One Structure, Two Elements-LuGe2 Superconductor vs Ordinary Metallic Conductor LuSn2. A Case Study on How Site-Selective Germanium for Tin Atom Substitution Leads to Modulating of the Charge Distribution

[(thf)5Ln(Ge9{Si(SiMe3)3}2)] (Ln = Eu, Sm, Yb): Capping Metalloid Germanium Cluster with Lanthanides

We report the synthesis of three neutral complexes with different coordination modes of a di-silylated metalloid germanium cluster to divalent lanthanides [(thf)₅Ln(ηⁿ-Ge₉(Hyp)₂)] (Ln = Yb (1, n = 1); Eu (2, n = 2, 3), Sm (3, n = 2, 3); Hyp = Si(SiMe₃)₃) by the salt metathesis of LnI₂ with K₂[Ge₉(Hyp)₂] in THF. The complexes were characterized by elemental analysis, nuclear magnetic resonance and UV-vis-NIR spectroscopy, and single-crystal X-ray diffraction. In thf solution, the formation of contact or solvate-separated ion pairs depending on the concentration is assumed. Compound 2 exhibits a blue luminescence typical for Eu²⁺. The solid-state magnetic measurements of compounds 2 and 3 confirm the presence of divalent europium and samarium, respectively.

Irreversible Transition from GaO6 Octahedra to GaO4 Tetrahedra for Improved Electrochemical Stability in Ga-Doped Li(Ni0.9Co0.1)O2

Trace doping is an efficient way to improve the stability of nickel-rich layered cathodes for lithium-ion batteries, but the structural origin of such improvement, rather than a simple replacement, has been rarely explored. Herein, Ga dopants have been introduced into a nickel-rich host, LiNi_0.9Co_0.1O₂, by a combination of coprecipitation and the solid-state sintering method. Structural analyses based on high-resolution synchrotron powder diffraction data and X-ray absorption spectra suggest that Ga preferentially sits in the NiO₆ slabs, resulting in reduced Ni/Li cationic mixing and depressed lattice vibration. Due to the smaller dissociation energy of Ga-O bonds, some Ga³⁺ cations migrate first toward Li layers upon delithiation and form the GaO₄ tetrahedral symmetry irreversibly during the electrochemical process, which acts as a pillar in the subsequent electrochemical processes. As a result, the doped material exhibits both improved cycling performance and rate capability under a high operating voltage (4.5 V) due to the stabilized structure in the lithiation/delithiation process. This study illustrates how a dopant enhances the electrochemical stability in views of both pristine and charged structure and suggests that a positive effect is expected from the dopant favoring the tetrahedral symmetry (e.g., Al).