Formation and crystal-chemical studies of metastable and stable mullite-type aluminum borates

• Applicants: Professor Dr. Reinhard X. Fischer; Professor Dr. Thorsten Michael Gesing
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2012 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 210363705

• Final Report Year: 2016

Final Report Abstract

A brief summary of the highlights: The phase formations are described for the metastable aluminum borates. Continuously decreasing lattice parameters upon increasing boron content indicate a solid-solution series with composition of Al6-xBxO9.  It could be shown by distance least squares (DLS) calculations that the incorporation of 3-fold coordinated boron determined by NMR spectroscopy induces a local distortion of the oxygen atoms in neighboring AlO6 octahedra of the Al6-xBxO9 phases.  The crystal structure of Al4B2O9 was reevaluated. It could be shown by electron diffraction that structural details vary among different crystals and even inside a crystallite. There are domains with an ordered distribution of atoms and other domains with a probably disordered configuration as indicated by diffuse scattering. This explains that the X-ray and neutron diffraction experiments yielded disordered oxygen positions representing an average structure over all unit cells and domains.  Compositional series of (Al1-xGax)4B2O9 with limited solubilities were synthesized and described. The symmetry relationships between the two end members were derived and the thermal expansion behavior determined.  NMR spectroscopy and PGA analyses indicate that the Al/B ratio in the Al18B4O33 phase varies within narrow limits.

Publications

• Synthesis and characterization of mullite-type (Al1-xGax)4B2O9. Zeitschrift für Kristallographie 229 (2014) 699-708
  K. Hoffmann, M.M. Murshed, R.X. Fischer, H. Schneider, Th.M. Gesing
  
• Formation, stability and crystal structure of mullite-type Al6-xBxO9. Journal of Solid State Chemistry, Volume 243, November 2016, Pages 124-135
  K. Hoffmann, T.J.N. Hooper, M.M. Murshed, O. Dolotko, Z. Révay, A. Senyshyn, H. Schneider, J. V. Hanna, Th. M. Gesing, R. X. Fischer
  
• Crystal chemical characterization of mullite-type aluminum borate compounds. Journal of Solid State Chemistry, Volume 247, March 2017, Pages 173-187
  K. Hoffmann, T.J.N. Hooper, H. Zhao, U. Kolb, M.M. Murshed, M. Fischer, H. Lührs, G. Nénert, P. Kudejová, A. Senyshyn, H. Schneider, J.V. Hanna, Th. M. Gesing, R. X. Fischer
  
• Elucidating structural order and disorder phenomena in mullite-type Al4B2O9 by automated electron diffraction tomography. J. Solid State Chem. 249 (2017) 114-123
  H. Zhao, K. Hoffmann, Y. Krysiak, B. Barton, L. Molina-Luna, R. B. Neder, H.-J. Kleebe, Th. M. Gesing, H. Schneider, R. X. Fischer, U. Kolb