Nakatsuka A, Yoshiasa A, Yamanaka T
Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.
Acta Crystallogr B. 1999 Jun 1;55(Pt 3):266-272. doi: 10.1107/s0108768198012567.
Five single crystals of Y(3)Al(5-x)Ga(x)O(12) (0 </= 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 Ga(3+), which is larger than Al(3+), 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 Ga(3+). 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 Y(3)Al(5-x)Ga(x)O(12) 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, Ia3;d, Z = 8, Mo Kalpha, lambda = 0.71069 Å; at x = 0.0 (triyttrium pentaaluminium dodecaoxide): a(0) = 12.0062 (5) Å, V = 1730.7 (2) Å(3), D(x) = 4.56 Mg m(-3), M(r) = 593.613, µ = 21.21 mm(-1), F(000) = 2224, R = 0.029 for 294 reflections; at x = 1.0 (triyttrium tetraaluminium gallium dodecaoxide): a(0) = 12.0432 (7) Å, V = 1746.7 (3) Å(3), D(x) = 4.84 Mg m(-3), M(r) = 636.351, µ = 24.09 mm(-1), F(000) = 2368, R = 0.022 for 124 reflections; at x = 2.0 (triyttrium trialuminium digallium dodecaoxide): a(0) = 12.0926 (9) Å, V = 1768.3 (4) Å(3), D(x) = 5.10 Mg m(-3), M(r) = 679.089, µ = 26.85 mm(-1), F(000) = 2512, R = 0.018 for 144 reflections; at x = 3.0 (triyttrium dialuminium trigallium dodecaoxide): a(0) = 12.1552 (6) Å, V = 1795.9 (3) Å(3), D(x) = 5.34 Mg m(-3), M(r) = 721.827, µ = 29.43 mm(-1), F(000) = 2656, R = 0.018 for 184 reflections; at x = 4.0 (triyttrium aluminium tetragallium dodecaoxide): a(0) = 12.2123 (8) Å, V = 1821.3 (4) Å(3), D(x) = 5.58 Mg m(-3), M(r) = 764.565, µ = 31.97 mm(-1), F(000) = 2800, R = 0.014 for 159 reflections.
采用助熔剂法合成了5个Y(3)Al(5-x)Ga(x)O(12)（0≤x≤5）石榴石固溶体的单晶，其组成x分别为0.0、1.0、2.0、3.0和4.0。石榴石固溶体晶格常数的成分依赖性显示出与维加德定律有很大偏差。利用单晶X射线衍射对这些石榴石的阳离子分布进行研究表明，比Al(3+)大的Ga(3+)优先占据四面体（四配位）位置而非八面体（六配位）位置。基于结构精修得到的结果，对多面体畸变进行了几何分析。结果表明，随着Ga(3+)取代量的增加，多面体共享边处的阳离子-阳离子排斥力减小。此外，根据键强估算了阳离子-氧键中共价键的比例；结果表明，Ga-O键的共价性大于Al-O键。在Y(3)Al(5-x)Ga(x)O(12)石榴石固溶体中观察到的特殊阳离子分布很可能是由Ga-O键的强共价性引起的，同时也是为了降低阳离子-阳离子排斥力而诱导产生的。晶体数据：立方晶系，Ia3;d，Z = 8，Mo Kα，λ = 0.71069 Å；x = 0.0（十二铝酸三钇）：a(0) = 12.0062 (5) Å，V = 1730.7 (2) Å(3)，D(x) = 4.56 Mg m(-3)，M(r) = 593.613，μ = 21.21 mm(-1)，F(000) = 2224，294个反射的R = 0.029；x = 1.0（十二铝镓酸三钇）：a(0) = 12.0432 (7) Å，V = 1746.7 (3) Å(3)，D(x) = 4.84 Mg m(-3)，M(r) = 636.351，μ = 24.09 mm(-1)，F(000) = 2368，124个反射的R = 0.022；x = 2.0（二镓三铝酸三钇）：a(0) = 12.0926 (9) Å，V = 1768.3 (4) Å(3)，D(x) = 5.10 Mg m(-3)，M(r) = 679.089，μ = 26.85 mm(-1)，F(000) = 2512，144个反射的R = 0.018；x = 3.0（三镓二铝酸三钇）：a(0) = 12.1552 (6) Å，V = 1795.9 (3) Å(3)，D(x) = 5.34 Mg m(-3)，M(r) = 721.827，μ = 29.43 mm(-1)，F(000) = 2656，184个反射的R = 0.018；x = 4.0（四镓铝酸三钇）：a(0) = 12.2123 (8) Å，V = 1821.3 (4) Å(3)，D(x) = 5.58 Mg m(-3)，M(r) = 764.565，μ = 31.97 mm(-1)，F(000) = 2800，159个反射的R = 0.014。
