Polyhedral analysis is used to study the geometrical limits of the crystal structure on isomorphic substitution of rare earths.The rare earth sites in the structure may be distinguished into six groups on the basis of...Polyhedral analysis is used to study the geometrical limits of the crystal structure on isomorphic substitution of rare earths.The rare earth sites in the structure may be distinguished into six groups on the basis of mean ionic radii of the substituting cations in each rare earth site:Y,Y+Ca,REE,REE+Ca,Ce+Ca,Ce.The calcula- tion of volumes of 70 rare earth polyhedra indicates that the polyhedral volumes of the heavy rare earth are in a range of 13×10^(-3)～28×10^(-3) nm^3,the volumes of light rare earth polyhedra are in 20×10^(-3)～40×10^(-3) nm^3, and the polyhedral sizes suitable to isomorphic substitution of all REE are about 21×10^(-3)～29×10^(-3) nm^3.The size of REE polyhedra depends on mean ionic radii in REE sites,coordination number and linkage of polyhedra. In structures where exist two kinds of polyhedra,with the change of composition in REE site,both REE polyhedron and anion group polyhedron tend to be more distorted,leading to the change of structural symmetry. The contrary distortion tendencies of both polyhedra favours the stability of the structure.The phenomena may be interpreted by the potential energy distribution or the potential energy compensating mechanism of the crystal.展开更多
文摘Polyhedral analysis is used to study the geometrical limits of the crystal structure on isomorphic substitution of rare earths.The rare earth sites in the structure may be distinguished into six groups on the basis of mean ionic radii of the substituting cations in each rare earth site:Y,Y+Ca,REE,REE+Ca,Ce+Ca,Ce.The calcula- tion of volumes of 70 rare earth polyhedra indicates that the polyhedral volumes of the heavy rare earth are in a range of 13×10^(-3)～28×10^(-3) nm^3,the volumes of light rare earth polyhedra are in 20×10^(-3)～40×10^(-3) nm^3, and the polyhedral sizes suitable to isomorphic substitution of all REE are about 21×10^(-3)～29×10^(-3) nm^3.The size of REE polyhedra depends on mean ionic radii in REE sites,coordination number and linkage of polyhedra. In structures where exist two kinds of polyhedra,with the change of composition in REE site,both REE polyhedron and anion group polyhedron tend to be more distorted,leading to the change of structural symmetry. The contrary distortion tendencies of both polyhedra favours the stability of the structure.The phenomena may be interpreted by the potential energy distribution or the potential energy compensating mechanism of the crystal.
