4 research outputs found

    Trigonal-Bipyramidal Coordination in First Ammoniates of ZnF<sub>2</sub>: ZnF<sub>2</sub>(NH<sub>3</sub>)<sub>3</sub> and ZnF<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub>

    No full text
    Single crystals of ZnF<sub>2</sub>(NH<sub>3</sub>)<sub>3</sub> and ZnF<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub> were obtained under ammonothermal conditions (250 °C, 196 MPa and 500 °C, 136 MPa). Upon thermal decomposition of both ZnF<sub>2</sub>(NH<sub>3</sub>)<sub>3</sub> and ZnF<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub>, a microcrystalline powder of ZnF<sub>2</sub>(NH<sub>3</sub>) was obtained. ZnF<sub>2</sub>(NH<sub>3</sub>)<sub>3</sub> and ZnF<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub> represent probable intermediates in a conceivable ammonothermal synthesis of the semiconductor Zn<sub>3</sub>N<sub>2</sub> and manifest a rare trigonal-bipyramidal coordination of F<sup>–</sup> and NH<sub>3</sub> ligands around Zn<sup>2+</sup> according to single-crystal X-ray diffraction. Thermal analysis of all three compounds showed not only ZnF<sub>2</sub>(NH<sub>3</sub>) but also ZnF<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub> to be decomposition intermediates of ZnF<sub>2</sub>(NH<sub>3</sub>)<sub>3</sub> prior to the formation of ZnF<sub>2</sub>. All three compounds demonstrate hydrogen bonds, as indicated by the intensities and half-widths of the bands in the vibrational spectra and by short N–H···F distances in the crystal structures of ZnF<sub>2</sub>(NH<sub>3</sub>)<sub>3</sub> and ZnF<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub>. With ZnF<sub>2</sub>(NH<sub>3</sub>)<sub>3</sub>, ZnF<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub>, and ZnF<sub>2</sub>(NH<sub>3</sub>), we present the first ammoniates of ZnF<sub>2</sub>

    Ternary Metastable Nitrides ε‑Fe<sub>2</sub><i>TM</i>N (<i>TM</i> = Co, Ni): High-Pressure, High-Temperature Synthesis, Crystal Structure, Thermal Stability, and Magnetic Properties

    No full text
    High-pressure, high-temperature synthesis gives access to ternary metastable nitrides ε-Fe<sub>2</sub><i>TM</i>N (<i>TM</i> = Co, Ni) as bulk materials for the first time. Both ε-Fe<sub>2</sub>CoN and ε-Fe<sub>2</sub>NiN crystallize isostructural to ε-Fe<sub>3</sub>N as evidenced by X-ray powder diffraction data. The lattice parameters of the new compounds are slightly smaller than those of ε-Fe<sub>3</sub>N owing to the reduced atomic radii of the metal atoms. Energy-dispersive X-ray spectroscopy of metallographic samples show homogeneous metal ratios corresponding to compositions Fe<sub>1.99(6)</sub>Co<sub>1.01(6)</sub>N and Fe<sub>1.97(2)</sub>Ni<sub>1.03(2)</sub>N. Extended X-ray absorption fine spectra indicate that cobalt and nickel occupy iron positions. Thermal analysis measurements reveal decomposition of both ternary nitrides above 920 K. ε-Fe<sub>2</sub>CoN disintegrates into N<sub>2</sub> and iron–cobalt alloy, while ε-Fe<sub>2</sub>NiN decays into N<sub>2</sub>, iron–nickel alloy as well as α-Fe. The replacement of iron by cobalt or nickel essentially lowers the saturation magnetization from roughly 6.0 μ<sub>B</sub>/f.u. for ε-Fe<sub>3</sub>N to nearly 4.3 μ<sub>B</sub>/f.u. for ε-Fe<sub>2</sub>CoN and 3.1 μ<sub>B</sub>/f.u. for ε-Fe<sub>2</sub>NiN. In parallel, the Curie temperature decreases from 575(3) K for ε-Fe<sub>3</sub>N to 488(5) K for ε-Fe<sub>2</sub>CoN and 234(3) K for ε-Fe<sub>2</sub>NiN. Calculations of the formation enthalpies illustrate that the phases ε-Fe<sub>2</sub><i>TM</i>N (<i>TM</i> = Co, Ni) are thermodynamically unfavorable at ambient conditions which is consistent with our experimental observations. The substitution of one Fe by Co (Ni) yields one (two) more electrons per formula unit which reduces the magnetic interactions. First-principles analysis indicate that the replacement has a negligible influence on the electron occupation numbers and spin moments of the N and unsubstituted Fe sites, but decreases the local magnetic moments on the substituted Fe positions because the extra electrons occupy the minority-spin channel formed by states of the <i>TM</i> atoms

