Influence of temperature and high pressure on the structural and physical properties in Fe-based Charge Order systems – COFeIn

The project may be divided in several parts :
- Synthesis and characterizations of iron borates and phosphates, in which iron exhibits a mixed valence; using data previously reported in literature and our own knowledge as solid state chemists; looking for exciting properties as multiferroicity
- Investigation of new systems, based on “Fe, P, B, O” to obtain new compounds and to study their structures and properties
- Deeper studies on selected samples to investigate structures and properties versus temperature or high pressure.
The compounds were prepared by solid state reaction or by hydrothermal method. A huge panel of techniques was used in our laboratories or in large scale facilities in order to obtain a complete description of the structures, microstructures and properties of materials.
The aim of this work is the establishment of reliable relationships between structures and (magnetic and electrical) properties. This study is interesting for the fundamental aspects as well for the optimization of properties for potential applications.

The work performed in this context leaded to the characterization of new behaviors in a few iron phosphates and borates. For examples, several unexpected mixed valence iron hydroxyphophates and a nickel phosphite were grown by hydrothermal method, leading to complete studies that are currently being draft. The Cu or Mn for Fe substitution in the Fe3BO5 oxyborate has shown a strong and interesting effect on the structures and properties. A large part of the PhD thesis paid by this funding was devoted to this topic and several papers are submitted or in preparation.

This project, for which we had never prepared any material previously, appeared very interesting and promising. The selection of the types of iron-based compounds using the charge ordering criterion was relevant since it allowed to obtain new phases and some of the expected properties, and also to open new perspectives, concerning the type of studied materials (hydroxyphosphates, phosphites) as well as the studied properties (electrochemical for instance). It is thus obvious that this study will be continued and several requests for support (PICS, ANR) have been made in this perspective.

The results were disseminated through publications in international revues (with peer review referring); oral presentations and posters in scientific meetings (international, national or local) ; PhD thesis book and the website (developed and administrated by the Roman partner).

This project deals with the preparation and the study of iron based oxides (in form of poly- and single-crystals) presenting charge ordering, combining the +2 and +3 iron oxidation states. Indeed, charge ordering in transition metal oxides is a fundamental parameter for obtaining original magnetic and/or electric properties. That was largely shown within the framework of the studies on colossal magneto-resistance in manganese perovskites and it again seems at the origin of the ferroelectricity in CaMn7O12 or LuFe2O4.

Our project breaks up into several sections:
- We chose to revisit the systems of “iron borates and phosphates” presenting iron mixed valences on the basis of literature and of our experience of solid state chemists in this field, to highlight unusual properties (in which multiferroicity).
- The second research orientation is more creative and relates to the exploration of new systems, always on a “Fe, P, B, O” basis, to obtain new compounds of which we will characterize the structures and the properties.
- The (structural and of the properties) evolution of materials will be followed versus temperature, magnetic field and also of high pressures. Not only this last factor allows obtaining new phases diagrams, but it can lead to the stabilization of new crystalline buildings, and thus to new properties. This effect was already illustrated in a convincing way in the case of charge order compounds, with the appearance of an irreversible structural transition in LuFe2O4. A broad panel of techniques of laboratory and large facilities will be implemented for the most detailed and complete possible description of the structural, micro structural and properties of our materials.
- The purpose of this crystallo-chemistry work of is to establish relationships between the structures and the (magnetic and electric) properties of our compounds. This has as much interest from the fundamental point of view as for the optimization of the properties for potential applications.