Diamondoid

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Diamondoid materials refers to a class of materials that are isostructural to diamond and moissanite (zincblende structure).
Gem-gum-tec may put a special focus on diamondoid materials as there are several that are are typically …

  • very hard with their bonds behaving covalently like in a construction kit (see: Limits of construction kit analogy)
  • easily passivatable to make bearing surfaces that do not fuse together on contact (as all of hydrocarbons in organic chemistry)

If you want to be especially specific about the material class being diamondoid
then you may use the term diamond like compound or "diamond-like XY".

Alternate material classes of interest

There are other material classes of interest too though.

  • the unusually rich world of high performance titanium based gemstones
  • ultrahard ambient condition metastable high pressure modifications
    like e.g. stishovite and mechadensite (an ultrahard titanium stabilized quartz polymorph)
  • some oxoaxid salts especially silicates and phosphates as these are hardest and least water soluble (parts can be sealed tough)
  • various transition metal monoxides today in super simple rock salt structure (~ Mohs 5)
  • intentionally watersoluble stuff for degradability like periclase and many other
  • Many more. For a candidate list see: Base materials with high potential

To refer to the more general class of gemstones use may use the pattern "gemstone based XY" or "gem based XY" for short.
E.g: "gemstone based APM" (instead of gem-gum-tec which also emphasizes importance of mechanical metamaterials).

Examples of diamondoid materials

Prime examples for diamondoid materials include but are not limited to:

There are also hexagonal variants:

Pseudo diamondoid (linear oxygen bridges):

  • Goup II-VI compound hexagonal zinc oxide i.e. zincite - TO CHECK
  • Brommelite BeO (beryllium is rare)

Crystallographic faces

(100) the cube faces

Cubes are perhaps most natural to think of for the human mind,
but cubes donot match perfectly with the tetrahedral bond coordination of carbon and silicon atoms.

To quickly identify (110) surfaces surface:
There are two natural way to put a tetrahedron into a cube.
These two ways corresponds to the two orientations of carbon atoms in diamond.
On (110) surfaces …

  • two coplanar sp3 bonds face symmetrically slantedly up and
  • two coplanar sp3 bonds face slantedly down with the plane being 90° rotated around the surfaces normal vector
  • looking down this direction gives a very dense square symmetry projection of bonds
  • spot trouble passivating this surface with hydrogen

Surface passivation:
Fully hydroge passivated (100) surfaces would be hydrogen congested.
Putting two hydrogen atoms where just one atom should be in an undisturbed lattice.
There are several ways to Natural ways to resolve this are

  • passivation with a bridging chalcogen element (oxygen or sulfur) roughly matching the atom size of carbon or silicon respectively.
  • replacing that chalcogen with nothing i.e. with a direct bond. This moves carbons away from the unsidturbed lattice pints by a lot. It's a so callesd surface reconstructions.
  • approximating the (100) surface with a zig-zag surface of one of the other surfaces

Symmetry:

  • In principle the (100) direction features a 4 fold symmetry.
    Due to diamondoids having two orientations for the bond coordination the surface symmetry is reduced to two though.
  • There are merely six cube surfaces.

Construction relevant:

  • Normal surfaces to one (100) surface include four (100) and four (110) surfaces
  • One side bevel surfaces include four falter (110) surfaces and four steeper (111) surfaces (wiki-TODO: add angles)

(110) the rhombic dodecahedron faces

To quickly identify (110) surfaces surface:

  • check for two bonds lying in a plane coplanar to the surface and two bonds orthogonal to it
  • check for large channels when looking straight at the surface
  • check for twofold symmetry

These surfaces and their normal directions are kind of special.

  • The octet truss (simplest 3D space-fill with stiff deltahedrons) has the struts facing in (110)
  • Looking this way thorough the diamondoid crystal one sees channels.
    These are not even big enough fro helium to pass through though.

Symmetry:

  • The (110) surfaces have twofold symmetry.
  • There are twelve rhombic dodecahedron faces.

Construction relevant:

  • Normal surfaces to one (110) surface include two (100), two (110) and four (111) surfaces
  • One side bevel surfaces include four steeper (110) surfaces and two flatter (111) surfaces (wiki-TODO: add angles)

Surface passivation:
Surfaces form zigzag rows every second row being exposed higher and hydrogen passivated.
Carbon or silicon surfaces can be nitrogen and phosporus passivated respectively.
Boron and aluminum may work too at least sparingly as the electron deficiet complement.

(111) the octahedron faces

To quickly identify (110) surfaces surface:

  • This is trivial. Simply check if one of the four bond points straight up.
  • Check for threefold symmetry

Symmetry:

  • The (111) surfaces have threefold (sixfold if hexagonal).
  • There are eight octahedon faces.

Construction relevant:

  • Normal surfaces to one (111) surface include only six (110) surfaces (with alternalingly slanted grain)
  • One side bevel surfaces include three steeper (100) surfaces and three flatter (110) surfaces (wiki-TODO: add angles)
  • Cubic case: Other side bevel surfaces are the same but 60° turned around the vertical axis
  • Hexagonal case: Other side bevel surfaces are the same but 0° turned around the vertical axis

Cubic vs hexagonal:
Sphere stacking layering is coplanar to the (111) directions

  • there is cubic diamond with ABCABC layering
  • there is hexagonal diamond (lonsdaleite) with ABAB layering leaving only one special (111) direction

Nice trick: For structural nanosystem design one can use cubic diamond as a link for areas with hexagonal diamond featuring a different hexagonal axis.

Surface passivation:
This one is trivial. Just plug the open bonds with hydrogen. They are plenty spaced and all face up. This gives a higher surface roughness too though. So (111) contacting (111) might be specially prone to snapback dissipation. This might be intentionally desired in some cases though.

These surfaces have the lowest bond density and should be easiest to reliably cleave. (wiki-TODO: to check)

Wiegner-Seiz cells

Cubic diamond is fcc and has a rhombic dodecahedron with (110) surfaces as space filling Wiegner-Seiz cell.
Hexahonal diamond is hcp and has all of the vertices of the rombic dodecatetron on one (111) side
twisted by 60° giving the much less known … as space filling Wiegner Seiz cell.

One can in principle use blocks of diamond and lonsdaleite in Wigner-Seiz shell shape to build up block based structures. Or give larger scale structures like microcomponents this shape and align the majority of the material with diamonds main crystallographic axes.

Crossover from fcc to hcp and vie versa is trivial as rohombic (110) faces match up.
Crossover from fcc to bcc is more tricky (requires adapter blocks of natural choice) as …

  • it involves deciding for either upscaling or downscaling and …
  • centers of rhombic dodecahedron cells end op on the vertices of truncated octagedron cells (or vice versa)
  • (wiki-TODO: Eventually add a screenshot explaining natural the choice rhombic dodecahedron to truncated octahedron spacefill crossover)

Related

More a list of diamondoid compounds here:

External links

Printable part to give some orientation about the relative placement of crystallographic faces to each other
https://www.printables.com/model/430598-truncated-rhombicuboctahedron-bead

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