Platinum-Dysprosium Alloys as Oxygen Electrodes in Alkaline Media: An Experimental and Theoretical Study.

Platinum-dysprosium (Pt-Dy) alloys prepared by the arc melting technique are assessed as potential electrodes for the oxygen reduction reaction (ORR) using voltammetry and chronoamperometry in alkaline media. A relatively small change (10 at.%) in the alloy composition brought a notable difference in the alloys' performance for the ORR. Pt40Dy60 electrode, i.e., the electrode with a lower amount of Pt, was identified to have a higher activity towards ORR as evidenced by lower overpotential and higher current densities under identical experimental conditions. Furthermore, DFT calculations point out the unique single-atom-like coordination and electronic structure of Pt atoms in the Pt40Dy60 surface as responsible for enhanced ORR activity compared to the alloy with a higher Pt content. Additionally, Pt-Dy alloys showed activity in the oxygen evolution reaction (OER), with the OER current density lower than that of pure Pt.

La2Pd3Ge5 and Nd2Pd3Ge5 Compounds: Chemical Bonding and Physical Properties.

The two La2Pd3Ge5 and Nd2Pd3Ge5 compounds, crystallizing in the oI40-U2Co3Ge5 crystal structure, were targeted for analysis of their chemical bonding and physical properties. The compounds of interest were obtained by arc melting and characterized by differential thermal analysis, scanning electron microscopy, and X-ray diffraction both on powder and on a single crystal (for the La analogue), to ensure the high quality of the samples and accurate crystallographic data. Chemical bonding was studied by analyzing the electronic structure and effective QTAIM charges of La2Pd3Ge5. A significant charge transfer mainly occurs from La to Pd so that Ge species assume tiny negative charges. This result, together with the -(I)COHP analysis, suggests that, in addition to the expected homopolar Ge bonds within zigzag chains, heteropolar interactions between Ge and the surrounding La and Pd occur with multicenter character. Covalent La-Pd interactions increase the complexity of chemical bonding, which could not be adequately described by the simplified, formally obeyed, Zintl-Klemm scheme. Electric resistivity, specific heat, magnetization, and magnetic susceptibility as a function of temperature indicate for both compounds a metallic-like behavior. For Nd2Pd3Ge5, two low-temperature phase transitions are detected, leading to an antiferromagnetic ground state.

Polar-Covalent Bonding Beyond the Zintl Picture in Intermetallic Rare-Earth Germanides.

A comparative chemical bonding analysis for the germanides La2 MGe6 (M=Li, Mg, Al, Zn, Cu, Ag, Pd) and Y2 PdGe6 is presented, together with the crystal structure determination for M=Li, Mg, Cu, Ag. The studied compounds adopt the two closely related structure types oS72-Ce2 (Ga0.1 Ge0.9 )7 and mS36-La2 AlGe6 , containing zigzag chains and corrugated layers of Ge atoms bridged by M species, with La/Y atoms located in the biggest cavities. Chemical bonding was studied by means of the quantum chemical position-space techniques QTAIM (quantum theory of atoms in molecules), ELI-D (electron localizability indicator), and their basin intersections. The new penultimate shell correction (PSC0) method was introduced to adapt the ELI-D valence electron count to that expected from the periodic table of the elements. It plays a decisive role to balance the Ge-La polar-covalent interactions against the Ge-M ones. In spite of covalently bonded Ge partial structures formally obeying the Zintl electron count for M=Mg2+ , Zn2+ , all the compounds reveal noticeable deviations from the conceptual 8-N picture due to significant polar-covalent interactions of Ge with La and M ≠ Li, Mg atoms. For M=Li, Mg a formulation as a germanolanthanate M[La2 Ge6 ] is appropriate. Moreover, the relative Laplacian of ELI-D was discovered to reveal a chemically useful fine structure of the ELI-D distribution being related to polyatomic bonding features. With the aid of this new tool, a consistent picture of La/Y-M interactions for the title compounds was extracted.

A new glance on R2MGe6 (R = rare earth metal, M = another metal) compounds. An experimental and theoretical study of R2PdGe6 germanides.

