Chemistry and physical properties of normal valence and hypervalent polar chalcogenides

Abstract : This thesis has its focus on the chemistry and physical properties of normal valence and hypervalent polar chalcogenides (PCs). The motivation for this study lies in the synthesis of new compounds with complex crystal structures. It aims at understanding the relationship between crystal structure, chemical bonding and physical properties through experimental and theoretical analyses. PCs are of special interest since they are at the interface between metals and nonmetals. The chemistry at this interface is governed by the interplay between covalent, metallic and ionic bonding, which makes it interesting and challenging to understand the relationship between crystal structure and chemical bonding. The main reason for aiming at structural complexity is to target new materials with low thermal conductivities – a key requirement for efficient thermoelectric materials. Thermoelectrics are capable of converting waste heat into electricity, which is of considerable economic and environmental interest. Previous discoveries of efficient, chalcogenide-based thermoelectrics such as PbTe, Bi2Te3, CsBi4Te6 and superionic Cu2-xSe motivated the exploratory search for new Cu- and Pn-chalcogenides (Pn = Sb, Bi) and to study related materials such as BaBiTe3 (cf. chapter V). One route towards complex crystal structures is to use a charge transfer from cations such as Sr or Ba to an anionic framework in order to create complex anionic, covalent networks (e.g. channels or layers) which can lead towards interesting physical properties. Stereoactive lone pairs can increase the structural complexity through distortions of the coordination polyhedra, which is one reason for studying systems containing Pn atoms such as Sb and Bi. Probing the physical properties and studying the crystal structure and chemical bonding of both, new and known polar chalcogenides with complex crystal structures resulted in interesting new discoveries, i.e. new compounds and crystal structures as well as unexpected physical properties. The thesis is separated in normal valence compounds, which can be entirely described by classical two-center two-electron (2c-2e) bonds (i.e. where the electrons are fully localized) and those, which contain hypervalent bonds and networks in which the electrons are partially delocalized. It contains four main parts: the study of 1) A0.5CuZrSe3 et ACuYSe3 (A = Sr, Ba) belonging to a family of compounds known as the “1113 family”, 2) Ba2FePnSe5 (Pn = Sb, Bi), 3) Ba4Cu8Se13 and 4) BaBiTe3-xSex (x = 0, 0.05, 1 and 3).
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Stefan Maier. Chemistry and physical properties of normal valence and hypervalent polar chalcogenides. Other. Normandie Université, 2017. English. ⟨NNT : 2017NORMC247⟩. ⟨tel-01717571⟩

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