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>
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
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>
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 ironÂ(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
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)