Bonding, Ion Mobility, and Rate-Limiting Steps in Deintercalation Reactions with ThCr₂Si₂-type KNi₂Se₂

Here, we study the nature of metal–metal bonding in the ThCr₂Si₂ structure type by probing the rate-limiting steps in the oxidative deintercalation of KNi₂Se₂. For low extents of oxidation, alkali ions are removed exclusively to form K_1–<i>x</i>Ni₂Se₂. For greater extents of oxidation, the rate of the reaction decreases dramatically, concomitant with the extraction of both potassium and nickel to form K_1–<i>x</i>Ni_2–<i>y</i>Se₂. The appreciable mobility of transition metal ions is unexpected, but illustrates the relative energy scales of different defects in the ThCr₂Si₂ structure type. Furthermore, the fully oxidized compounds, K_0.25Ni_1.5Se₂, spontaneously convert from the tetrahedral [NiSe₄]-containing ThCr₂Si₂ structure to a vacancy-ordered NiAs structure with [NiSe₆] octahedra. From analysis of the atom positions and kinetic data, we have determined that this transformation occurs by a continuous, low-energy pathway via subtle displacements of Ni atoms and buckling of the Se sublattice. These results have profound implications for our understanding of the stability, mobility, and reactivity of ions in materials

Bonding, Ion Mobility, and Rate-Limiting Steps in Deintercalation Reactions with ThCr₂Si₂-type KNi₂Se₂

Here, we study the nature of metal–metal bonding in the ThCr₂Si₂ structure type by probing the rate-limiting steps in the oxidative deintercalation of KNi₂Se₂. For low extents of oxidation, alkali ions are removed exclusively to form K_1–<i>x</i>Ni₂Se₂. For greater extents of oxidation, the rate of the reaction decreases dramatically, concomitant with the extraction of both potassium and nickel to form K_1–<i>x</i>Ni_2–<i>y</i>Se₂. The appreciable mobility of transition metal ions is unexpected, but illustrates the relative energy scales of different defects in the ThCr₂Si₂ structure type. Furthermore, the fully oxidized compounds, K_0.25Ni_1.5Se₂, spontaneously convert from the tetrahedral [NiSe₄]-containing ThCr₂Si₂ structure to a vacancy-ordered NiAs structure with [NiSe₆] octahedra. From analysis of the atom positions and kinetic data, we have determined that this transformation occurs by a continuous, low-energy pathway via subtle displacements of Ni atoms and buckling of the Se sublattice. These results have profound implications for our understanding of the stability, mobility, and reactivity of ions in materials

Bonding, Ion Mobility, and Rate-Limiting Steps in Deintercalation Reactions with ThCr₂Si₂-type KNi₂Se₂

Here, we study the nature of metal–metal bonding in the ThCr₂Si₂ structure type by probing the rate-limiting steps in the oxidative deintercalation of KNi₂Se₂. For low extents of oxidation, alkali ions are removed exclusively to form K_1–<i>x</i>Ni₂Se₂. For greater extents of oxidation, the rate of the reaction decreases dramatically, concomitant with the extraction of both potassium and nickel to form K_1–<i>x</i>Ni_2–<i>y</i>Se₂. The appreciable mobility of transition metal ions is unexpected, but illustrates the relative energy scales of different defects in the ThCr₂Si₂ structure type. Furthermore, the fully oxidized compounds, K_0.25Ni_1.5Se₂, spontaneously convert from the tetrahedral [NiSe₄]-containing ThCr₂Si₂ structure to a vacancy-ordered NiAs structure with [NiSe₆] octahedra. From analysis of the atom positions and kinetic data, we have determined that this transformation occurs by a continuous, low-energy pathway via subtle displacements of Ni atoms and buckling of the Se sublattice. These results have profound implications for our understanding of the stability, mobility, and reactivity of ions in materials

Bonding, Ion Mobility, and Rate-Limiting Steps in Deintercalation Reactions with ThCr₂Si₂-type KNi₂Se₂