    Three Oxidation States of Manganese in the Barium Hexaferrite BaFe<sub>12–<i>x</i></sub>Mn<sub><i>x</i></sub>O<sub>19</sub>

    No full text
    The coexistence of three valence states of Mn ions, namely, +2, +3, and +4, in substituted magnetoplumbite-type BaFe<sub>12–<i>x</i></sub>Mn<sub><i>x</i></sub>O<sub>19</sub> was observed by soft X-ray absorption spectroscopy at the Mn-L<sub>2,3</sub> edge. We infer that the occurrence of multiple valence states of Mn situated in the pristine purely iron­(III) compound BaFe<sub>12</sub>O<sub>19</sub> is made possible by the fact that the charge disproportionation of Mn<sup>3+</sup> into Mn<sup>2+</sup> and Mn<sup>4+</sup> requires less energy than that of Fe<sup>3+</sup> into Fe<sup>2+</sup> and Fe<sup>4+</sup>, related to the smaller effective Coulomb interaction of Mn<sup>3+</sup> (d<sup>4</sup>) compared to Fe<sup>3+</sup> (d<sup>5</sup>). The different chemical environments determine the location of the differently charged ions: with Mn<sup>3+</sup> occupying positions with (distorted) octahedral local symmetry, Mn<sup>4+</sup> ions prefer octahedrally coordinated sites in order to optimize their covalent bonding. Larger and more ionic bonded Mn<sup>2+</sup> ions with a spherical charge distribution accumulate at tetrahedrally coordinated sites. Simulations of the experimental Mn-L<sub>2,3</sub> XAS spectra of two different samples with <i>x</i> = 1.5 and <i>x</i> = 1.7 led to Mn<sup>2+</sup>:Mn<sup>3+</sup>:Mn<sup>4+</sup> atomic ratios of 0.16:0.51:0.33 and 0.19:0.57:0.24

    Ti-Substituted BaFe<sub>12</sub>O<sub>19</sub> Single Crystal Growth and Characterization

    No full text
    Ti-substituted barium hexaferrite BaFe<sub>12</sub>O<sub>19</sub> single crystals BaFe<sub>12–<i>x</i></sub>Ti<sub><i>x</i></sub>O<sub>19</sub> with <i>x</i> up to 1.3 and sizes 2–8 mm were grown by spontaneous crystallization from molten sodium carbonate flux. The distribution of Ti on different crystallographic sites was determined from single crystal X-ray diffraction data. For low Ti contents up to <i>x</i> = 0.8 the unit cell expands; on further increase of the Ti amount the unit cell starts to shrink. This behavior for low Ti contents is most likely due to a reduction of Fe<sup>3+</sup> to Fe<sup>2+</sup> for charge balance. At higher Ti concentrations, supposedly vacancies in the transition metal substructure are formed. An increasing Ti concentration results in a monotonous reduction of the Curie temperature from 452 to 251 °C and the saturation magnetization at room temperature from 64.8 to 24.8 emu/g for powder samples and from 70.0 to 60.1 emu/g for single crystals (for <i>x</i> up to 0.78)
    corecore