12 research outputs found
Computational Design of Rare-Earth-Free Magnets with the Ti<sub>3</sub>Co<sub>5</sub>B<sub>2</sub>âType Structure
The prolific Ti<sub>3</sub>Co<sub>5</sub>B<sub>2</sub> structure
type has produced exciting materials with tunable magnetic properties,
ranging from soft magnetic Ti<sub>2</sub>FeRh<sub>5</sub>B<sub>2</sub>, to semihard magnetic Ti<sub>2</sub>FeRu<sub>4</sub>RhB<sub>2</sub> and hard magnetic Sc<sub>2</sub>FeRu<sub>3</sub>Ir<sub>2</sub>B<sub>2</sub>. Density functional theory (DFT) was employed to investigate
their spinâorbit coupling effect, spin exchange, and magnetic
dipoleâdipole interactions in order to understand their magnetic
anisotropy and relate it to their various coercivities, with the objective
of being able to predict new materials with large magnetic anisotropy.
Our calculations show that the contribution of magnetic dipoleâdipole
interactions to the magnetocrystalline anisotropy energy (MAE) in
Ti<sub>3</sub>Co<sub>5</sub>B<sub>2</sub>-type compounds is much weaker
than the spinâorbit coupling effect, and Sc<sub>2</sub>FeRu<sub>3</sub>Ir<sub>2</sub>B<sub>2</sub> has, by far, the largest MAE and
strong intrachain and interchain FeâFe spin exchange coupling,
thus confirming its hard magnetic properties. We then targeted materials
containing the more earth-abundant and less expensive Co, instead
of Rh, Ru or Ir, so that our study started with Ti<sub>3</sub>Co<sub>5</sub>B<sub>2</sub>, which we found to be nonmagnetic. In the next
step, substitutions on the Ti sites in Ti<sub>3</sub>Co<sub>5</sub>B<sub>2</sub> led to new potential quaternary phases with the general
formula T<sub>2</sub>Tâ˛Co<sub>5</sub>B<sub>2</sub> (T = Ti,
Hf; TⲠ= Mn, Fe). For Hf<sub>2</sub>MnCo<sub>5</sub>B<sub>2</sub>, we found a large MAE (+0.96 meV/f.u.) but relatively weak interchain
MnâMn spin exchange interactions, whereas for Hf<sub>2</sub>FeCo<sub>5</sub>B<sub>2</sub>, there is a relatively smaller MAE
(+0.17 meV/f.u.) but strong FeâFe interchain and intrachain
spin exchange interactions. Therefore, these two Co-rich phases are
predicted to be new rare-earth-free, semihard to hard magnetic materials
Electronic Pseudogap-Driven Formation of New Double-Perovskite-like Borides within the Sc<sub>2</sub>Ir<sub>6â<i>x</i></sub>T<sub><i>x</i></sub>B (T = Pd, Ni; <i>x</i> = 0â6) Series
Analysis of the electronic density
of states of the hypothetical ternary double-perovskite-like phases
âSc<sub>2</sub>T<sub>6</sub>B (T = Ir, Pd, Ni)â reveals
the presence of deep and large pseudogaps between 61 and 68 valence
electrons (VE) as well as a strong peak at 69 VEs. Subsequently, crystal
orbital Hamilton population (COHP) bonding analysis shows that the
heteroatomic TâB and ScâT interactions are optimized
in Sc<sub>2</sub>Ir<sub>6</sub>B (63 VE) but not in âSc<sub>2</sub>Pd<sub>6</sub>B (69 VE)â and âSc<sub>2</sub>Ni<sub>6</sub>B (69 VE)â, thus indicating less stability for
these VE-richer phases. These findings point out the possibility of
discovering new double-perovskite-like borides through chemical substitution
and lead to the study of the Sc<sub>2</sub>Ir<sub>6â<i>x</i></sub>Pd<sub><i>x</i></sub>B and Sc<sub>2</sub>Ir<sub>6â<i>x</i></sub>Ni<sub><i>x</i></sub>B (<i>x</i> = 0â6; VE = 63â69) series,
for which powder samples and single crystals were synthesized by arc
melting the elements. Superstructure reflections were observed in
the powder diffractograms of Sc<sub>2</sub>Ir<sub>6â<i>x</i></sub>Pd<sub><i>x</i></sub>B and Sc<sub>2</sub>Ir<sub>6â<i>x</i></sub>Ni<sub><i>x</i></sub>B for <i>x</i> = 0â5 and VE = 63â68,
thereby showing that these phases crystallize in the double-perovskite-like
Ti<sub>2</sub>Rh<sub>6</sub>B-type structure (space group <i>Fm</i>3Ě
<i>m</i>, <i>Z</i> = 4). Single-crystal
and Rietveld refinement results confirm and extend these findings
because Pd (or Ni) is found to mix exclusively with Ir in all quaternary
compositions. For <i>x</i> = 6, no superstructure reflections
were observed, in accordance with the theoretical expectation for
the 69 VE phases
Drastic Change of Magnetic Interactions and Hysteresis through Site-Preferential Ru/Ir Substitution in Sc<sub>2</sub>FeRu<sub>5â<i>x</i></sub>Ir<sub><i>x</i></sub>B<sub>2</sub>
