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Study of new layered transition metal sulphides synthesised under high pressure

Final Activity Report Summary - NELATRAMES (Study of new layered transition metal sulphides synthesised under high pressure)

In this project, we studied the following three aspects that were related to new transition metal sulphides:

1. we searched for new perovskite sulphides with perovskite structure using high pressure synthesis
2. we performed synthesis and study of the physical properties of new layered oxysulphides
3. we studied a pressure-induced phase transition of delafossite-derived sulphides.

As to the first aspect, our studies gave no evidence of the formation of sulphides with perovskite structure up to 10 GPa; however our density functional theory (DFT) ab initio calculations suggested that the perovskite phase was stable at larger pressures, of approximately 14 GPa, and further experiments were required in this pressure range.

As to the second project component, we synthesised for the first time single crystal of the quasi two-dimensional compound Sr6V9S22O2 and we carried out a first study of the two-dimensional electrical transport and magnetic properties. This study put into evidence the large anisotropy of the electrical resistivity along with a remarkably large magnetoresistance, of a magnitude of 6 % at 5 K, 5 T. These properties were promising in view of the development of novel electronic devices, especially when considering the large flexibility of two-dimensional transition metal sulphides, as compared to their oxide counterpart. A paper reporting the above results was under preparation by the time of the project completion. A submission to ‘Physical Review B’ was programmed after having refined the crystal structure using single crystal x-ray diffraction (XRD) single data. This task was done in collaboration with P. Bordet, an expert in the field and member of the ‘Centre National de la Recherche Scientifique’ (CNRS) in Grenoble.

As to point three, our XRD study under high pressure unveiled an unusual pressure-induced structural phase transition in AuCrS2, which involved anomalous c axis expansion, probably due to strong covalent Au-S linear bond. This finding suggested the existence of an electronic instability associated with the linear S-Au-S bonds, which might correspond to new structural and physical properties of the family of delafossite-related compounds under high pressure. In order to obtain conclusive results as to the structural changes occurring at the transition, we submitted a proposal for obtaining new beamtime at ID27 of the European synchrotron radiation facility (ESRF). We estimated that the new data should enable to complete this ongoing work and envisaged the publication of a paper after the experiment at the ESRF that was scheduled in the October 2008 to January 2009 period.