Extending the application of metavalent bonding and its tailoring via disorder

Cheng, Yudong; Wuttig, Matthias (Thesis advisor); Mazzarello, Riccardo (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

Chalcogenides are well known for their interesting properties which enable a wide range of applications ranging from phase change materials (PCMs) to thermoelectrics and topological insulators. The remarkable applications have been attributed to an unconventional property portfolio including the Born effective charge (Z*), the optical dielectric constant (ε∞) and the Grüneisen parameter for transverse optical mode (γTO), as well as the effective coordination number (ECoN). For materials like GeTe, this property portfolio has been attributed to a bonding mechanism coined metavalent bonding (MVB) which differs significantly from conventional chemical bonds such as ionic, metallic and covalent bonds. Therefore, given the intimate link between chemical bonding mechanisms and properties, understanding this novel metavalent bonding will not only broaden our knowledge but also facilitate materials design. For this purpose, understanding this novel bonding mechanism and extending this concept to more materials will be of vital importance. This work will focus on two related topics. Firstly, inspired by the prevailing of MVB in some IV-VI solids, the existence of this novel bonding mechanism in a new group of materials (V2VI3 solids) has been systematically studied. As is known, a number of V2VI3 compounds (Bi2Te3, Bi2Se3, Sb2Te3 and β-As2Te3) show remarkable properties and applications. To see if these properties can be related to MVB, different techniques including atom probe tomography (APT) and density functional theory calculation have been employed to reveal the chemical bonding mechanism in V2VI3 compounds by comparing with their iso-electronic compounds (Bi2S3, Sb2Se3, and Sb2S3) and iso-structural compounds (GaSe and β-In2Se3). Laser-assisted APT reveals a highly unconventional bond breaking behavior (high probabilities of multiple events) for the four V2VI3 compounds showing remarkable applications. Detailed analysis of the multiple events has demonstrated that this novel bond breaking process in the four V2VI3 compounds is incompatible with present explanations but an intrinsic property of MVB solids. Interestingly, it has also been proven that the four V2VI3 compounds are characterized by a pronounced electronic coupling across the van der Waals-like layer. The combination of this unique bond breaking behavior and five property-based indicators demonstrate that Bi2Te3, Bi2Se3, Sb2Te3 and β-As2Te3 all employ metavalent bonding. Furthermore, all the MVB materials are localized in a unique region of an electronic map. After extending the application of MVB into some V2VI3 solids, the pronounced property contrast between MVB materials and their corresponding amorphous states inspire us to study the relationship between disorder and MVB. Therefore, two chemical bonding indicators, i.e. the Born effective charge (Z*) and the dielectric function (ε(ω)) of two MVB compounds (GeTe, GeSb2Te4), and their disorder-dependences have been discussed. Interestingly, the Z* and optical properties of crystalline GeTe and GeSb2Te4 are several times higher than the corresponding amorphous phase. Furthermore, GeSb2Te4 exhibits a clear disorder dependence in the crystalline state. This disorder-dependence holds exclusively for materials utilizing MVB because of the characteristic medium-range electronic interaction and p-orbital alignment. This orbital alignment is highly sensitive to disorder, especially the structural disorder, which consequently leads to the weakening of the two MVB indicators and hence the strength of MVB.