engleski

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

Cr-doped mullites were prepared from single-phase precursors containing up to 9.60 wt.% Cr_2O_3 using a sol-gel technique followed by thermal treatment. PIXE (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 (delta c/c=0.089, delta a/a=0.061, delta b/b=0.045 % per mole Cr_2O_3). A second phase, namely theta-(Al, Cr)_2O_3 was revealed by XRD in the sample containing 9.60 wt.% Cr_2O_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_2O_3 content less than ~5wt.%) 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 (theta-(Al, Cr)_2O_3) 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.

Ceramics; Sol-gel growth; X-ray diffration; EPR spectroscopy; Crystal structure

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