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

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

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