Crystal structure of Cr-mullite

A Rietveld refinement of Cr-doped mullite, Cr(0.5)A(3.92)Si(1.58)O(9.79), o rthorhombic, space group Pbam [a = 7.56712(6) Angstrom, b = 7.70909(6) Angs trom, c = 2.90211(2) Angstrom, V = 169.30 Angstrom(3)] revealed Cr to resid e predominantly in the octahedrally coordinated M1 site. The mean M1-O dist ance of 1.935 A is that expected from Al3+ and Cr3+ molar fractions obtaine d from the chemical analysis and structure refinement. The small displaceme nt factors of the T* and Oc* sites indicate deficiencies in the electron de nsities which could be compensated by assuming additional Cr atoms. The pre dominant Cr incorporation into the M1 site causes an expansion of the octah edral bonds which is directly related to the observed lengthening of the c edge. The strong expansion of the long and elastic octahedral M1-Od bond in Cr-doped mullite, which would affect the a and b edges, is partly absorbed by a shortening of tetrahedral bonds.