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.

All nitrogen atoms in both α and β phase have all three bonds in the same plane for sp2 orbitals.

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

• Base materials with high potential
• Nitrogen & Silicon

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• 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

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• 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

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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

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• https://en.wikipedia.org/wiki/Silicon_nitride
• https://www.mineralienatlas.de/lexikon/index.php/MineralData?lang=en&language=english&mineral=Nierite

Matrials project

alpha-Si3N4 (trigonal)
https://legacy.materialsproject.org/materials/mp-2245#
https://next-gen.materialsproject.org/materials/mp-2245

beta-Si3N4 (hexagonal)
https://legacy.materialsproject.org/materials/mp-988/#
https://next-gen.materialsproject.org/materials/mp-988/

c/gamma-Si3N4 (cubic, spinel structure)
https://legacy.materialsproject.org/materials/mp-2075/#
https://next-gen.materialsproject.org/materials/mp-2075/ (not hausmannite structure!)