Here, we study the nature of metal–metal bonding in the ThCr₂Si₂ structure type by probing the rate-limiting steps in the oxidative deintercalation of KNi₂Se₂. For low extents of oxidation, alkali ions are removed exclusively to form K_1–<i>x</i>Ni₂Se₂. For greater extents of oxidation, the rate of the reaction decreases dramatically, concomitant with the extraction of both potassium and nickel to form K_1–<i>x</i>Ni_2–<i>y</i>Se₂. The appreciable mobility of transition metal ions is unexpected, but illustrates the relative energy scales of different defects in the ThCr₂Si₂ structure type. Furthermore, the fully oxidized compounds, K_0.25Ni_1.5Se₂, spontaneously convert from the tetrahedral [NiSe₄]-containing ThCr₂Si₂ structure to a vacancy-ordered NiAs structure with [NiSe₆] octahedra. From analysis of the atom positions and kinetic data, we have determined that this transformation occurs by a continuous, low-energy pathway via subtle displacements of Ni atoms and buckling of the Se sublattice. These results have profound implications for our understanding of the stability, mobility, and reactivity of ions in materials

Bonding, Ion Mobility, and Rate-Limiting Steps in Deintercalation Reactions with ThCr₂Si₂-type KNi₂Se₂

Here, we study the nature of metal–metal bonding in the ThCr₂Si₂ structure type by probing the rate-limiting steps in the oxidative deintercalation of KNi₂Se₂. For low extents of oxidation, alkali ions are removed exclusively to form K_1–<i>x</i>Ni₂Se₂. For greater extents of oxidation, the rate of the reaction decreases dramatically, concomitant with the extraction of both potassium and nickel to form K_1–<i>x</i>Ni_2–<i>y</i>Se₂. The appreciable mobility of transition metal ions is unexpected, but illustrates the relative energy scales of different defects in the ThCr₂Si₂ structure type. Furthermore, the fully oxidized compounds, K_0.25Ni_1.5Se₂, spontaneously convert from the tetrahedral [NiSe₄]-containing ThCr₂Si₂ structure to a vacancy-ordered NiAs structure with [NiSe₆] octahedra. From analysis of the atom positions and kinetic data, we have determined that this transformation occurs by a continuous, low-energy pathway via subtle displacements of Ni atoms and buckling of the Se sublattice. These results have profound implications for our understanding of the stability, mobility, and reactivity of ions in materials

Synthesis and Structure of Three New Oxychalcogenides: A₂O₂Bi₂Se₃ (A = Sr, Ba) and Sr₂O₂Sb₂Se₃

Three new compounds have been added to the alkali chalcogenide and oxychalcogenide families: Sr₂O₂Bi₂Se₃, Ba₂O₂Bi₂Se₃, and Sr₂O₂Sb₂Se₃ were synthesized by direct combination of SrO or BaO with Bi₂Se₃ or Sb₂Se₃. The structure, determined from laboratory X-ray powder diffraction data, consists of double chains of edge-sharing BiSe₄O square pyramids. Temperature-dependent resistance data reveal all three compounds to be insulators, while heat capacity data of Ba₂O₂Bi₂Se₃ and Sr₂O₂Bi₂Se₃, in conjunction with the literature reports, reveal low energy phonon modes due to bismuth lone pair effects. We propose a specific materials design principle connecting electron count to structural dimensionality by comparison to related chalcogenides, including the BiS₂ superconductors

Control of the Iridium Oxidation State in the Hollandite Iridate Solid Solution K_1–<i>x</i>Ir₄O₈

The synthesis and physical properties of the K_1–<i>x</i>Ir₄O₈ (0 ≤ <i>x</i> ≤ 0.7) solid solution are reported. The structure of KIr₄O₈, solved with single-crystal X-ray diffraction at <i>T</i> = 110 K, is found to be tetragonal, space group <i>I</i>4/<i>m</i>, with <i>a</i> = 10.0492(3) Å and <i>c</i> = 3.14959(13) Å. A highly anisotropic displacement parameter is found for the potassium cation. Density functional theory calculations suggest that this anisotropy is due to a competition between atomic size and bond valence. KIr₄O₈ has a significant electronic contribution to the specific heat, γ = 13.9 mJ mol-Ir^–1 K^–2, indicating an effective carrier mass of m*/m_e ≈ 10. Further, there is a magnetic-field-dependent upturn in the specific heat at <i>T</i> < 3 K, suggestive of a magnetically sensitive phase transition below <i>T</i> < 1.8 K. Resistivity and magnetization measurements show that both end-members of the solid solution, KIr₄O₈ and K_1–<i>x</i>Ir₄O₈ (<i>x</i> ≈ 0.7), are metallic, with no significant trends in the temperature-independent contributions to the magnetization. These results are interpreted and discussed in the context of the importance of the variability of the oxidation state of iridium. The differences in physical properties between members of the K_1–<i>x</i>Ir₄O₈ (0 ≤ <i>x</i> ≤ 0.7) series are small and appear to be insensitive to the iridium oxidation state

Karola Filng, Herbert Heuss, Frank Sparing, Od „rasne znanosti“ do logora - Romi u II. svjetskom ratu (1. dio), Zagreb: Ibis grafika, 2006., 125 str.

