Acta Crystallographica Section B

Structural Science

Volume 55, Part 3 (June 1999)

research papers

Acta Cryst. (1999). B55, 266-272 [ doi:10.1107/S0108768198012567 ]

Cation distribution and crystal chemistry of Y₃Al_5-xGa_xO₁₂ (0 x 5) garnet solid solutions

A. Nakatsuka, A. Yoshiasa and T. Yamanaka

Abstract: Five single crystals of Y₃Al_5-xGa_xO₁₂ (0 [less-than or equal to] x 5) garnet solid solutions with the compositions x = 0.0, 1.0, 2.0, 3.0 and 4.0 were synthesized using a flux method. The compositional dependence of the lattice constants of the garnet solid solutions shows a large deviation from Vegard's law. Investigation of the cation distributions of these garnets using single-crystal X-ray diffraction shows that Ga³⁺, which is larger than Al³⁺, preferentially occupies the tetrahedral (four-coordinate) site rather than the octahedral (six-coordinate) site. On the basis of the results obtained from structure refinements, geometric analyses of the polyhedral distortions were carried out. The results imply that the cation-cation repulsive force across the polyhedral shared edges decreases with increasing substitution of Ga³⁺. Moreover, the proportion of covalent bonding in the cation-oxygen bonds was estimated from the bond strength; the results indicate that the covalency of the Ga-O bond is greater than that of the Al-O bond. The peculiar cation distributions observed in the Y₃Al_5-xGa_xO₁₂ garnet solid solutions are most probably caused by the strong covalency of the Ga-O bond and also simultaneously induced by the need to decrease the cation-cation repulsive force. Crystal data: cubic, Ia3d, Z = 8, Mo K [alpha] , [lambda] = 0.71069 Å; at x = 0.0 (triyttrium pentaaluminium dodecaoxide): a₀ = 12.0062 (5) Å, V = 1730.7 (2) Å³, D_x = 4.56 Mg m^-3, M_r = 593.613, [mu] = 21.21 mm^-1, F(000) = 2224, R = 0.029 for 294 reflections; at x = 1.0 (triyttrium tetraaluminium gallium dodecaoxide): a₀ = 12.0432 (7) Å, V = 1746.7 (3) Å³, D_x = 4.84 Mg m^-3, M_r = 636.351, [mu] = 24.09 mm^-1, F(000) = 2368, R = 0.022 for 124 reflections; at x = 2.0 (triyttrium trialuminium digallium dodecaoxide): a₀ = 12.0926 (9) Å, V = 1768.3 (4) Å³, D_x = 5.10 Mg m^-3, M_r = 679.089, [mu] = 26.85 mm^-1, F(000) = 2512, R = 0.018 for 144 reflections; at x = 3.0 (triyttrium dialuminium trigallium dodecaoxide): a₀ = 12.1552 (6) Å, V = 1795.9 (3) Å³, D_x = 5.34 Mg m^-3, M_r = 721.827, [mu] = 29.43 mm^-1, F(000) = 2656, R = 0.018 for 184 reflections; at x = 4.0 (triyttrium aluminium tetragallium dodecaoxide): a₀ = 12.2123 (8) Å, V = 1821.3 (4) Å³, D_x = 5.58 Mg m^-3, M_r = 764.565, [mu] = 31.97 mm^-1, F(000) = 2800, R = 0.014 for 159 reflections.