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

Abstract

Aluminum borate compounds studied here belong to the family of mullite-type materials with the characteristic chains of edge-sharing octahedra. Depending on the individual phases the octahedral chains are linked by different arrangements of AlO4- AlO5-, BO3- and eventually BO4-polyhedra. This work is focused on the conditions of phase formation in the system Al2O3-B2O3, on the thermal and compositional stability ranges of the related phases and on their individual crystal-chemical characterization. The formation of aluminum borate compounds synthesized from sol-gel derived precursors was investigated dependent on the initial Al/B ratio, with a special focus on the compositional range between the thermodynamically stable polymorphs Al4B2O9 and Al18B4O33. Upon increasing initial boron content a decrease of the formation temperature is observed, as revealed by thermal analyses. The formation of Al6-xBxO9 compounds was observed for the compositional range of 1.09 a A x a A 2 and the conditions of synthesis were optimized. The lattice parameters represent an orthorhombic metric with a a b, decreasing in all three directions with increasing boron content, most pronounced in c-direction. Based on Distance Least Squares (DLS) refinements, an improved model for Al6-xBxO9 is presented, simulating a local geometry avoiding long B-O distances. In this model two octahedral chains are linked by planar BO3 groups, yielding split positions for the oxygen atoms and strongly distorted octahedral chains. The crystal structure of Al4B2O9 was re-evaluated by electron diffraction experiments, resolving the question of oxygen disorder in the channels of the crystal structure. Inside crystallites the structural details vary. Domains are found with an ordered distribution of oxygen atoms without any significant signal for the second postulated channel oxygen atom O5, and other domains with a probable disordered configuration of the atoms O5 and O10. Diffuse scattering along the b-direction is assigned to a superstructure with a threefold b-axis. For a series of samples with Al4B2O9 structure a slightly increasing cell volume upon increasing initial boron content is observed. This is suggested to be caused by minor structural differences, which is supported by the results of Nuclear Magnetic Resonance (NMR) spectroscopy, revealing a small increase of the BO4/BO3 ratio upon increasing initial boron content. A new study of (Al1-xGax)4B2O9 compounds is presented is this thesis. For the first time the influence of foreign cations on the Al4B2O9 and Ga4B2O9 structure was investigated, representing a substitution limit of about 70 mol-% Ga3 and Al3 in the Al4B2O9 and Ga4B2O9 structure, respectively. It is demonstrated that the thermal stability of a given member is a function of Al/Ga ratio in the crystal structure: increasing substitution of gallium reduces the decomposition temperature of Al4B2O9 successively, whereas the incorporation of aluminum improves the thermal stability of Ga4B2O9. The compositional range for Al18B4O33/Al20B4O36 was investigated, based on a series of samples prepared along different synthesis routes and with various initial Al/B ratios. Combining the results of NMR spectroscopy and prompt gamma activation analysis (PGAA) a solid solution is assumed, expressed as Al20-xB4 xO36 with Al substituting B in the range of about 1 a 3%. This is supported by powder diffraction data refinements, observing vacancies on the Al2 site by combining the data of neutron and X-ray powder diffraction. Furthermore, the results of X-ray diffraction experiments indicate disorder effects for samples synthesized from sol-gel precursors with a high amount of aluminum. This is supported by 27Al NMR spectroscopy, representing a differing AlO4/AlO5 ratio for these samples, compared to samples synthesized with an excess of boron or prepared from solid-state reactions. The decomposition process of Al20-xB4 xO36 is observed to start at 1473 K, revealed by thermal analysis and X-ray diffraction experiments. Complete decomposition takes place during a long-term experiment at 1673 K, confirming an incongruent melting of the Al-rich aluminum borate phase, yielding Al2O3 and liquid.