin-orbital entanglement fully or partially removes orbital degeneracy, reducing or avoiding the normally ubiquitous Jahn-Teller effect. As we illustrate for the honeycomb-lattice iridates and double perovskites, this leads to enhanced quantum fluctuations of the spin-orbital entangled states and the chance to promote exotic spin liquid and multipolar ordered ground states. Connections to experiments, materials, and future directions are discussed.

Topological crystalline insulators in transition metal oxides.

Topological crystalline insulators possess electronic states protected by crystal symmetries, rather than time-reversal symmetry. We show that the transition metal oxides with heavy transition metals are able to support nontrivial band topology resulting from mirror symmetry of the lattice. As an example, we consider pyrochlore oxides of the form A2M2O7. As a function of spin-orbit coupling strength, we find two Z2 topological insulator phases can be distinguished from each other by their mirror Chern numbers, indicating a different topological crystalline insulators. We also derive an effective k·p Hamiltonian, similar to the model introduced for Pb(1-x)Sn(x)Te, and discuss the effect of an on-site Hubbard interaction on the topological crystalline insulator phase using slave-rotor mean-field theory, which predicts new classes of topological quantum spin liquids.

Trace element geochemistry of the Mt Vulture carbonatites, southern Italy

The Mt Vulture carbonatites are the only carbonatite occurrence in the southern Apennines. We present new trace element data for these rocks in order to evaluate the factors influencing rare earth element (REE) and other trace element fractionations and their REE grade. This study focuses on massive hyalo-alvikites from two lava flows and one dike, which have different relative abundances of silicate and carbonate (i.e. Si/Ca). These differences are also evident from CaO/(CaO + MgO + FeO(T) + MnO) and Sr/Ba ratios. The REE grade of the Mt Vulture carbonatites is very similar to that of the global average for calcio-carbonatites. R-mode factor analysis shows that most of the trace element variance reflects the relative roles of carbonate and silicate minerals in influencing trace element distributions. Silicates largely control heavy rare earth element (HREE), transition metal, Zr, and Th abundances, whereas carbonate minerals control light rare earth element (LREE), Ba, and Pb abundances. In addition, apatite influences LREE concentrations. Increasing silica contents are accompanied by decreases in (La/Yb)N and (La/Sm)N ratios and less marked LREE enrichment. In contrast, higher carbonate contents are associated with increases in (La/Yb)N and (La/Sm)N. The Si/Ca ratio has little influence on Eu anomalies and middle rare earth element (MREE) to HREE fractionations. Apatite has a negligible effect on inter-REE fractionations amongst the carbonatites.

Scalable Functionalized Graphene Nano-platelets as Tunable Cathodes for High-performance Lithium Rechargeable Batteries

High-performance and cost-effective rechargeable batteries are key to the success of electric vehicles and large-scale energy storage systems. Extensive research has focused on the development of (i) new high-energy electrodes that can store more lithium or (ii) high-power nano-structured electrodes hybridized with carbonaceous materials. However, the current status of lithium batteries based on redox reactions of heavy transition metals still remains far below the demands required for the proposed applications. Herein, we present a novel approach using tunable functional groups on graphene nano-platelets as redox centers. The electrode can deliver high capacity of ~250 mAh g−1, power of ~20 kW kg−1 in an acceptable cathode voltage range, and provide excellent cyclability up to thousands of repeated charge/discharge cycles. The simple, mass-scalable synthetic route for the functionalized graphene nano-platelets proposed in this work suggests that the graphene cathode can be a promising new class of electrode.