Speaker
Apply to be<br> considered for a student <br> award (Yes / No)?
yes
Main supervisor (name and email)<br>and his / her institution
Prof. G. R Hearne, grhearne@uj.ac.za
University of Johannesburg
Level for award<br> (Hons, MSc, <br> PhD)?
PhD
Would you like to <br> submit a short paper <br> for the Conference <br> Proceedings (Yes / No)?
yes
Abstract content <br> (Max 300 words)<br><a href="http://events.saip.org.za/getFile.py/access?resId=0&materialId=0&confId=34" target="_blank">Formatting &<br>Special chars</a>
Charge order (CO) refers to the spatial localization of charge carriers on lattice sites with a certain periodicity, resulting in a mixed-valence superstructure within the crystal lattice [1, 2]. Recent work has found evidence of a superstructure in Fe2OBO3, with Fe2+ and Fe3+ valence states close to integer values [3]. This makes Fe2OBO3 the archetypal ionic CO compound.
In this study the effect of pressure on the magneto-electronic and structural properties of Fe2OBO3 have been investigated by 57Fe Mössbauer spectroscopy (MS) and synchrotron x-ray diffraction (XRD) at room temperature [4, 5]. CO is drastically altered at about 11 GPa with a concomitant electron hopping relaxation, Fe2+<-> Fe3+, as evidenced by MS. Above 16 GPa, the CO has completely ‘melted’ and electron hopping between distinct Fe sites is dominant. Temperature dependent resistivity measurements up to 22 GPa demonstrate that the electronic structure remains gapped up to this pressure [4].
Analysis of the in-situ XRD data indicates that the ambient pressure P21/c monoclinic crystal structure gradually transforms into the high pressure (HP) Pmcn orthorhombic phase, starting at 6 GPa to completion at 22 GPa[4]. Moreover, the unit cell is found to be more compressible along the a-axis (10% over the pressure range up to 30 GPa) compared to the other directions in the unit cell with 3% compressibility over the same pressure range.
The change in volume for the P21/c -> Pmcn phase transition is limited to 1% and the bulk moduli of the two structures are quite similar. The CO phase transition at HP was found to be reversible from decompression measurements.
We present our rationalization of the magnetic-electronic ground state of the HP phase where CO is disrupted: MS results, the anisotropic compression and separate resistivity pressure measurements suggest the existence of an ‘exotic’ dimer Mott insulator phase.
[1] D. Reznik et al., Nature 440, 1170 (2006).
[2] Y.-D. Chuang et al., Science 292, 1509 (2001).
[3] M. Angst et al., PRL 99, 086403 (2007).
[4] G. Diguet et al., PRB 89, 035132(2014).
[5] G. R. Hearne et al. , PRB 86, 195134 (2012).
