X-Ray Diffraction and Near Edge Studies of Iron Oxides and Alumina at High Temperatures Using Aerodynamic Levitation and Laser Heating

A new high temperature chamber using aerodynamic levitation and laser heating has been used to study spheres of oxide materials at temperatures up to 2100°C. Near edge studies of Fe₃O₄ (MP : 1597°C) and FeO (MP 1377°C) have been performed by fluorescent XANES and in situ X-ray Diffraction (XRD). Both iron oxides reveal a shift in the Fe K-absorption edge on melting to lower energies by between 2 and 3 eV. This is accompanied by an increase in the threshold resonance. Experiments with neutral and oxidising levitating gases show the same behaviour and suggest a physical origin. A possible explanation is an increase in the Fe-O bond length with a consequent reduction in physical density on melting. This would naturally result in a narrower energy gap which would lead to the observed shift in the absorption threshold to lower energies in passing from the solid to the liquid state. This would also account for the rise in the threshold resonance on melting. By performing high temperature XRD, the thermal expansion coefficients α of Al₂O₃ (MP 2054°C) have been obtained crystallographically by measuring the increase of the unit cell parameters. XRD curves have been recorded at several temperatures from 800°C up to the melting point. The measured coefficients are in agreement with literature data. On the technical side, some problems relating to the levitation state and temperature measurements are discussed indicating the need for improvement of the chamber.