Yuemei Zhang

Graphene- and Phosphorene-like Boron Layers with Contrasting Activities in Highly Active Mo2B4 for Hydrogen Evolution

Two different boron layers, flat (graphene-like) and puckered (phosphorene-like), found in the crystal structure of Mo2B4 show drastically different activities for hydrogen evolution, according to Gibbs free energy calculations of H-adsorption on Mo2B4. The graphene-like B layer is highly active, whereas the phosphorene-like B layer performs very poorly for hydrogen evolution. A new Sn-flux synthesis permits the rapid single-phase synthesis of Mo2B4, and electrochemical analyses show that it is one of the best hydrogen evolution reaction active bulk materials with good long-term cycle stability under acidic conditions. Mo2B4 compensates its smaller density of active sites if compared with highly active bulk MoB2 (which contains only the more active graphene-like boron layers) by a 5-times increase of its surface area.

Boron: Enabling Exciting Metal-Rich Structures and Magnetic Properties

Borons unique chemical properties and its reactions with metals have yielded the large class of metal borides with compositions ranging from the most boron-rich YB66 (used as monochromator for synchrotron radiation) up to the most metal-rich Nd2Fe14B (the best permanent magnet to date). The excellent magnetic properties of the latter compound originate from its unique crystal structure to which the presence of boron is essential. In general, knowing the crystal structure of any given extended solid is the prerequisite to understanding its physical properties and eventually predicting new synthetic targets with desirable properties. The ability of boron to form strong chemical bonds with itself and with metallic elements has enabled us to construct new structures with exciting properties. In recent years, we have discovered new boride structures containing some unprecedented boron fragments (trigonal planar B4 units, planar B6 rings) and low-dimensional substructures of magnetically active elements (ladders, scaffolds, chains of triangles). The new boride structures have led to new superconducting materials (e.g., NbRuB) and to new itinerant magnetic materials (e.g., Nb6Fe1-xIr6+xB8). The study of boride compounds containing chains (Fe-chains in antiferromagnetic Sc2FeRu5B2), ladders (Fe-ladders in ferromagnetic Ti9Fe2Rh18B8), and chains of triangles (Cr3 chains in ferrimagnetic and frustrated TiCrIr2B2) of magnetically active elements allowed us to gain a deep understanding of the factors (using density functional theory calculations) that can affect magnetic ordering of such low-dimensional magnetic units. We discovered that the magnetic properties of phases containing these magnetic subunits can be drastically tuned by chemical substitution within the metallic nonmagnetic network. For example, the small hysteresis (measure of magnetic energy storage) of Ti2FeRh5B2 can be successively increased up to 24-times by gradually substituting Ru for Rh, a result that was even surpassed (up to 54-times the initial value) for Ru/Ir substitutions. Also, the type of long-range magnetic interactions could be drastically tuned by appropriate substitutions in the metallic nonmagnetic network as demonstrated using both experimental and theoretical methods. It turned out that Ru-rich and valence electron poor metal borides adopting the Ti3Co5B2 or the Th7Fe3 structure types have dominating antiferromagnetic interactions, while in Rh-rich (or Ir-rich) and valence electron rich phases ferromagnetic interactions prevail, as found, for example, in the Sc2FeRu5-xRhxB2 and FeRh6-xRuxB3 series. Fascinatingly, boron clusters (e.g., B6 rings) even directly interact in some cases with the magnetic subunits, an interaction which was found to favor the Fe-Fe magnetic exchange interactions in the ferromagnetic Nb6Fe1-xIr6+xB8. Using less expensive transition metals, we have recently predicted new itinerant magnets, the experimental proof of which is still pending. Furthermore, new structures have been discovered, all of which are being studied experimentally and computationally with the aim of finding new superconductors, magnets, and mechanically hard materials. A new direction is being pursued in our group, as binary and ternary transition metal borides show great promise as efficient water splitting electrocatalysts at the micro- and nanoscale.

Valence State Driven Site Preference in the Quaternary Compound Ca5MgAgGe5: An Electron-Deficient Phase with Optimized Bonding

The quaternary phase Ca5Mg0.95Ag1.05(1)Ge5 (3) was synthesized by high-temperature solid-state techniques, and its crystal structure was determined by single-crystal diffraction methods in the orthorhombic space group Pnma-Wyckoff sequence c(12) with a = 23.1481(4) Å, b = 4.4736(1) Å, c = 11.0128(2) Å, V = 1140.43(4) Å(3), Z = 4. The crystal structure can be described as linear intergrowths of slabs cut from the CaGe (CrB-type) and the CaMGe (TiNiSi-type; M = Mg, Ag) structures. Hence, 3 is a hettotype of the hitherto missing n = 3 member of the structure series with the general formula R(2+n)T2X(2+n), previously described with n = 1, 2, and 4. The member with n = 3 was predicted in the space group Cmcm-Wyckoff sequence f(5)c(2). The experimental space group Pnma (in the nonstandard setting Pmcn) corresponds to a klassengleiche symmetry reduction of index two of the predicted space group Cmcm. This transition originates from the switching of one Ge and one Ag position in the TiNiSi-related slab, a process that triggers an uncoupling of each of the five 8f sites in Cmcm into two 4c sites in Pnma. The Mg/Ag site preference was investigated using VASP calculations and revealed a remarkable example of an intermetallic compound for which the electrostatic valency principle is a critical structure-directing force. The compound is deficient by one valence electron according to the Zintl concept, but LMTO electronic structure calculations indicate electronic stabilization and overall bonding optimization in the polyanionic network. Other stability factors beyond the Zintl concept that may account for the electronic stabilization are discussed.

