Layer structure

from Wikipedia, the free encyclopedia
The mica mineral phlogopite with a typical layer structure.

A layer structure can be used when the interactions between the building blocks within a level (layer) are greater than the interactions between the levels. This is the case with some crystal structures . Polar , covalent bonds are usually present within the layers , which results in a two-dimensional link. There are relatively weak Van der Waals forces between the layers . This is shown in different atomic distances and leads to good cleavage parallel to the layers. Well-known examples of compounds with a layer structure are mica , graphite and montmorillonite .

Examples of compounds with a layer structure

Intercalation of metal atoms between layers of graphite.

graphite

Graphite consists of layers of carbon atoms and can be cited as the simplest example of a layer structure. The distance between the carbon atoms within a layer is 142 pm , the distance between the layers is significantly larger at 335 pm. The layers are held together by weak van der Waals forces. The layer structure explains the conductivity of graphite, which is about 5000 times greater parallel to the layer planes than perpendicular to the planes.

Transition metal halides

Three-dimensional cadmium iodide structure, consisting of stacked layer packages.

Cadmium iodide structure (CdI 2 )

Cadmium crystallizes in a sheet structure in the hexagonal crystal system and is name-giving for the CdI 2 - structure type . The structure is made up of stacked layer systems. Within these layers there are covalent bonds with ion formation components, with the cadmium atoms coordinated in an octahedral manner. The iodide ions form a hexagonal close packing of spheres, in which only every second octahedral gap layer is occupied by cadmium ions. The anion layers appear in the layer sequence ABAB.

Examples of compounds which crystallize in the cadmium iodide structure are titanium (II) chloride , titanium (II) iodide and magnesium bromide .

Cadmium chloride structure (CdCl 2 )

The cadmium chloride structure is also a layered structure. The cadmium ions occupy octahedral gaps every second layer and the chlorine atoms are densely packed cubically . While in the cadmium iodide structure the layers are exactly one above the other, in the cadmium chloride structure they are shifted from one another (layer sequence ABCABC).

Representatives of the cadmium chloride structure are, for example, magnesium chloride , nickel (II) bromide and zinc iodide .

Metal disulfides

Many metal disulfides crystallize in the CdI 2 structure and are made up of a sulfide layer, a metal layer and again a sulfide layer (S – M – S). The metal atoms occupy every second octahedral layer of the hexagonal closest packing of anions. This applies, for example, to the compounds titanium (IV) sulfide , zirconium (IV) sulfide and platinum (IV) sulfide .

Some metal disulfides also crystallize in the MoS 2 structure, where the metal atoms occupy trigonal-prismatic gaps between anion layers stacked in pairs (AA, BB or CC). Examples are niobium (IV) sulfide and tungsten (IV) sulfide .

Iodine

As can already be seen from the scale shape of the solid iodine , iodine forms a layer structure in the solid state. The iodine molecules are all in one plane within the layers. The distance between two iodine atoms and neighboring iodine molecules within a layer is 20 to 80 μm smaller than the distance between the layers. Within the layers, the iodine molecules come much closer (up to 349.6 pm) than twice the van der Waals radius (430 pm), but not as close as in normal covalent bonds. There are therefore considerable electronic interactions between the iodine molecules, which determine the semiconductor properties and the metallic luster of solid iodine. Solid bromine and chlorine have the same structure as solid iodine, but with a less drastically shortened distance between the halogen molecules .

  • Solid, crystalline iodine

  • Structure of a layer in solid iodine

  • Three-dimensional structure of solid iodine

Layered silicates

→ Main article: layered silicates

In the phyllosilicates each SiO 4 - tetrahedra linked in a plane above three corners with neighboring tetrahedrons. As a result, two-dimensional layers of the [Si 4 O 10 ] 4− anion are formed. If only van der Waals forces occur between the layers (e.g. with talc and kaolinite ), the result is soft minerals with layers that can easily be moved against each other. If the layers are held together by cations , this leads to greater hardness and, at the same time, good cleavage parallel to the layers. This is the case, for example, with the structure of mica.

Applications

Schematic structure of a lithium-ion cell (positive electrode: LiCoO 2 ; negative electrode: Li-graphite)

Solids with a layer structure such as graphite or molybdenum (IV) sulfide are often used as lubricants . The carbon or sulfide layers can shift in a leaf-like manner parallel to the layers.

Another main area of ​​application is the possibility of storing substances between the layers ( intercalation ). Such intercalation compounds have important practical applications in many areas, e.g. B. the lithium-ion battery , where lithium atoms are intercalated between the graphite layers. Potassium and polybromide-graphite intercalation compounds are also used as electrical conductors . In addition to inorganic substances, a number of organic compounds can also be embedded in substances with layered structures. An application example for this is the adsorption of ferrocene on individual molecular molybdenum (IV) sulfide layers. The adsorbate can then be drawn onto a glass substrate as an electrically conductive film.

Individual evidence

  1. ↑ Entry on layer structures. In: Römpp Online . Georg Thieme Verlag, accessed on November 15, 2018.
  2. ^ A b c Erwin Riedel, Christoph Janiak : Inorganische Chemie . 8th edition. De Gruyter, Berlin 2011, ISBN 978-3-11-022567-9 , pp. 138-139 .
  3. ↑ a b James E. Huheey, Ellen A. Keiter, Richard L. Keiter: Inorganic Chemistry: Principles of Structure and Reactivity . 3. Edition. De Gruyter, Berlin 2003, ISBN 3-11-017903-2 , p. 879-880 .
  4. ^ A b Michael Binnewies, Maik Finze, Manfred Jäckel, Peer Schmidt, Helge Willner: General and Inorganic Chemistry . 3. Edition. Springer Spectrum, Berlin 2016, p. 485-486 , doi : 10.1007 / 978-3-662-45067-3 .
  5. ↑ Michael Binnewies, Maik Finze, Manfred Jäckel, Peer Schmidt, Helge Willner: General and Inorganic Chemistry . 3. Edition. Springer Spectrum, Berlin 2016, p. 90-91 , doi : 10.1007 / 978-3-662-45067-3 .
  6. ↑ James E. Huheey, Ellen A. Keiter, Richard L. Keiter: Inorganic Chemistry: Principles of Structure and Reactivity . 3. Edition. De Gruyter, Berlin 2003, ISBN 3-11-017903-2 , p. 296-298 .
  7. ↑ James E. Huheey, Ellen A. Keiter, Richard L. Keiter: Inorganic Chemistry: Principles of Structure and Reactivity . 3. Edition. De Gruyter, Berlin 2003, ISBN 3-11-017903-2 , p. 980-981 .
  8. ^ AF Holleman, Egon Wiberg: Textbook of inorganic chemistry . 101st edition. De Gruyter, Berlin 1995, ISBN 3-11-012641-9 , p. 447-448 .
  9. ↑ Entry on silicates. In: Römpp Online . Georg Thieme Verlag, accessed on November 16, 2018.
  10. ↑ Erwin Riedel , Christoph Janiak: Inorganic Chemistry . 8th edition. De Gruyter, Berlin 2011, ISBN 978-3-11-022567-9 , pp. 540 .
  11. ↑ Marc-Denis Weitze, Christina Berger: Materials for mobility . In: Technology in focus . Springer, Berlin, Heidelberg 2013, ISBN 978-3-642-29540-9 , pp. 119-135 , doi : 10.1007 / 978-3-642-29541-6_5 .