Barbosalite Fe3(PO4)2(OH)2 is a hydroxy-phosphate (potentially important in electrochemical and electro-catalytic applications) which has a monoclinic crystal structure at ambient conditions. Corner sharing (Fe3+)O4(OH)2 octahedra form chains, cross-linked laterally to each other by face-sharing (Fe3+)-(Fe2+)-(Fe3+)-… octahedral linear sequences [1,2]. In these sequences the (Fe2+)O4(OH)2 octahedron always shares two faces with two neighboring (Fe3+)O4(OH)2 octahedra, or Fe2+ is vacant from the site and such vacancies are randomly distributed in the sequence (disordered vacancies). This renders a mixed-valence configuration involving an Fe3+:Fe2+ ratio of 2:1 and magnetic ordering is onset at ~173 K. The stability and robustness of this charge order (CO) in these sequences is investigated by means of pressurization, in analogy to pressure studies on other CO compounds Fe2OBO3 and LuFe2O4 where interesting high pressure ground states evolve [3]. In barbosalite both the IR absorption and Raman spectra indicate appreciable changes occur in the OH stretch mode up to ~17 GPa. XRD data indicate P21 (monoclinic) ® (3 GPa) ® P212121 ® (8-10 GPa) ® P43212 (tetragonal) structural adjustments occur at the indicated transition pressures. We have complemented these studies with Fe Mössbauer spectroscopy (MS) as a direct probe of the Fe valences and magnetism under in-situ pressure conditions in the DAC up to 30 GPa. Distinct valence states are readily discerned at low pressures up to ~6 GPa by characteristic MS signatures of Fe2+ and Fe3+ in the spectral profiles. At 8-10 GPa lineshape distortions, especially related to the Fe2+ contribution, are evident and become more pronounced up to 18 GPa. Spectral simulations using a model involving back-and-forth electron hopping of the minority spin between Fe2+ and neighboring Fe3+ sites are in accord with the evolved distorted lineshapes. By ~22 GPa and beyond, lineshape simulations suggest that electron hopping rates are a few MHz. Therefore the MS data suggests that pressurization leads to charge fluctuations in the (Fe3+)-(Fe2+)-(Fe3+)-… sequences, manifest as progressive disruption of the existent site-centered CO at low pressure. Fe2+ minority spin delocalization over the extent of the (Fe3+)-(Fe2+)-(Fe3+)-… units occurs with increasing frequency upon compression beyond ~10 GPa. Additionally, electrical resistivity measurements to 30 GPa currently in progress may yield further insights. The MS pressure data has been analyzed using this electron-exchange model to monitor the pressure evolution of relevant hyperfine interaction parameters of the original Fe2+ and Fe3+ sites. This provides information on the compressional response of the local electronic and structural environment of these sites as well as (Fe2+)-to-(Fe3+) electron-exchange rates. This is then related to the interplay between pressure-induced site-centered CO disruption in the face-sharing octahedral sequences and changes in hydrogen bonding seen in the IR absorption and Raman responses.
REFERENCES
[1] G. J. Redhammer, G. Tippelt, G. Roth, et al.. Phys. Chem. Minerals 27, 419, (2000).
[2] D. Rouzies and J. M. M. Millet. Hypefine Interactions 77, 11, (1993).
[3] G. R. Hearne, E. Carleschi, W. N. Sibanda, et al.. Phys. Rev. B 93, 105101, (2016).