Raman-Spectroscopy, X-Ray-Diffraction, And Phase Relationship Determinations With A Versatile Heating Cell For Measurements Up To 3600-K (Or 2700-K In Air)
A simple, rapid, and inexpensive heating-wire technique is used for physical observations at high temperatures. The upper limit is 2000 K in air with platinum-iridium or platinum-rhodium wires and 2700 K with iridium; temperatures up to 3600 K can be achieved under an inert atmosphere with tungsten wires. Raman spectroscopy measurements made up to 1900 K by this technique suggest that the high-temperature harmonic vibrational behavior of corundum (alpha-Al2O3) results from the cancellation of anharmonic effects. Powder x-ray diffraction experiments with synchrotron radiation show that perovskite (CaTiO3) changes from orthorhombic symmetry to cubic between 1330 and 1530 K, with an intermediate tetragonal phase likely, consistent with lambda-type transitions recorded by recent calorimetric measurements. Finally, observations of CaAl2Si2O8 polymorphism has shown the existence of a new metastable phase.