The
quinary members of the complex boride series Sc<sub>2</sub>FeRu<sub>5â<i>x</i></sub>Ir<sub><i>x</i></sub>B<sub>2</sub> were synthesized by arc melting the elements
and characterized by powder and single-crystal X-ray diffraction as
well as metallographic and energy-dispersive X-ray analyses. The use
of a 4d/5d mixture allows distinguishing these elements with X-ray
diffraction methods, thus enabling the study of site preference and
its influence on the magnetic properties.The magnetic measurements
reveal several changes of magnetic ordering
within the series: from antiferromagnetism (Sc<sub>2</sub>FeRu<sub>5</sub>B<sub>2</sub>) to ferromagnetism (Sc<sub>2</sub>FeRuIr<sub>4</sub>B<sub>2</sub>) and finally to metamagnetism (Sc<sub>2</sub>FeIr<sub>5</sub>B<sub>2</sub>). Within the quinary series, the magnetic
moments continuously increase with increasing amounts of Ir in one
(8<i>j</i>) of two possible Wyckoff sites. The members with <i>x</i> = 2 and 3 represent the first hard magnetic borides of
transition metals
Unexpected Competition between Antiferromagnetic and Ferromagnetic States in Hf<sub>2</sub>MnRu<sub>5</sub>B<sub>2</sub>: Predicted and Realized
Materials
âdesignâ is increasingly gaining importance in the solid-state
materials community in general and in the field of magnetic materials
in particular. Density functional theory (DFT) predicted the competition
between ferromagnetic (FM) and antiferromagnetic (AFM) ground states
in a ruthenium-rich Ti<sub>3</sub>Co<sub>5</sub>B<sub>2</sub>-type
boride (Hf<sub>2</sub>MnRu<sub>5</sub>B<sub>2</sub>) for the first
time. Vienna ab initio simulation package (VASP) total energy calculations
indicated that the FM model was marginally more stable than one of
the AFM models (AFM1), indicating very weak interactions between magnetic
1D Mn chains that can be easily perturbated by external means (magnetic
field or composition). The predicted phase was then synthesized by
arc-melting and characterized as Hf<sub>2</sub>Mn<sub>1â<i>x</i></sub>Ru<sub>5+<i>x</i></sub>B<sub>2</sub> (<i>x</i> = 0.27). Vibrating-scanning magnetometry shows an AFM
ground state with <i>T</i><sub>N</sub> â 20 K under
low magnetic field (0.005 T). At moderate-to-higher fields, AFM ordering
vanishes while FM ordering emerges with a Curie temperature of 115
K. These experimental outcomes confirm the weak nature of the interchain
interactions, as predicted by DFT calculations
Spin Frustration and Magnetic Ordering from One-Dimensional Stacking of Cr<sub>3</sub> Triangles in TiCrIr<sub>2</sub>B<sub>2</sub>
Spin-frustrated
chains of Cr<sub>3</sub> triangles are found in the new metal boride
TiCrIr<sub>2</sub>B<sub>2</sub> 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 Îź<sub>B</sub> at 5 T), and density functional theory (DFT) calculations propose
a canted, nonlinear magnetic ground-state ordering in the new phase.
TiCrIr<sub>2</sub>B<sub>2</sub> crystallizes in the hexagonal Ti<sub>1+<i>x</i></sub>Os<sub>2â<i>x</i></sub>RuB<sub>2</sub> structure type (space group <i>P</i>6Ě
2<i>m</i>, No. 189, Pearson symbol <i>hP</i>18). The structure
contains trigonal planar B<sub>4</sub> boron fragments with BâB
distances of 1.76(3) Ă
alternating along the <i>c</i>-direction with Cr<sub>3</sub> triangles with intra- and intertriangle
CrâCr distances of 2.642(9) and 3.185(1) Ă
, respectively.
Magnetization measurements of TiCrIr<sub>2</sub>B<sub>2</sub> reveal
ferrimagnetic behavior and a large, negative Weiss constant of â750
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 Cr<sub>3</sub> triangles
Chemical Tuning of Magnetic Properties through Ru/Rh Substitution in Th<sub>7</sub>Fe<sub>3</sub>âtype FeRh<sub>6â<i>n</i></sub>Ru<sub><i>n</i></sub>B<sub>3</sub> (<i>n</i> = 1â5) Series
The new quaternary boride series
FeRh<sub>6â<i>n</i></sub>Ru<sub><i>n</i></sub>B<sub>3</sub> (<i>n</i> = 1â5) was synthesized
by arc melting and characterized by
powder and single-crystal X-ray diffraction (XRD), energy-dispersive
X-ray analysis, and superconducting quantum interference device magnetometry.
