Mixed-anion compounds – a novel material design concept

Each crystalline material found in the earth is often composed of several different positively charged ions (cations) but only one negatively charge ion (anion) type. The latter usually constitutes the base for their names: oxygen for oxides, sulfur for sulfides, and chlorine for chlorides. The reason for only finding one anion type in each material is caused by the different densities of the compound groups, e.g. sulfides are denser than oxides and are found deeper in the earth?s crust. Hence, Nature cannot easily make the chemistry based on two different anions, because they seldom meet in the Earth. However, in the lab, it is possible to do this bi-anion chemistry, as such is the focus of this project: To explore the inorganic chemistry of oxochlorides, oxofluorides, oxosulfides, as similar. The chemical synthesis to make bi-anion compounds is therefore often a challenge, and the ordering on anions can give the crystals dimensionality. Moreover, the direct surrounding of cations can be completely new and unexplored. This project is looking for novel compound containing two anions to understand the exotic properties that are caused by the specific mixture of anions.

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This project has two main goals; first, to develop a synthetic platform for a new class of multi-anion compounds with design of local structure as well as dimensionality of the backbone of the crystal structure; and second: to utilize neutrons (in combination with X-rays and electrons) to describe the crystal structure of the new compounds, at the local and average level, including temperature induced (disorder) transitions. In this way new science is combined with capacity building on neutron based methods, thereby strengthening the competence as well as activity level at UiO with respect to future use of ESS in Lund, 2024 onwards. This project will use large scale neutron facilities, at ISIS, SINQ, J-park and SNS/ORNL. Bi-anionic compounds are surprisingly rare, however, this could partly be due to the major differences in anion properties calling for specific synthesis approaches. We will use computational modeling (DFT simulations) to screen possible compounds, with basis in structure analogy to (single anion) compounds reported in data bases, and predict stability and atomic arrangement. On this basis, new compounds will be synthesized, by various approaches; soft chemical, ceramic routes, chemical transport reactions, reactive ball milling, high- pressure approaches and more. Their physical properties will then be studied. The combination of elements in the target compounds will be selected as to possibly provide basis for new ferroelectric, cooperative magnetic and multiferroic materials. Neutron scattering will provide good anion contrast (in most cases; partly in combination with synchrotron X-rays, electron PEDT, or MAS-NMR), excellent 3d-element contrast, and excellent basis for studies of disorder and dynamics by means of QENS and INS, supplement by optical and nuclear spectroscopy, whereas local order/disorder will be described on basis of neutron/X-ray PDF. One PhD and one researcher will obtain state of the art training in several neutron methods.