Coexistence of ferroelectricity and ferromagnetism in tantalum clusters

Advanced Ir-Based Alloy Electrocatalysts for Proton Exchange Membrane Water Electrolyzers

Proton exchange membrane water electrolyzer (PEMWE) coupled with renewable energy to produce hydrogen is an important part of clean energy acquisition in the future. However, the slow kinetics of the oxygen evolution reaction (OER) hinder the large-scale application of PEM water electrolysis technology. To deal with the problems existing in the PEM electrolyzer and improve the electrolysis efficiency, substantial efforts are invested in the development of cost-effective and stable electrocatalysts. Within this scenario, the different OER reaction mechanisms are first discussed here. Based on the in-depth understanding of the reaction mechanism, the research progress of low-iridium noble metal alloys is reviewed from the aspects of special effects, design strategies, reaction mechanisms, and synthesis methods. Finally, the challenges and prospects of the future development of high-efficiency and low-precious metal OER electrocatalysts are presented.

Unusual Inertness of a Ta8+ Cluster in Dinitrogen Reactions

Clusters serve as the optimal model to elucidate the structure-property relationship of materials, bridging condensed matter and individual atoms. The pursuit of exceptionally stable clusters has garnered significant interest. The distinctive electronic configuration and symmetrical geometry generally provide a consistent rationale for their stability. However, this principle does not quite correspond to the behavior of all transition metal clusters. Utilizing our customized apparatus, we successfully produced pure tantalum clusters Tan+ (n = 1-16) and examined their reactions with dinitrogen under sufficient gas-collision conditions. Significantly, with the introduction of N2 gas reactants, the Ta8+ cluster became the predominant species. Comprehensive theoretical analyses indicate that the inertness of Ta8+ is due to not only its unique electronic configuration and superatomic feature but also its unfavorable N2 adsorption dynamics and N≡N activation kinetics on the cluster. We demonstrate the contributions of frontier orbitals, the natural population of charges, and their interactions with lone-pair electrons of N2, together with the rate coefficients derived from Rice-Ramsperger-Kassel-Marcus (RRKM) theory. This study provides comprehensive insights into the cluster stability and activity, which can be used as a reference for the development of gas separation materials that are resistant to N2.

Observation and mechanism of cryo N2 cleavage by a tantalum cluster

We explore the cryogenic kinetics of N2 adsorption to Ta4+ and the infrared signatures of [Ta4(N2)m]+ complexes, m = 1-5. This is accomplished by N2 exposure of isolated ions within a cryogenic ion trap. We find stepwise addition of numerous N2 molecules to the Ta4+ cluster. Interestingly, the infrared signatures of the [Ta4(N2)1]+ and [Ta4(N2)2]+ products are special: there are no NN stretching bands. This is consistent with cleavage of the first two adsorbed dinitrogen molecules. DFT calculations reveal intermediates and barriers along reaction paths of N2 cleavage in support of these experimental findings. We indicate the identified multidimensional path of N2 cleavage as an across edge-above surface (AEAS) mechanism: initially end-on coordinated N2 bends towards a neighboring Ta-atom which yields a second intermediate, with a μ2 bonded N2 across an edge of the Ta4+ tetrahedron core. Further rearrangement above a Ta-Ta-Ta surface of the Ta4+ tetrahedron results in a μ3 bonded N2 ligand. This intermediate relaxes swiftly by ultimate NN cleavage unfolding into the final dinitrido motif. Submerged activation barriers below the entrance channel confirm spontaneous cleavage of the first two dinitrogen molecules (-59 and -33 kJ mol-1, respectively), while cleavage of the third N2 ligand is kinetically hindered (+55 kJ mol-1). We recognize that substoichiometric N2 exposure allows for spontaneous activation by Ta4+, while higher N2 exposure causes self-poisoning.

Structures of Small Tantalum Cluster Anions: Experiment and Theory

We present a study of the structural evolution of tantalum cluster anions Ta_n^-, 6 ≤ n ≤ 13 using a combination of trapped ion electron diffraction (TIED) experiments with a variety of electronic structure methods. A genetic algorithm has been employed to establish a set of likely structures for each cluster, their geometries and energetics have been studied by density functional theory (DFT), random phase approximation, and two-component (2C) DFT methods, which include spin-orbit coupling. We find octahedral structures for Ta₆^- and Ta₈^- as well as structures based on the pentagonal bipyramid (Ta₇^- and Ta₉^-). Ta₁₀^--Ta₁₂^- are defective icosahedral structures and Ta₁₃^- is a distorted icosahedron. For most clusters, we find a good agreement between the theoretically predicted ground-state structures, especially those determined by the 2C method and the TIED results.

Probing the properties of size dependence and correlation for tantalum clusters: geometry, stability, vibrational spectra, magnetism, and electronic structure

A comprehensive investigation on the equilibrium geometry, relative stability, vibrational spectra, and magnetic and electronic properties of neutral tantalum clusters (Ta_n, n = 2–17) was performed using density functional theory (DFT). We perform a study of the size dependence and correlations among those descriptors of parameters, and showed these could provide a novel way to confirm and predict experimental results. Some new isomer configurations that have never been reported before for tantalum clusters were found. The growth behaviors revealed that a compact geometrical growth route is preferred and develops a body-centered-cubic (BCC) structure with the cluster size increasing. The perfectly fitted functional curve, strong linear evolution, and obvious odd–even oscillation behavior proved their corresponding properties depended on the cluster size. Multiple demonstrations of the magic number were confirmed through the correlated relationships with the relative stability, including the second difference in energy, maximum hardness, and minimum polarizability. An inverse evolution trend between the energy gap and electric dipole moment and strong linear correlation between ionization potentials and polarizability indicated the strong correlation between the magnetic and electronic properties. Vibrational spectroscopy as a fingerprint was used to distinguish the ground state among the competitive geometrical isomers close in energy. The charge density difference isosurface, density of states, and molecular orbitals of selected representative clusters were analyzed to investigate the difference and evolutional trend of the relative stability and electronic structure. In addition, we first calculated the ionization potential and magnetic moment and compared these with the current available experimental data for tantalum clusters.

A comprehensive investigation on the equilibrium geometry, relative stability, vibrational spectra, and magnetic and electronic properties of neutral tantalum clusters (Ta_n, n = 2–17) was performed using density functional theory (DFT).

Evolution of the structural, energetic, and electronic properties of the 3d, 4d, and 5d transition-metal clusters (30 TMn systems for n = 2–15): a density functional theory investigation

Subnanometric transition-metal (TM) clusters have attracted great attention due to their unexpected physical and chemical properties, leastwise compared to their bulk counterparts.

Multiferroic rhodium clusters

Simultaneous magnetic and electric deflection measurements of rhodium clusters (Rh(N), 6 ≤ N ≤ 40) reveal ferromagnetism and ferroelectricity at low temperatures, while neither property exists in the bulk metal. Temperature-independent magnetic moments (up to 1 μ(B) per atom) are observed, and superparamagnetic blocking temperatures up to 20 K. Ferroelectric dipole moments on the order of 1D with transition temperatures up to 30 K are observed. Ferromagnetism and ferroelectricity coexist in rhodium clusters in the measured size range, with size-dependent variations in the transition temperatures that tend to be anticorrelated in the range n = 6-25. Both effects diminish with size and essentially vanish at N = 40. The ferroelectric properties suggest a Jahn-Teller ground state. These experiments represent the first example of multiferroic behavior in pure metal clusters.