Difference between revisions of "Isostructural bending"

Latest revision as of 20:39, 13 July 2017

By substituting compatible elements in a crystal structure (a specific structure type to be more concrete) one introduces stresses due to the different diameters of the new atoms. In thin rods and plates that are a only a few atomic layers across these stresses can partially relax by bending (strains). This can be useful for making cylindrical geometries like axle housings and helical geometries like guiding screws.

Examples

Carbon atoms in diamond or lonsdaleite (== hexagonal diamond) can be replaced with silicon atoms at any ratio. (In case of a 1:1 ratio one has the gemstone compound called moissanite.)

When there's a pseudo phase diagram between two different compounds with the same structure-type then in most cases it should be possible to do some checkerboard patterning. As a concrete example one can replace titanium with silicon in the rutile structure type (C4). This is effectively moving around in the rutile <=> stishovite pseudo phase diagram. In case of the rutile structure there are a lot of compatible elements thus one can extend the diagram to a triangle, a tetrahedron, a hyper-tetrahedron and so on.

Where to take care

Care must be taken though. Going all the way from a specific structure type like e.g. SiO₂ to the same structure type of CO₂ one ends up with a compound that is likely an explosive in bulk since CO₂ very much likes to be a molecular gas with strong and low energy C=O double bonds.

Finding compatible elements

To find compatible elements one can first try go down the group in the periodic table.
Here's an example preserving the rutile structure. (We are starting with silicon and jump over the unstable compound of rutile-structure-CO₂). The following sequence is: SiO₂ (stishovite), GeO₂ (argutite), SnO₂ (cerrusite), PbO₂ (plattnerite).

This especially holds for non metals. In case of the transition metals:

• 1) going sidewards is often possible too.
  Example preserving rutile structure: TiO₂ (rutile), VO₂ (no natural mineral present?), CrO₂ (no natural mineral present?), MnO₂ (pyrolysite)
  No (IV) oxides for iron and following elements (and scandium preceding titanium).
• 2) the elements of the higher periods get exceedingly rare (main exception is zirconium Zr -- ZrO₂ baddeleyite)
  and sometimes pretty toxic (that correlation may not be a coincidence)

Related

• Bending induced by surface passivation.
• Structural type