The R2PdGe6 series (R = rare earth metal) was structurally characterized, and the results achieved were extended for a comprehensive study on R2MGe6 (M = another metal) compounds, employing symmetry-based structural rationalization and energy calculations. Directly synthesized R2PdGe6 exists for almost all R-components (R = Y, La-Nd, Sm and Gd-Lu) and even if with La is probably metastable. Several single crystal X-ray analyses (R = Y, Ce, Pr, Nd, Er and Lu) indicated oS72-Ce2(Ga0.1Ge0.9)7 as the correct structure. The alternative In-flux method, once optimized, produced three good quality R2PdGe6 single crystals: La2PdGe6 and Pr2PdGe6 turned out to be mS36-La2AlGe6-type non-merohedrally twinned crystals and Yb2PdGe6 is of oS72-Ce2(Ga0.1Ge0.9)7-type. The vacancy ordering phenomenon was considered as a possible cause of the symmetry reduction relations connecting the most frequently reported 2 : 1 : 6 structural models (oS18, oS72 and mS36) with the oS20-SmNiGe3 aristotype. The detected twin formation is consistent with the symmetry relations, which are discussed even considering the validity of the different structural models. DFT total energy calculations were performed for R2PdGe6 (R = Y and La) in the three abovementioned structural models, and for La2MGe6 (M = Pt, Cu, Ag and Au) in the oS18 and oS72 modifications. The results indicate that the oS18-Ce2CuGe6 structure, prevalently proposed in the literature, is associated with the highest energy and thus it is not likely to be realized in these series. The oS72 and mS36 polytypes are energetically equivalent, and small changes in the synthetic conditions could easily stabilize any of them, in agreement with experimental results obtained by direct and flux syntheses.

Yb9+xCuMg4-x (x = 0.034): A κ-Phase Formed by Lanthanoids.

Atom order in the crystal structures of Yb2Cu2-xMg (x = 0.17; Mo2FeB2-type; P4/mbm; a = 0.75592(2) nm; c = 0.40282(1) nm) and Yb9+xCuMg4-x (x = 0.034; Hf9Mo4B-type; P63/mmc; a = 1.0169(5) nm; c = 1.0290(5) nm) was determined from powder and X-ray single-crystal counter data analyses supported by electron probe microanalyses. Among the group of the so-called κ-phases, Yb9+xCuMg4-x is the first representative formed by a lanthanoid element. The structure of this κ-phase can be viewed as a typical network of corner-connected empty Yb6-octahedra, which encompass Yb6Mg6-icosahedra (filled by a mix of Mg/Yb atoms) and Yb6-trigonal prisms centered by Cu atoms to complete the three-dimensional metal framework. From another point of view, the same structure is considered as built from infinite polyicosahedral columns of Yb9Mg4 composition with Cu atoms located in trigonal prismatic interstices, highlighting similarities with other Yb-rich Yb-Cu-Mg phases. Density functional theory (DFT) calculations classify Yb9CuMg4 as a polar intermetallic. Metallic-like behavior is inferred from the Sommerfeld constant, γ = 49.2 mJ/mol·K(2), derived from the electronic density of states, calculated at the Fermi level. DFT integration of the f-density of states indicates almost completely filled f-states, revealing 13.6 and 13.7 electrons in the valence band for Yb1 and Yb2 atoms, respectively, close to the Yb(2+) ground state ((1)S0) for both Yb atoms. Magnetic susceptibility data recorded on the same compound are consistent with a nonmagnetic divalent Yb(2+) state. Temperature-dependent heat capacity data display a metallic behavior characterized by a small Sommerfeld constant γ = 64.8 mJ/mol·K(2) and a rather low Debye temperature ΘD = 140 K as typical for soft materials.

Crystal Chemistry of the New Families of Interstitial Compounds R6Mg23C (R = La, Ce, Pr, Nd, Sm, or Gd) and Ce6Mg23Z (Z = C, Si, Ge, Sn, Pb, P, As, or Sb).

The crystal chemical features of the new series of compounds R6Mg23C with R = La-Sm or Gd and Ce6Mg23Z with Z = C, Si, Ge, Sn, Pb, P, As, or Sb have been studied by means of single-crystal and powder X-ray diffraction techniques. All phases crystallize with the cubic Zr6Zn23Si prototype (cF120, space group Fm3̅m, Z = 4), a filled variant of the Th6Mn23 structure. While no Th6Mn23-type binary rare earth-magnesium compound is known to exist, the addition of a third element Z (only 3 atom %), located into the octahedral cavity of the Th6Mn23 cell (Wyckoff site 4a), stabilizes this structural arrangement and makes possible the formation of the ternary R6Mg23Z compounds. The results of both structural and topological analyses as well as of LMTO electronic structure calculations show that the interstitial element plays a crucial role in the stability of these phases, forming a strongly bonded [R6Z] octahedral moiety spaced by zeolite cage-like [Mg45] clusters. Considering these two building units, the crystal structure of these apparently complex intermetallics can be simplified to the NaCl-type topology. Moreover, a structural relationship between RMg3 and R6Mg23C compounds has been unveiled; the latter can be described as substitutional derivatives of the former. The geometrical distortions and the consequent symmetry reduction that accompany this transformation are explicitly described by means of the Bärnighausen formalism within group theory.

Vacancy ordering as a driving factor for structural changes in ternary germanides: the new R2Zn(1-x)Ge6 series of polar intermetallics (R = rare-earth metal).

