Systematical, experimental investigations on LiMgZ (Z= P, As, Sb) wide band gap semiconductors

This work reports on the experimental investigation of the wide band gap compounds LiMgZ (Z = P, As, Sb), which are promising candidates for opto-electronics and anode materials for Lithium batteries. The compounds crystallize in the cubic (C1_b) MgAgAs structure (space group F-43m). The polycrystalline samples were synthesized by solid state reaction methods. X-ray and neutron diffraction measurements show a homogeneous, single-phased samples. The electronic properties were studied using the direct current (DC) method. Additionally UV-VIS diffuse reflectance spectra were recorded in order to investigate the band gap nature. The measurements show that all compounds exhibit semiconducting behavior with direct band gaps of 1.0 eV to 2.3 eV depending on the Z element. A decrease of the peak widths in the static 7Li nuclear magnetic resonance (NMR) spectra with increasing temperature was observed, which can directly be related to an increase of Li ion mobility.Comment: 8 page

Mg2Si nanoparticle synthesis for high pressure hydrogenation

The Mg-Si-H system is economically favorable as a hydrogen storage medium for renewable energy systems while moving toward sustainable energy production. Hydrogen desorption from MgH2 in the presence of Si is achievable, forming magnesium silicide (Mg2Si). However, absorbing hydrogen into Mg2Si remains problematic due to severe kinetic limitations. The objective of this study is to reduce these kinetic limitations by synthesizing Mg2Si nanoparticles to limit the migration distance for magnesium atoms from the Mg2Si matrix to produce MgH2 and Si, thus improving the reversibility of the Mg-Si-H system. Mg2Si nanoparticles were synthesized using a reduction reaction undertaken by solid-liquid mechanochemical ball milling. Particle size was controlled by adding a reaction buffer (lithium chloride) to the starting reagents to restrict particle growth during milling. The reaction buffer was removed from the nanoparticles using tetrahydrofuran and small-angle X-ray scattering revealed an average Mg2Si particle size of ~10 nm, the smallest Mg2Si nanoparticles synthesized to date. High-pressure hydrogen measurements were undertaken above thermodynamic equilibrium at a range of temperatures to attempt hydrogen absorption into the Mg2Si nanoparticles. X-ray diffraction results indicate that partial hydrogen absorption took place. Under these absorption conditions bulk Mg2Si cannot absorb hydrogen, demonstrating the kinetic benefit of nanoscopic Mg2Si

Topological Insulators

The recent discovery of a new class of materials, the so-called topological insulators [1–5]. has generated a great interest in the fields of condensed matter physics and materials science [1]. In principle, according to their band structure, compounds can be divided into metals and insulators. Recently a new class of the so-called topological states has emerged, the Quantum Spin Hall (QSH) state in two and three dimensions. The respective materials are called "topological insulators". The 3D topological insulators have a full insulating gap in the bulk, but a topological protected gapless surface or edge states on the boundary [6–8]. Additionally the 2D topological insulators (e.g. HgTe [9, 10], are metallic in the bulk, but can be designed as topological insulators in quantum well structures with a trivial semiconductors such as CdTe. A topological insulator can easily be identified by a few simple rules: the presents of a large spin orbit coupling, an odd number of band inversions between the conduction and the valence band by increasing the average nuclear charge, and a sign change of the symmetry of the molecular orbitals [11]. Similiar features are favorable for thermoelectric properties, thus topological insulators may be good thermoelectric materials and vice versa. Here we present a short introduction to topological insulators and give examples of compound classes where both topological insulators and good thermoelectric properties can be found