ガーネット構造における近接陽イオン間斥力の重要性

Abstract

Crystal chemistry and structure of garnets are reviewed with special attention to the cation-cation repulsion across the shared edges of polyhedra. The cation-cation repulsion plays an important role in the stability of garnet structure, and this effect is the most prominent between tetrahedral and dodecahedral cations. The magnitude of the cation-cation repulsion is closely related to the degree of polyhedral distortion, based on the difference in length between the shared and the unshared edges in polyhedra, due to the structural geometric restriction. Especially, the cation-cation repulsion increases in garnet solid solutions owing to the decrease in the shielding effect when the structural geometric restriction brought by the substitution of the cations with different sizes forcibly stretches the shared edges compared with the unshared ones. This is a factor to destabilize the crystal structure. Both the tendency of cation distribution in Y3Fe5O12 (YIG)-Y3Ga5O12 (YGG) system and the symmetry in Mg3Al2Si3O12-MgSiO3 system change with the compositional variations in these solid solutions. These changes are the good examples of the destabilization of the crystal structures due to the effect of the cation-cation repulsion.