Single-crystal structure refinement showed the distribution of the
iron atoms in two of three possible crystallographic 4d metal sites
in the structure (Th<sub>7</sub>Fe<sub>3</sub>-type, space group <i>P</i>6<sub>3</sub><i>mc</i>). Rietveld refinements
of the powder XRD data indicated single-phase synthesis of all the
members. A linear decrease of the lattice parameters and the unit
cell volume with increasing Ru content was found, indicating Vegardâs
behavior. Susceptibility measurements show decreasing Curie temperature
and magnetic moment (Îź<sub>a</sub><sup>5T</sup>) recorded at
5 T with increasing Ru content from <i>T</i><sub>C</sub> = 295 K and Îź<sub>a</sub><sup>5T</sup> = 3.35 Îź<sub>B</sub> (FeRh<sub>5</sub>RuB<sub>3</sub>) to <i>T</i><sub>C</sub> = 205 K and Îź<sub>a</sub><sup>5T</sup> = 0.70 Îź<sub>B</sub> (FeRhRu<sub>5</sub>B<sub>3</sub>). The measured coercivities
lie between 1.0 and 2.2 kA/m indicating soft to semihard magnetic
materials
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Titanium Sulfides as Intercalation-Type Cathode Materials for Rechargeable Aluminum Batteries
We
report the electrochemical intercalationâextraction of
aluminum (Al) in the layered TiS<sub>2</sub> and spinel-based cubic
Cu<sub>0.31</sub>Ti<sub>2</sub>S<sub>4</sub> as the potential cathode
materials for rechargeable Al-ion batteries. The electrochemical characterizations
demonstrate the feasibility of reversible Al intercalation in both
titanium sulfides with layered TiS<sub>2</sub> showing better properties.
The crystallographic study sheds light on the possible Al intercalation
sites in the titanium sulfides, while the results from galvanostatic
intermittent titration indicate that the low Al<sup>3+</sup> diffusion
coefficients in the sulfide crystal structures are the primary obstacle
to facile Al intercalationâextraction
Spin Frustration and Magnetic Ordering from One-Dimensional Stacking of Cr<sub>3</sub> Triangles in TiCrIr<sub>2</sub>B<sub>2</sub>
Spin-frustrated
chains of Cr<sub>3</sub> triangles are found in the new metal boride
TiCrIr<sub>2</sub>B<sub>2</sub> 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 Îź<sub>B</sub> at 5 T), and density functional theory (DFT) calculations propose
a canted, nonlinear magnetic ground-state ordering in the new phase.
TiCrIr<sub>2</sub>B<sub>2</sub> crystallizes in the hexagonal Ti<sub>1+<i>x</i></sub>Os<sub>2â<i>x</i></sub>RuB<sub>2</sub> structure type (space group <i>P</i>6Ě
2<i>m</i>, No. 189, Pearson symbol <i>hP</i>18). The structure
contains trigonal planar B<sub>4</sub> boron fragments with BâB
distances of 1.76(3) Ă
alternating along the <i>c</i>-direction with Cr<sub>3</sub> triangles with intra- and intertriangle
CrâCr distances of 2.642(9) and 3.185(1) Ă
, respectively.
Magnetization measurements of TiCrIr<sub>2</sub>B<sub>2</sub> reveal
ferrimagnetic behavior and a large, negative Weiss constant of â750
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 Cr<sub>3</sub> triangles
Graphene- and Phosphorene-like Boron Layers with Contrasting Activities in Highly Active Mo<sub>2</sub>B<sub>4</sub> for Hydrogen Evolution
Two different boron
layers, flat (graphene-like) and puckered (phosphorene-like),
found in the crystal structure of Mo<sub>2</sub>B<sub>4</sub> show
drastically different activities for hydrogen evolution, according
to Gibbs free energy calculations of H-adsorption on Mo<sub>2</sub>B<sub>4</sub>. 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
Mo<sub>2</sub>B<sub>4</sub>, 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. Mo<sub>2</sub>B<sub>4</sub> compensates its smaller density of active sites
if compared with highly active bulk MoB<sub>2</sub> (which contains
only the more active graphene-like boron layers) by a 5-times increase
of its surface area
Synthesis and Theoretical Investigations of the Solid Solution CeRu<sub>1â<i>x</i></sub>Ni<sub><i>x</i></sub>Al (<i>x</i> = 0.1â0.95) Showing Cerium Valence Fluctuations
Members of the solid solution series
of CeÂRu<sub>1â<i>x</i></sub>Ni<sub><i>x</i></sub>Al can be obtained directly by arc melting of the
elements. The presented compounds with 0.1 ⤠<i>x</i> ⤠0.85 crystallize in the orthorhombic space group <i>Pnma</i> (No. 62) in the LaNiAl structure type, while for 0.9
⤠<i>x</i> ⤠1, the hexagonal ZrNiAl-type
structure is found. The orthorhombic members exhibit an anomaly in
the trend of the lattice parameters as well as an interesting behavior
of the magnetic susceptibility, suggesting that the cerium cations
exhibit no local moment. Besides the mixed-valent nature of the cerium
cations, valence fluctuations along with a change in the cerium oxidation
state depending on the nickel content have been found. The oxidation
state has been determined from the magnetic data and additionally
by XANES. Density functional theory calculations have identified the
shortest CeâRu interaction as decisive for the stability of
the orthorhombic solid solution