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8.2: Structures related to NaCl and NiAs

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    34793
  • Calcite Structure

    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. These compounds include:

    • FeS2 (pyrite, "fools gold"): S22- (disulfide) and Fe2+
    • CaC2 (a salt-like carbide): Ca2+ and linear C22- anions
    • CaCO3 (calcite, limestone, marble): Ca2+ and triangular CO32-.

    CaCO3-calcite-structure.png

    Figure 8.2.1: Calcite structure

    The calcite (CaCO3) crystal structure is shown at the right. Triangular CO32- ions fill octahedral holes between the Ca2+ ions (white spheres) in a distorted NaCl lattice. As in NaCl, each ion is coordinated by six of the other kind and the Ca ions occupy the corners and faces of the rhombohedral unit cell. From this image we can see why the CaCO3structure has a lower symmetry than that of NaCl. The fourfold rotation symmetry of the NaCl unit cell is lost when the spherical Cl- ions are replaced by triangular CO32- ions. Because of this symmetry lowering, transparent crystals calcite are birefringent, meaning that their refractive indices are different along the two principal crystal directions. This gives rise to the phenomenon of double refraction, illustrated below.

    200px-Calcite.jpg

    Figure 8.2.2: Calcite crystals are birefringent.

    NiAs structure

    The NaCl structure can be described a face-centered cubic lattice with all of the octahedral holes filled. What if we start with a hexagonal-close packed lattice rather than a face-centered cubic lattice?

    220px-Strukturformel_Nickelarsenid.png

    Figure 8.2.3: Nickel arsenide crystal structure. The Ni6As trigonal prisms are shaded gray. One octahedron of six As atoms surrounding a Ni atom is shown in the center of the figure.

    This is the structure adopted by NiAs and many other transition metal sulfides, phosphides, and arsenides. The cations are shown in gray while the anions are light blue in the figure at the right. The cations are in octahedral coordination, so each cation is coordinated to six anions. The anions are also coordinated to six cations, but they occupy trigonal prismatic sites. In terms of layer stacking, the NiAs structure is AcBcAcBc..., where the A and B sites (the hcp lattice) are occupied by the As atoms, and the c sites, which are eclipsed along the layer stacking axis, are occupied by Ni. Unlike the NaCl structure, where the anion and cation sites are interchangeable, NiAs has unique anion and cation sites. The layer stacking sequence for NiAs is shown below:

    ------------ A

    - - -c- - - -

    ------------ B

    - - -c- - - -

    ------------ A

    - - -c- - - -

    ------------ B

    - - -c- - - -

    The NiAs structure cannot be adopted by ionic compounds because of the eclipsing cations, because the cation-cation repulsions would be internally destabilizing for an ionic compound. This structure is mainly adopted by covalent and polar covalent MX compounds, typically with "soft" X anions (S, Se, P, As,....) and low-valent transition metal cations. For example, some compounds with the NiAs structure are: MS, MSe, MTe (M=Ti, V, Fe, Co, Ni). Often these are nonstoichiometric or complex stoichiometries with ordered vacancies (Cr7S8, Fe7S8).

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