Stabilization of γ-Fe 2 O 3 nanoparticles to heat by doping Nb and its property studied by Mossbauer spectroscopy

Nb 5+ -doped gamma - Fe 2 O 3 was synthesized by sol-gel method followed by calcination at various temperatures from 200 to 700 degrees C, analyzed using Powder X-ray Diffraction (PXRD), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray Spectroscopy (EDS) and Mo <spacing diaeresis>ssbauer Spectroscopy. PXRD results show the formation of gamma - Fe 2 O 3 at 300 degrees C and it begins to transform to alpha-Fe 2 O 3 at 350 degrees C for pure and 2 at.% Nb-doped samples. As the Nb doping increases (>= 3.8 at.%), the transformation was suppressed up to 600 degrees C. TEM EDS confirms the incorporation of Nb to gamma - Fe 2 O 3 lattice in 9.1 at.% Nb sample calcined at 500 degrees C. At elevated temperature of 700 degrees C, gamma - Fe 2 O 3 went through full transformation to alpha-Fe 2 O 3 by expelling large amount of Nb to the surface forming FeNbO 4 . TEM EDS confirms the compositional change during this process with reduced Nb content in alpha-Fe 2 O 3 and Nb-rich nanoparticle on the surface of alpha-Fe 2 O 3 . Superparamagnetic properties were observed for Mo <spacing diaeresis>ssbauer spectra when the Nb doping increased, which is caused by decreased particle size. The slight increase in internal magnetic field also indicated the transformation from gamma - Fe 2 O 3 to alpha-Fe 2 O 3 . High Nb doping with 20 and 40 at.% shows the formation of FeNbO 4 at lower temperature where limited or almost no Nb substitution in alpha-Fe 2 O 3 was detected by EDS.