Insights into the design of thermoelectric Mg_(3)Sb_(2) and its analogs by combining theory and experiment

Over the past two decades,we have witnessed a strong interest in developing Mg3Sb2 and related CaAl2Si2-type materials for lowand intermediate-temperature thermoelectric applications.In this review,we discuss how computations coupled with experiments provide insights for understanding chemical bonding,electronic transport,point defects,thermal transport,and transport anisotropy in these materials.Based on the underlying insights,we examine design strategies to guide the further optimization and development of thermoelectric Mg3Sb2-based materials and their analogs.We begin with a general introduction of the Zintl concept for understanding bonding and properties and then reveal the breakdown of this concept in AMg2X2 with a nearly isotropic three-dimensional chemical bonding network.For electronic transport,we start from a simple yet powerful atomic orbital scheme of tuning orbital degeneracy for optimizing p-type electrical properties,then discuss the complex Fermi surface aided by high valley degeneracy,carrier pocket anisotropy,and light conductivity effective mass responsible for the exceptional n-type transport properties,and finally address the defect-controlled carrier density in relation to the electronegativity and bonding character.Regarding thermal transport,we discuss the insight into the origin of the intrinsically low lattice thermal conductivity in Mg3Sb2.Furthermore,the anisotropies in electronic and thermal transport properties are discussed in relation to crystal orbitals and chemical bonding.Finally,some specific challenges and perspectives on how to make further developments are presented.

Second Generation Nitrogen Doped Titania Nanoparticles:A Comprehensive Electronic and Microstructural Picture

A state-of-the-art overview of N-dopant characterizations in nano-TiO_(2)second-generation photocatalysts is provided.The related literature is very rich and sometimes offers contrasting interpretations.Here we critically discuss up-to-date literature results and our own findings,as retrieved by several experimental(BET,HR-TEM,XPS,DRS,HR-XRPD,EXAFS,electrochemical tools)and theoretical(periodic DFT)techniques.Our intent is to pull together outcomes from very different and complementary sources to make an as much as possible coherent picture of the morphological,electrochemical and electronic properties of N-TiO_(2)materials.It is commonly accepted that critical issues to be considered in the design of high-performing N-TiO_(2)photocatalysts are synthetic strategy,defect concentration and chemical nature of the lattice point defects.We focus on the latter two issues,with emphasis on sol-gel prepared materials,according to the specific area of expertise of our group.The problem of the chemical nature of guest N species into the lattice is crucial,as substitutional(N_(s))or interstitial(N_(i))nitrogen place their valence states just above the valence band or deeper into the band gap.Overall,we show how synergism among experimental and theoretical techniques is decisive to disentangle structural,electronic and morphological effects in complex N-doped TiO_(2)matrices.