Charge density waves in mixed anionic rare-earth pnictide chalcogenides

• Applicant: Dr. Christopher Benndorf
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 448091910

The project aims at the investigation of mixed anionic rare-earth pnictide chalcogenides RE/Pn/Ch (RE = rare-earth metal; Pn = P, As, Sb, Bi; Ch = S, Se, Te) with special focus on the possibility of charge density wave (CDW) formation. The substances of interest are characterized by low dimensional, quasi 2D crystal structures, a typical requirement for the formation of CDW. For example, REBiTe and RESbS (RE = La-S) adopt the layered PbFCl-type structure or its distortion variants. The substances are distinguished by spatial separation of the pnictide and chalcogenide anions and no attractive bonding between these species is observable. The structures can be described as an alternating stacking of [RECh+] and [Pn-] layers with predominant ionic interaction between them. Within the [Pn-] units homo atomic bonding is present suggesting the formation of polyanions responsible for CDW states. Previous investigations indicate that CDW formation is essentially affected by the rare-earth metal’s electronic contribution, measurements of the electrical conductivity in the temperature range of 3 K to 300 K showed semiconducting behavior for CeBiTe and, in contrast, a metal-to-insulator transition – typical for CDW conductors – for NdBiTe at approximately 150 K. Besides the synthesis of novel rare-earth pnictide chalcogenides with CDW states we aim to optimize the properties of known substances by substitution experiments on all components accompanied by the manipulation of the Fermi level to enforce the formation of CDW. The detection of possible CDW states will be done by temperature dependent diffraction experiments on single crystals and bulk powders by using X-rays and synchrotron radiation as well as electrons in a transmission electron microscope (TEM). Simultaneous measurement of the compounds temperature dependent electrical conductivity, heat capacity and, if necessary, the magnetic susceptibility will give evidence of CDW formation. Structure determination using microcrystalline samples will be carried out using electron crystallography techniques in a TEM. The scientific result obtained in the project will form the basis for further investigations of CDW conductors and will increase the understanding of the structural origin of CDW with special focus on aspects of their crystal as well as solid-state chemistry.

• DFG Programme: Research Grants