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?
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.
In the trigonal α case the slightly nontrivial bonding topology
(caused by nitrogens three covalent bonds being sp3 and all tilted to one side away from the remaining lone pair)
makes for no good cleavage planes which might make this material
particularly tough especially compared to diamondoid like structures (zincblende & wurzite)
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