Among oxide compounds with direct metal–metal bonding, the Y₅Mo₂O₁₂ (<i>A</i>₅<i>B</i>₂O₁₂) structural family of compounds has a particularly intriguing low-dimensional structure due to the presence of bioctahedral <i>B</i>₂O₁₀ dimers arranged in one-dimensional edge-sharing chains along the direction of the metal–metal bonds. Furthermore, these compounds can have a local magnetic moment due to the noninteger oxidation state (+4.5) of the transition metal, in contrast to the conspicuous lack of a local moment that is commonly observed when oxide compounds with direct metal–metal bonding have integer oxidation states resulting from the lifting of orbital degeneracy typically induced by the metal–metal bonding. Although a monoclinic <i>C</i>2/<i>m</i> structure has been previously proposed for <i>Ln</i>₅Mo₂O₁₂ (<i>Ln</i> = La–Lu and Y) members of this family based on prior single crystal diffraction data, it is found that this structural model misses many important structural features. On the basis of synchrotron powder diffraction data, it is shown that the <i>C</i>2/<i>m</i> monoclinic unit cell represents a superstructure relative to a previously unrecognized orthorhombic <i>Immm</i> subcell and that the superstructure derives from the ordering of interchangeable Mo₂O₁₀ and LaO₆ building blocks. The superstructure for this reason is typically highly faulted, as evidenced by the increased breadth of superstructure diffraction peaks associated with a coherence length of 1–2 nm in the <i>c</i>* direction. Finally, it is shown that oxygen vacancies can occur when <i>Ln</i> = La, producing an oxygen deficient stoichiometry of La₅Mo₂O_11.55 and an approximately 10-fold reduction in the number of unpaired electrons due to the reduction of the average Mo valence from +4.5 to +4.05, a result confirmed by magnetic susceptibility measurements. This represents the first observation of oxygen vacancies in this family of compounds and provides an important means of continuously tuning the magnetic interactions within the one-dimensional octahedral chains of this system

Superconductivity–Electron Count Relationship in Heusler Phasesthe Case of LiPd₂Si

We report superconductivity in the full Heusler compound LiPd2Si (space group Fm3̅m, No. 225) at a critical temperature of Tc = 1.3 K and a normalized heat capacity jump at Tc, ΔC/γTc = 1.1. The low-temperature isothermal magnetization curves imply type-I superconductivity, as previously observed in LiPd2Ge. We show, based on density functional theory calculations and using the molecular orbital theory approach, that while LiPd2Si and LiPd2Ge share the Pd cubic cage motif that is found in most of the reported Heusler superconductors, they show distinctive features in the electronic structure. This is due to the fact that Li occupies the site which, in other compounds, is filled with an early transition metal or a rare-earth metal. Thus, while a simple valence electron count–property relationship is useful in predicting and tuning Heusler materials, inclusion of the symmetry of interacting frontier orbitals is also necessary for the best understanding

Bonding and Electronic Nature of the Anionic Framework in LaPd₃S₄

Double Dirac materials are a topological phase of matter in which a non-symmorphic symmetry enforces greater electronic degeneracy than normally expected – up to eightfold. The cubic palladium bronzes NaPd3O4 and LaPd3S4 are built of Pd3X4 (X = O, S) anionic frameworks that are ionically bonded to A cations (A = Na, La). These materials were recently identified computationally as harboring eightfold fermions. Here we report the preparation of single crystals and electronic properties of LaPd3S4. Measurements down to T = 0.45 K and in magnetic fields up to μ0H = 65 T are consistent with normal Fermi liquid physics of a Dirac metal in the presence of dilute magnetic impurities. This interpretation is further confirmed by analysis of specific heat, magnetization measurements and comparison to density functional theory (DFT) calculations. Through a bonding analysis of the DFT electronic structure of NaPd3O4 and LaPd3S4, we identify the origin of the stability of the anionic Pd3X4 framework at higher electron counts for X = S than X = O, and propose chemical tuning strategies to enable shifting the 8-fold fermion points to the Fermi level