On the bases of the study of comparative crystal chemistry of silicates it has been concluded that the octahedra and square pyramids of Ti-0 and Zr-0 play functional role of tetrahedra of Si-0 in the construction of c...On the bases of the study of comparative crystal chemistry of silicates it has been concluded that the octahedra and square pyramids of Ti-0 and Zr-0 play functional role of tetrahedra of Si-0 in the construction of crystal structures. Therefore, those silicates may be named titano-and zircono-silicates. Because of the functional similarity of coordination polyhedra, the structures of cristobalite and feldspar have been compared with those of perovskite and garnet, respectively. As a new concept, the functional replacement of tetrahedra by octahedra and/or pyramids is defined by the authors of this paper for favorable comparison of relative crystal structures.展开更多
Accurate crystal structure determination of complex phosphate Mg-fillowite ? Na2Ca(Mn4Mg2Fe)7[PO4]6 has been finished. Mg-fillowite occurs in muscovite-pegmatite in the Altay area, Xinjiang, China. It is of the trigon...Accurate crystal structure determination of complex phosphate Mg-fillowite ? Na2Ca(Mn4Mg2Fe)7[PO4]6 has been finished. Mg-fillowite occurs in muscovite-pegmatite in the Altay area, Xinjiang, China. It is of the trigonal lattice, with space group R-3, unit cell a=1.5143(3) nm, c=4.3191(2) nm, V=8.5736 nm3, Z = 18. The R-factor of the determination accuracy is R (I > 2σ ( I )) = 0.0776. The cationic polyhedra consist of two kinds of structural units: compound col- umn and screw column. In three-dimensional space, they are linked with each other through corner sharing or edge sharing to build the entire structure. There are 45 atoms in an asymmetric unit and 720 atoms in a unit cell. In terms of types and connection manners of the coordination polyhedra, the degree of complicity is rarely seen in all mineral crystal structures. Study of such a structure is significant for the classification of phosphate and the building rule of coordination polyhedra in a complex crystal structure.展开更多
By using the Lagrange's intermediate value theorem,it is derived mathematically that the structur-al distortion of a coordination polyhedron may lead to an increase in bond-valence sum of the cen-tral atom of ion ...By using the Lagrange's intermediate value theorem,it is derived mathematically that the structur-al distortion of a coordination polyhedron may lead to an increase in bond-valence sum of the cen-tral atom of ion .The applicabilities of the bond-valence model are discussed in the following two cases:the modeling of crystal structure ,and the indication of distortion degree of a coordination polyhedron.Also it is shown that a distorted polyhedron should be in favor of a longer average bond length or a smaller coordination number.展开更多
基金This project was supported by the National Natural Science Foundation of China (Grant No. 49872019)
文摘On the bases of the study of comparative crystal chemistry of silicates it has been concluded that the octahedra and square pyramids of Ti-0 and Zr-0 play functional role of tetrahedra of Si-0 in the construction of crystal structures. Therefore, those silicates may be named titano-and zircono-silicates. Because of the functional similarity of coordination polyhedra, the structures of cristobalite and feldspar have been compared with those of perovskite and garnet, respectively. As a new concept, the functional replacement of tetrahedra by octahedra and/or pyramids is defined by the authors of this paper for favorable comparison of relative crystal structures.
基金supported by the National Natu-ral Science Foundation of China(Gant Nos.4024201 3 and 40472025).
文摘Accurate crystal structure determination of complex phosphate Mg-fillowite ? Na2Ca(Mn4Mg2Fe)7[PO4]6 has been finished. Mg-fillowite occurs in muscovite-pegmatite in the Altay area, Xinjiang, China. It is of the trigonal lattice, with space group R-3, unit cell a=1.5143(3) nm, c=4.3191(2) nm, V=8.5736 nm3, Z = 18. The R-factor of the determination accuracy is R (I > 2σ ( I )) = 0.0776. The cationic polyhedra consist of two kinds of structural units: compound col- umn and screw column. In three-dimensional space, they are linked with each other through corner sharing or edge sharing to build the entire structure. There are 45 atoms in an asymmetric unit and 720 atoms in a unit cell. In terms of types and connection manners of the coordination polyhedra, the degree of complicity is rarely seen in all mineral crystal structures. Study of such a structure is significant for the classification of phosphate and the building rule of coordination polyhedra in a complex crystal structure.
文摘By using the Lagrange's intermediate value theorem,it is derived mathematically that the structur-al distortion of a coordination polyhedron may lead to an increase in bond-valence sum of the cen-tral atom of ion .The applicabilities of the bond-valence model are discussed in the following two cases:the modeling of crystal structure ,and the indication of distortion degree of a coordination polyhedron.Also it is shown that a distorted polyhedron should be in favor of a longer average bond length or a smaller coordination number.
