Computational Design of Rare-Earth-Free Magnets with the Ti₃Co₅B₂‑Type Structure

The prolific Ti₃Co₅B₂ structure type has produced exciting materials with tunable magnetic properties, ranging from soft magnetic Ti₂FeRh₅B₂, to semihard magnetic Ti₂FeRu₄RhB₂ and hard magnetic Sc₂FeRu₃Ir₂B₂. Density functional theory (DFT) was employed to investigate their spin–orbit coupling effect, spin exchange, and magnetic dipole–dipole 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 dipole–dipole interactions to the magnetocrystalline anisotropy energy (MAE) in Ti₃Co₅B₂-type compounds is much weaker than the spin–orbit coupling effect, and Sc₂FeRu₃Ir₂B₂ has, by far, the largest MAE and strong intrachain and interchain Fe–Fe 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 Ti₃Co₅B₂, which we found to be nonmagnetic. In the next step, substitutions on the Ti sites in Ti₃Co₅B₂ led to new potential quaternary phases with the general formula T₂T′Co₅B₂ (T = Ti, Hf; T′ = Mn, Fe). For Hf₂MnCo₅B₂, we found a large MAE (+0.96 meV/f.u.) but relatively weak interchain Mn–Mn spin exchange interactions, whereas for Hf₂FeCo₅B₂, there is a relatively smaller MAE (+0.17 meV/f.u.) but strong Fe–Fe 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 Sc₂Ir_6–<i>x</i>T_<i>x</i>B (T = Pd, Ni; <i>x</i> = 0–6) Series

Analysis of the electronic density of states of the hypothetical ternary double-perovskite-like phases “Sc₂T₆B (T = Ir, Pd, Ni)” 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 T–B and Sc–T interactions are optimized in Sc₂Ir₆B (63 VE) but not in “Sc₂Pd₆B (69 VE)” and “Sc₂Ni₆B (69 VE)”, 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 Sc₂Ir_6–<i>x</i>Pd_<i>x</i>B and Sc₂Ir_6–<i>x</i>Ni_<i>x</i>B (<i>x</i> = 0–6; VE = 63–69) series, for which powder samples and single crystals were synthesized by arc melting the elements. Superstructure reflections were observed in the powder diffractograms of Sc₂Ir_6–<i>x</i>Pd_<i>x</i>B and Sc₂Ir_6–<i>x</i>Ni_<i>x</i>B for <i>x</i> = 0–5 and VE = 63–68, thereby showing that these phases crystallize in the double-perovskite-like Ti₂Rh₆B-type structure (space group <i>Fm</i>3̅<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 Sc₂FeRu_5–<i>x</i>Ir_<i>x</i>B₂

The quinary members of the complex boride series Sc₂FeRu_5–<i>x</i>Ir_<i>x</i>B₂ 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 (Sc₂FeRu₅B₂) to ferromagnetism (Sc₂FeRuIr₄B₂) and finally to metamagnetism (Sc₂FeIr₅B₂). 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 Hf₂MnRu₅B₂: Predicted and Realized

Materials “design” 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 Ti₃Co₅B₂-type boride (Hf₂MnRu₅B₂) 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 Hf₂Mn_1–<i>x</i>Ru_5+<i>x</i>B₂ (<i>x</i> = 0.27). Vibrating-scanning magnetometry shows an AFM ground state with <i>T</i>_N ≈ 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 Cr₃ Triangles in TiCrIr₂B₂

Spin-frustrated chains of Cr₃ triangles are found in the new metal boride TiCrIr₂B₂ 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 μ_B at 5 T), and density functional theory (DFT) calculations propose a canted, nonlinear magnetic ground-state ordering in the new phase. TiCrIr₂B₂ crystallizes in the hexagonal Ti_1+<i>x</i>Os_2–<i>x</i>RuB₂ structure type (space group <i>P</i>6̅2<i>m</i>, No. 189, Pearson symbol <i>hP</i>18). The structure contains trigonal planar B₄ boron fragments with B–B distances of 1.76(3) Å alternating along the <i>c</i>-direction with Cr₃ triangles with intra- and intertriangle Cr–Cr distances of 2.642(9) and 3.185(1) Å, respectively. Magnetization measurements of TiCrIr₂B₂ reveal ferrimagnetic behavior and a large, negative Weiss constant of −750 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 Cr₃ triangles

Chemical Tuning of Magnetic Properties through Ru/Rh Substitution in Th₇Fe₃‑type FeRh_6–<i>n</i>Ru_<i>n</i>B₃ (<i>n</i> = 1–5) Series

The new quaternary boride series FeRh_6–<i>n</i>Ru_<i>n</i>B₃ (<i>n</i> = 1–5) 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 (Th₇Fe₃-type, space group <i>P</i>6₃<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 Vegard’s behavior. Susceptibility measurements show decreasing Curie temperature and magnetic moment (μ_a^5T) recorded at 5 T with increasing Ru content from <i>T</i>_C = 295 K and μ_a^5T = 3.35 μ_B (FeRh₅RuB₃) to <i>T</i>_C = 205 K and μ_a^5T = 0.70 μ_B (FeRhRu₅B₃). The measured coercivities lie between 1.0 and 2.2 kA/m indicating soft to semihard magnetic materials

Titanium Sulfides as Intercalation-Type Cathode Materials for Rechargeable Aluminum Batteries

We report the electrochemical intercalation–extraction of aluminum (Al) in the layered TiS₂ and spinel-based cubic Cu_0.31Ti₂S₄ 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 TiS₂ 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 Al³⁺ diffusion coefficients in the sulfide crystal structures are the primary obstacle to facile Al intercalation–extraction

Spin Frustration and Magnetic Ordering from One-Dimensional Stacking of Cr₃ Triangles in TiCrIr₂B₂

Spin-frustrated chains of Cr₃ triangles are found in the new metal boride TiCrIr₂B₂ 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 μ_B at 5 T), and density functional theory (DFT) calculations propose a canted, nonlinear magnetic ground-state ordering in the new phase. TiCrIr₂B₂ crystallizes in the hexagonal Ti_1+<i>x</i>Os_2–<i>x</i>RuB₂ structure type (space group <i>P</i>6̅2<i>m</i>, No. 189, Pearson symbol <i>hP</i>18). The structure contains trigonal planar B₄ boron fragments with B–B distances of 1.76(3) Å alternating along the <i>c</i>-direction with Cr₃ triangles with intra- and intertriangle Cr–Cr distances of 2.642(9) and 3.185(1) Å, respectively. Magnetization measurements of TiCrIr₂B₂ reveal ferrimagnetic behavior and a large, negative Weiss constant of −750 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 Cr₃ triangles

Graphene- and Phosphorene-like Boron Layers with Contrasting Activities in Highly Active Mo₂B₄ for Hydrogen Evolution

Two different boron layers, flat (graphene-like) and puckered (phosphorene-like), found in the crystal structure of Mo₂B₄ show drastically different activities for hydrogen evolution, according to Gibbs free energy calculations of H-adsorption on Mo₂B₄. 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 Mo₂B₄, 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. Mo₂B₄ compensates its smaller density of active sites if compared with highly active bulk MoB₂ (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_1–<i>x</i>Ni_<i>x</i>Al (<i>x</i> = 0.1–0.95) Showing Cerium Valence Fluctuations

Members of the solid solution series of Ce­Ru_1–<i>x</i>Ni_<i>x</i>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 Ce–Ru interaction as decisive for the stability of the orthorhombic solid solution