Structural Aspects, Physical Properties and Reactivity of the Intermetallic Compounds AlMo3, Al8Mo3 and Al9-xFexMo3
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Abstract
This dissertation focusses on the synthesis and characterization of the intermetallic compounds AlMo3, Al8Mo3 and Al8FeMo3 as well as corresponding composite samples and the substitution series Al9-xFexMo3 (0.33 ≤ xFe < 1). The studied materials have been obtained by arc melting as well as spark plasma sintering (SPS). Structural aspects, physical properties and the reactivity of the intermetallic phases have been investigated.
The thermal and electrochemical oxidation behavior have been clarified via in-situ and ex situ powder X-ray diffraction (p-XRD), simultaneous difference thermal analysis / thermogravimetric analysis (DTA/TGA) and infrared spectroscopy (IR). Onset temperatures, intermediate and final compounds occurring in the oxidation process have been identified. Superficially structured materials have been obtained by moderate anodization and analyzed by scanning electron microscopy (SEM) as well as energy-dispersive X-ray analysis (EDX). Upon oxidation of the intermetallic phases, potentially catalytically active compounds MoO3, Al2(MoO4)3 and (Fe1 xAlx)2(MoO4)3, typically embedded in Al2O3 matrices, are formed.
Al8FeMo3 and the substitution series Al9-xFexMo3 (0.33 ≤ xFe < 1) crystallize in the tetragonal Al3Ti-type structure. In-situ and ex-situ p-XRD and differential scanning calorimetry (DSC) have shown statistically distributed orthorhombic modulations of the crystal structure as function of the sample’s genesis, the temperature and the chemical composition. From 57Fe-Mössbauer spectroscopy (MS), a flexible charge contribution of iron has been derived to be the essential feature in stabilizing the Al3Ti-type structure within a range of 0.67 ≤ xFe ≤ 1.
The aspects of anomalous electron-phonon coupling and outstanding magnetic properties have been studied for the series Al9-xFexMo3. Above a critical temperature of TC ≈ 93 K, Pauli paramagnetism occurs. Below TC, members with 0.5 ≤ xFe < 1 exhibit a rare form of metamagnetism, revealing giant negative magnetization in magnetic fields below 10 Oe. Ferromagnetism is observed in larger applied magnetic fields. Based on 57Fe-Mössbauer spectroscopic data, magnetic fluctuations are evident and assigned to screened magnetic moments of Fe. The magnetic properties presumably correlate to multibands of iron and molybdenum close to the Fermi-level.