Spin Frustration and Magnetic Ordering from One-Dimensional Stacking of Cr3 Triangles in TiCrIr2B2.

Spin-frustrated chains of Cr3 triangles are found in the new metal boride TiCrIr2B2 by synergistic experimental and theoretical investigations. Although magnetic ordering is found at 275 K, competing ferro- and anti-ferromagnetic interactions coupled with spin frustration induce a rather small total magnetic moment (0.05 μB at 5 T), and density functional theory (DFT) calculations propose a canted, nonlinear magnetic ground-state ordering in the new phase. TiCrIr2B2 crystallizes in the hexagonal Ti1+xOs2-xRuB2 structure type (space group P6̅2m, No. 189, Pearson symbol hP18). The structure contains trigonal planar B4 boron fragments with B-B distances of 1.76(3) Å alternating along the c-direction with Cr3 triangles with intra- and intertriangle Cr-Cr distances of 2.642(9) and 3.185(1) Å, respectively. Magnetization measurements of TiCrIr2B2 reveal ferrimagnetic behavior and a large, negative Weiss constant of -750 K. DFT calculations demonstrate a strong site preference of Cr for the triangle sites, as well as magnetic frustration due to indirect anti-ferromagnetic interactions within the Cr3 triangles.

Magnetic ordering in tetragonal 3d metal arsenides M2As (M = Cr, Mn, Fe): an ab initio investigation.

The electronic and magnetic structures of the tetragonal Cu2Sb-type 3d metal arsenides (M2As, M = Cr, Mn, Fe) were examined using density functional theory to identify chemical influences on their respective patterns of magnetic order. Each compound adopts a different antiferromagnetic (AFM) ordering of local moments associated with the 3d metal sites, but every one involves a doubled crystallographic c-axis. These AFM ordering patterns are rationalized by the results of VASP calculations on several magnetically ordered models using a × a × 2c supercell. Effective exchange parameters obtained from SPRKKR calculations indicate that both direct and indirect exchange couplings play essential roles in understanding the different magnetic orderings observed. The nature of nearest-neighbor direct exchange couplings, that is, either ferromagnetic (FM) or AFM, were predicted by analysis of the corresponding crystal orbital Hamilton population (COHP) curves obtained by TB-LMTO calculations. Interestingly, the magnetic structures of Fe2As and Mn2As show tetragonal symmetry, but a magnetostrictive tetragonal-to-orthorhombic distortion could occur in Cr2As through AFM Cr1-Cr2 coupling between symmetry inequivalent Cr atoms along the a-axis, but FM coupling along the b-axis. A LSDA+U approach is required to achieve magnetic moment values for Mn2As in better agreement with experimental values, although computations always predict the moment at the M1 site to be lower than that at the M2 site. Finally, a rigid-band model applied to the calculated DOS curve of Mn2As correctly assesses the magnetic ordering patterns in Cr2As and Fe2As.

Synthesis, crystal and electronic structures, physical properties and 121Sb and 151Eu Mössbauer spectroscopy of the alumo-antimonide Zintl-phase Eu5Al2Sb6

Eu5Al2Sb6 was synthesized from the elements in niobium ampoules. It crystallizes in the orthorhombic crystal system (a = 1027.55(7), b = 1200.28(6), c = 1324.22(7) pm) with space group Pnma (no. 62), isostructural to Sr5Al2Sb6, and can be described as a Zintl phase. The Al atoms are tetrahedrally surrounded by Sb atoms, forming branched strands. Besides Sb3− anions, antimony Sb24− dumbbells can also be found in the crystal structure. Eu5Al2Sb6 [≡(Eu2+)5(Al3+)2(Sb3−)4(Sb24−)] exhibits three magnetic ordering phenomena at T1 = 12.3(1), T2 = 10.6(1) and T3 = 3.0(1) K, obtained by heat capacity measurements. While T3 is of antiferromagnetic nature, T1 and T2 correspond to canted antiferromagnetic transitions. Resistivity investigations indicate that the title compound is a semiconductor, in line with the band structure calculations. Spin exchange parameters, calculated using mapping analysis, confirm that the magnetic phase transition at TN = 3.5(1) is associated with the ordering of Eu1 atoms. The divalent character has been confirmed by 151Eu Mossbauer spectroscopic studies. Measurements conducted at 5 K show a hyperfine field splitting for two of the three Eu sites, underlining the magnetic data. Additionally, 121Sb Mossbauer spectroscopic studies have been conducted on Eu5Al2Sb6 and isostructural Sr5Al2Sb6.