Structural studies of Cr-doped mullite derived from single-phase precursors

Abstract

Abstract Cr-doped mullites were prepared from single-phase precursors containing up to 9.60 wt% Cr 2 O 3 using a sol–gel technique followed by thermal treatment. Particle induced X-ray emission spectroscopy and X-ray powder diffraction were used to characterize the samples. Mullites were orthorhombic, space group Pbam . Cr doping caused the increase of unit-cell parameters. Strongest expansion was noticed along c -axis followed by a and b (Δ c / c =0.089, Δ a / a =0.061, Δ b / b =0.045% per mole Cr 2 O 3 ). A second phase, namely θ-(Al,Cr) 2 O 3 , was revealed by XRD in the sample containing 9.60 wt% Cr 2 O 3 . The structure of mullites was refined by the Rietveld method, location of Cr 3+ was performed by the EPR spectroscopy. At low chromium doping level (Cr 2 O 3 content less than ∼5 wt%) Cr 3+ ions were substituted for Al 3+ in the AlO 6 octahedra of the mullite structure (M1 site). For higher doping level, Cr 3+ ions were additionally substituted for Al 3+ in the AlO 6 octahedra of the second phase [θ-(Al,Cr) 2 O 3 at 1400 °C, or α-(Al,Cr) 2 O 3 at 1600 °C] which segregated in the system. Substitution of Cr 3+ for Al 3+ on M1 site in the mullite structure resulted in increase of average distances in (M1)O 6 octahedron and decrease of average distances in T*O 4 tetrahedron, while average distances in TO 4 tetrahedron stayed almost constant.