Synthesis and structural characterization of the new compounds R2Zn1-xGe6 (R = La-Nd, Sm, Gd-Ho) is reported. A structural change was revealed along this series by careful analysis of single-crystal X-ray diffraction data. For light rare earths up to Tb the orthorhombic oS72-Ce2(Ga0.1Ge0.9)7 model was established; instead, the Dy compound represents a new structure type (P21/m, mP34, Z = 4, a = 7.9613(3) Å, b = 8.2480(4) Å, c = 10.5309(5) Å, ß = 100.861(1)°) being a superstructure of the mS36-La2AlGe6 prototype. The established structural models support the increase of Zn deficiency along the series, suggested by microprobe analysis, and its key role in governing structural changes. The vacancy ordering criterion was applied as a successful approach to find a general scheme including the structures of the ∼R2MGe6 compounds known up to now (R = rare-earth metal, M = transition metal, Mg, Al, Ga) and highlighting the subtle structural differences within this family. According to this scheme, these structures are obtained from a common aristotype (oS20-SmNiGe3) via symmetry reduction based on group-subgroup relations accompanied by ordering of vacancies. This approach was optimized with the help of the ToposPro software and extended to the R2Zn3Ge6 series, enriched with new members (R = Sm, Gd-Ho) during this work. Electronic structure calculations on La2ZnGe6 confirm the presence of infinite covalent germanium zigzag chains and three-bonded corrugated layers connected via Zn atoms to form a polyanionic network stabilized by La atoms.

New ternary germanides La4Mg5Ge6 and La4Mg7Ge6: crystal structure and chemical bonding.

The synthesis, structural characterization, and chemical-bonding peculiarities of the two new polar lanthanum-magnesium germanides La(4)Mg(5)Ge(6) and La(4)Mg(7)Ge(6) are reported. The crystal structures of these intermetallics were determined by single-crystal X-ray diffraction analysis. The La(4)Mg(5)Ge(6) phase crystallizes in the orthorhombic Gd(4)Zn(5)Ge(6) structure type [Cmc2(1), oS60, Z = 4, a = 4.5030(7) Å, b = 20.085(3) Å, c = 16.207(3) Å, wR2 = 0.0451, 1470 F(2) values, 93 variables]. The La(4)Mg(7)Ge(6) phase represents a new structure type with a monoclinic unit cell [C2/m, mS34, Z = 2, a = 16.878(3) Å, b = 4.4702(9) Å, c = 12.660(3) Å, ß = 122.25(3)°, wR2 = 0.0375, 1466 F(2) values, 54 variables]. Crystallographic analysis together with linear muffin-tin orbital band structure calculations reveals the presence of strongly bonded 3D polyanionic [Mg-Ge] networks balanced by positively charged La atoms in both stoichiometric compounds. The La(4)Mg(5)Ge(6) compound is related to Zintl phases, showing a prominent density of states pseudogap at the Fermi level. A distinctive feature of the La(4)Mg(5)Ge(6) structure is the presence of Ge-Ge covalent dumbbells; in La(4)Mg(7)Ge(6), the higher Mg content generates a polyanionic network consisting exclusively of Mg-Ge heterocontacts. Nevertheless, the frameworks of the two phases are structurally similar, as is highlighted in this work.

Anti-mackay polyicosahedral clusters in La-Ni-Mg ternary compounds: synthesis and crystal structure of the La(43)Ni(17)Mg(5) new intermetallic phase.

The crystal structure of the complex La(43)Ni(17)Mg(5) ternary phase was solved and refined from X-ray single crystal diffraction data. It is characterized by a very large unit cell and represents a new structure type: La(43)Ni(17)Mg(5) - orthorhombic, Cmcm, oS260, a = 10.1895(3), b = 17.6044(14), c = 42.170(3) A, Z = 4, wR1 = 0.0598, wR2 = 0.0897, 4157 F(2) values, 176 variables. The crystal structures of the La-rich La-Ni-Mg intermetallic phases La(4)NiMg, La(23)Ni(7)Mg(4), and La(43)Ni(17)Mg(5) have been comparatively analyzed. The constitutive fragments of these structures are binary polyicosahedral core-shell clusters of Mg(4)La(22) and Mg(5)La(24) compositions together with binary polytetrahedral clusters of nickel and lanthanum atoms. The structures of the Mg-La clusters are described in detail as a unique feature of the analyzed intermetallic phases; the dodecahedral Voronoi polyhedra are proposed as a useful tool to characterize polyicosahedral clusters. The arrangements of the building units in the studied phases show some regularities; particularly the i(4)3, i(5)3 and L-i(4) units, made up of polyicosahedral clusters and analogous to the Kreiner i(3) and L units, are proposed as structural blocks.