Raman Spectroscopic Study of Solid Solution Spinel Oxides
Brian D. Hosterman
Abstract
Solid solution spinel oxides of composition MgxNi1?xCr2O4, NiFexCr2?xO4, and FexCr3?xO4 were synthesized and characterized using x-ray diffraction and Raman spectroscopy. Frequencies of the Raman-active modes are tracked as the metal cations within the spinel lattice are exchanged. This gives information about the dependence of the lattice vibrations on the tetrahedral and octahedral cations. The highest-frequency Raman-active mode, A1g, is unaffected by substitution of the divalent tetrahedral cation, whereas the lower frequency vibrations are more strongly affected by substitution of the tetrahedral cation. The change in wavenumber of many phonons is nonlinear upon cation exchange. All detected modes of MgxNi1?xCr2O4 and FexCr3?xO4 exhibit one-mode behavior. Additional modes are detected in the NiFexCr2?xO4 series due to cation inversion of the spinel lattice. Results from the FexCr3?xO4 spinels are then applied to identifying the corrosion layers of three stainless steel samples exposed to lead-bismuth eutectic in a high temperature, oxygen-controlled environment. The Raman spectrum of the outer corrosion layer in all steels is identified as Fe3O4. The wavenumber of the A1g mode for the inner corrosion layer indicates an iron chromium spinel oxide. Micro-Raman spectroscopy proves capable of determining structural and compositional differences between complex corrosion layers of stainless steels.