The anions are also coordinated to six cations, but they occupy trigonal prismatic sites. The cations are in octahedral coordination, so each cation is coordinated to six anions. The cations are shown in gray while the anions are light blue in the figure at the right. This is the structure adopted by NiAs and many other transition metal sulfides, phosphides, and arsenides. One octahedron of six As atoms surrounding a Ni atom is shown in the center of the figure. The Ni 6As trigonal prisms are shaded gray. What if we start with a hexagonal-close packed lattice rather than a face-centered cubic lattice? The NaCl structure can be described a face-centered cubic lattice with all of the octahedral holes filled. This gives rise to the phenomenon of double refraction. Because of this symmetry lowering, transparent crystals calcite are birefringent, as illustrated below.Ĭalcite crystals are birefringent, meaning that their refractive indices are different along the two principal crystal directions. The fourfold rotation symmetry of the NaCl unit cell is lost when the spherical Cl - ions are replaced by triangular CO 3 2 - ions. From this image we can see why the CaCO 3 structure has a lower symmetry than that of NaCl. As in NaCl, each ion is coordinated by six of the other kind. Triangular CO 3 2 - ions fill octahedral holes between the Ca 2 + ions (black spheres) in a distorted NaCl lattice. The calcite (CaCO 3) crystal structure is shown above. The rhombohedral unit cell of the calcite crystal structure. CaCO 3 (calcite, limestone, marble): Ca 2 + and triangular CO 3 2. CaC 2 (a salt-like carbide): Ca 2 + and linear C 2 2 - anions.FeS 2 (pyrite, "fools gold"): S 2 2 - (disulfide) and Fe 2 +.There are a number of compounds that have structures similar to that of NaCl, but have a lower symmetry (usually imposed by the geometry of the anion) than NaCl itself.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |