Yb₅Ga₂Sb₆: A Mixed Valent and Narrow-Band Gap Material in the RE₅M₂X₆ Family

A new compound Yb₅Ga₂Sb₆ was synthesized by the metal flux technique as well as high frequency induction heating. Yb₅Ga₂Sb₆ crystallizes in the orthorhombic space group <i>Pbam</i> (no. 55), in the Ba₅Al₂Bi₆ structure type, with a unit cell of <i>a</i> = 7.2769(2) Å, <i>b</i> = 22.9102(5) Å, <i>c</i> = 4.3984(14) Å, and <i>Z</i> = 2. Yb₅Ga₂Sb₆ has an anisotropic structure with infinite anionic double chains (Ga₂Sb₆)^10– cross-linked by Yb²⁺ and Yb³⁺ ions. Each single chain is made of corner-sharing GaSb₄ tetrahedra. Two such chains are bridged by Sb₂ groups to form double chains of 1/∞ [Ga₂Sb₆^10–]. The compound satisfies the classical Zintl–Klemm concept and is a narrow band gap semiconductor with an energy gap of around 0.36 eV calculated from the electrical resistivity data corroborating with the experimental absorption studies in the IR region (0.3 eV). Magnetic measurements suggest Yb atoms in Yb₅Ga₂Sb₆ exist in the mixed valent state. Temperature dependent magnetic susceptibility data follows the Curie–Weiss behavior above 100 K and no magnetic ordering was observed down to 2 K. Experiments are accompanied by all electron full-potential linear augmented plane wave (FP-LAPW) calculations based on density functional theory to calculate the electronic structure and density of states. The calculated band structure shows a weak overlap of valence band and conduction band resulting in a pseudo gap in the density of states revealing semimetallic character

Tuning the Antibacterial Activity of Amphiphilic Neamine Derivatives and Comparison to Paromamine Homologues

Aminoglycosides are antibiotic drugs that act through binding to rRNA. In the search for antimicrobial amphiphilic aminoglycosides targeting bacterial membranes, we report here on the discovery of three dialkyl derivatives of the small aminoglycoside neamine active against susceptible and resistant Gram-positive and Gram-negative bacteria. One of these derivatives (R = 2-naphthylpropyl), which has good activity against MRSA and VRSA, showed a low toxicity in eukaryotic cells at 10 μM. The synthesis of amphiphilic paromamine and neamine homologous derivatives pointed out the role of the 6'-amine function of the neamine core in the antibacterial effects. The optimal number of lipophilic substituents to be attached to the neamine core and the corresponding required lipophilicity determined here should permit a more selective targeting of bacterial membranes relative to eukaryotic membranes. This work revealed the existence of windows of lipophilicity necessary for obtaining strong antibacterial effects that should be of interest in the field of antibacterial amphiphilic aminoglycosides