Electron density study of garnets:: Y3X2Al3O12; X = Al and X = (Al, Cr)

The electron density distribution in two garnets Y3Al2Al3O12 [YAG] and Y3X2Al3O12: X = (Al0.71Cr0.29) [YCG] have been studied with single-crystal X-ray diffraction methods through scattering factor refinement procedures. YAG has broad electron density residuals of 0.3 e/Angstrom(3) height extending between the octahedral Al site and the coordinated oxygen atoms, YCG shows residuals around the Y and oxygen sites indicating significant anharmonicity, along with electron density deformation of the X-site characteristic of octahedral Cr3+ sites. Partial substitution of Al by Cr in the X site does not increase the unshared O-O distance or the far Y-O distance in the adjacent Y site. [Cubic Ia (3) over bar d, Z = 8. For YAG, a = 12.0095(4) Angstrom; D-x = 4.553 g/cm(3); F 000 = 2224; final R after anharmonic refinement: 0.0085 for 281 equivalent-averaged data. For YCG, a = 12.0405(5) Angstrom; D-x = 4.630 g/cm(3); F 000 = 2276; final R after anharmonic refinement: 0.0107 for 274 equivalent-averaged data.] (C) 1997 Academic Press.