Al flux synthesis of the oxidation-resistant quaternary phase REFe ₄Al₉Si₆ (RE = Tb, Er)

Two rare earth iron aluminum silicides, REFe₄Al ₉Si₆ (RE = Tb, Er), were synthesized in liquid Al at temperatures below 850°C. They crystallize in the tetragonal space group P4₂/nmc (no. 137) with cell dimensions of a = 8.718(1) Å and c = 15.171(3) Å for the Tb analogue. The structure, which is highly intricate and represents a rare structural arrangement, is based on that of NdRh₄Al_15.4. It can be understood in terms of highly corrugated layers of merged Al₆ rings stacking to form a three-dimensional framework. The Fe and Si atoms are situated in various sites in the framework. The RE atoms have a very high coordination number (20) and sit in remaining pockets formed by the bonding arrangements in the structure. Magnetic measurements show that the rare earth ions are in a 3+ state, whereas Mössbauer measurements show that the Fe atoms do not exhibit a magnetic moment and are more reduced than in elemental Fe. The possible insights gained from these results into the metallurgical processing of advanced aluminum matrix alloys are discussed. Thermal gravimetric analysis experiments in air show that REFe₄Al₉Si₆ is resistant to oxidation up to 900°C, which is attributable to an alumina/silica surface scale.