Silicon nitride
Nierite (trigonal α-Si3N4) - proven naturally occuring mineral
Nierite is the gemstone form of Si3N4 silicon nitride (Mohs 9, trigonal).
It has been synthesized in a bulk nanocrystalline form (see external links) which
shows that a larger single crystal would be transparent too.
This material seems to be one of the very best Base materials with high potential
for gemstone based atomically precise manufacturing and technology.
Hexagonal h/β-Si3N4 "h/β-Nierite"
(wiki-TODO: Find & link to structure data.)
Cubic c/γ-Si3N4 or "c/γ-Nierite"
This has been synthesized as a transparent macroscopic (though nanopolycrystalline) piece as of 2017.
(wiki-TODO: Find & link to structure data.)
Questions:
- Higher symmetry cubic structure might have advantages for nanomachine designs?
- Is there a way to do a natural transition from cubic to hexagonal similar to as there is one from diamond to lonsdaleite?
Verbal description of the structure:
Basically a sparse network of Si4N4 cubes that share their silicon corners
thereby making all these corner silicon atoms unusually 6-coordinated.
The nitrogen edges of these cubes are connected via single normally 4-coordinated silicon atoms.
But this makes the nitrogen atoms unusually 4-coordinated without a clear lone pair direction.
Electron delocalization and high coordination might contribute to this materials high hardness.
Abundance and accessibility
The material combines elements that are highly abundant and highly accessible.
To the point of basically for free unless going to geoengineering level scales.
There is …
- Highly abundant silicon. (Most of Earths crust is SiO2 with a few patches of CaCO3 atop)
- Highly accessible nitrogen. (78% of our atmosphere - FAPP not depleatable - unless geoengineering level scale processes)
Hardness and toughness
Its high hardness compared to most silicon dioxide SiO2 polymorphs
(except a few like stishovite and seifertite)
might be to a small part due to nitrogen forming three bonds in contrast to oxygen forming just bonds
thus giving a higher bond density. That would make up only for ~+1/3 and the Mohs scale is nonlinear.
Suggesting indivitual bonds being a lot stronger.
Particulatly in the lower symmetry trigonal α case the slightly nontrivial bonding topology
(perhaps partly caused by nitrogens three covalent bonds)
makes for no good cleavage planes which might make this material
particularly tough especially compared to diamondoid like structures (zincblende & wurzite)
Here the nitrogen atoms have their three bonds in a common plane speaking for sp2 hybid orbitals:
https://www.chemtube3d.com/SS-Si3N4/
Misc
Naturally found only in meteorites.
Related
- Beta carbon nitride – carbon rather than silicon, but that might give flammability and toxicity problems
External links
Pictures:
- https://www.sciencedirect.com/science/article/abs/pii/0022024879901234
- https://www.nature.com/articles/srep44755/figures/1 – (a) A photograph of a bulk nanocrystalline form of cubic c-Si3N4 synthesized at 15.6 GPa and 1800 °C. The division of the ruler (this side) is 1 mm. The thickness of the sample is 0.464 mm.
- Silicon nitride Si3N4 bearing balls 1–20 mm … not gem quality but as a black ceramic
- graphic showing trigonal alpha, hexagonal beta, and cubic gamma phase structure of Si3N4:
https://www.researchgate.net/figure/CUBIC-SILICON-NITRIDE-g-Si3N4-reprinted-with-permission-of-Ref-46-Copyright-C-2010_fig21_301314001
Cubic gamma structure:
- 3D preview (rhombic cell) & download cif: https://legacy.materialsproject.org/materials/mp-2075/
- cubic cell: https://next-gen.materialsproject.org/materials/mp-2075/
(not Hausmannite Mn3O4 structure as AI summary claimed there, hausmannite structure is tetragonal, more than a small distortin from cubic) - gamma phase structure 2D orthogonal projection: https://www.mdpi.com/2073-4352/14/6/549
- gamma phase structure 2D orthogonal projection: https://www.jaici.or.jp/solutions/interview/db/icsd-case8/
- gamma phase structure 2D diagonal projection (doped): https://pubs.rsc.org/en/content/articlelanding/2014/ra/c4ra10519b/unauth
- gamma phase structure 2D diagonal projection:
https://www.researchgate.net/figure/Crystal-structure-of-g-M3N4-MSi-Ge-The-blue-balls-represent-the-nitrogen-atoms-the_fig4_327892420 - Strukturbericht H11 Spinel structure: https://www.atomic-scale-physics.de/lattice/struk/h1_1.html