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"The Magic of Vacancies: Solid Solutions of Chalcogenido Metalates"
Dr. Günther Thiele
(Freie Universität Berlin)
The increasing demand of new technologies from microelectronics to superconductors, batteries, and spintronics for every day-life application encourages the introduction of new materials with improved properties and performance. Alkali metal sulfido metalates are well-known materials with a wide range of applications. Here, a novel class of these compounds is introduced with the stoichiometry of A2[M3Ch4] where A is an alkali metal (Na or K), M is a 3d transition metal (Mn … Zn), and Ch is either S or Se. The 2-3-4 compounds represent defect-variant structures of K2[Fe4Se4] with two-dimensionally extended anionic substructure of [MIICh4]6–. The compound class can be tailored by means of elemental combinations, where all components can be replaced by, or mixed with, a multitude of similar elements. This variety of elemental composition – a solid solution behavior – is used to tune the properties and the performance. The compounds display high dielectric constants (1120 at 1 kHz) as high-k materials as well as extraordinary ionic conductivities (24.37 mS∙cm–1 for K2[Fe3S4] at 295 K) which enables potential usage as solid-state electrolytes. The advantages include intrinsically increased charge densities and lifetime cycles, whilst reducing safety hazards and construction costs in ubiquitous mobile lithium-ion batteries, as well as next-generation sodium-/potassium-ion batteries. The center-metal vacancies in the structures induce additional charge transferring channels which result in increased conductivities. Moreover, the obtained compounds indicate large spontaneous magnetic exchange bias fields (35 mT for K2[Fe3S4] at 20 K), for potential spintronic applications. This phenomenon is a consequence of the coexistence of spin glass and antiferromagnetic orderings due to the center-metal vacancies in the lattice. The findings introduce a novel candidate for technological applications and could demonstrate the critical role
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