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Fe2OBO3 and LuFe2O4 are Fe-based 3d compounds known to be mixed-valence (Fe2+ and Fe3+) insulators at ambient conditions. These are relatively new charge ordering (CO) compounds that evidence strong magneto-electric coupling, besides offering the best potential for establishing the CO mechanism. Fe2OBO3 has monoclinic and orthorhombic crystal structures at ambient conditions and high pressure (HP) respectively. The compound orders ferrimagnetically at TM ~ 155K and has a CO temperature TCO ~ 320K. Whereas, LuFe2O4 has TM and TCO as 240K and 330K, respectively. At HP, these are anticipated to show new ground states (i.e., CO collapse, valence fluctuations or new CO states). For instance, in recent work on Fe2OBO3, a CO instability occurs at P ~16 GPa . In LuFe2O4, a pressure-induced structural transition (rhombohedral to orthorhombic) occurs in the range 5 – 10 GPa with indications of a new CO state occurring in the fully transformed sample at P > 8 GPa . Our interest is to explore in further detail the magneto-electronic ground-states of the HP phases of these two topical CO compounds, e.g., to check whether an insulator-metal transition ensues. This would provide crucial complementary information to our Fe Mössbauer-magnetic and XRD-structural probes of the new HP stabilized electronic phases.
The pressure response of electrical transport properties of polycrystalline powdered Fe2OBO3 and LuFe2O4 samples have been investigated by way of resistivity measurements at variable cryogenic temperatures from ambient pressure up to ~20 GPa in a diamond anvil cell. The DC four-probe resistivity was determined using the Van der Pauw method. At low pressure (LP) both samples display semiconducting behaviour, anticipated in the CO state which is prevalent below ambient temperatures. We are able to monitor the band-gap evolution of the LP (CO stabilized) phase. We will present our results on how the systems evolve towards their new electronic HP phases, as well as provide information on the nature of the carrier transport.
 G.R. Hearne et al., PRB 86, 195134 (2012).
 J. Rouquette et al., PRL 105, 237203 (2010).
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Prof. G.R. Hearne, firstname.lastname@example.org, University of Johannesburg