Binary gem-like compound

binary compounds that do not react or dissolve in water

• SiC silicon carbide mossanite - transparent when pure
• B₄C boron carbide
• SiB₄; SiB₆ ? silicon boride
• AlB₁₂ aluminium dodecaboride - hard
• β-C₃N₄ beta carbon nitride (possibly a health hazard if cyanide release can occur - to investigate)
• Si₃N₄ silicon nitride
• cubic BN cubic boron nitride - very similar to diamond (also cubic and hexagonal "allotropes")
• BP boron phosphide - transparent and chemically very stable
• CaB₆ calcium hexaboride - non water soluble earth alkali compound which is uncommon - irritating
• SiO₂ quartz (it's actually slightly water soluble) & allotropes like dense and hard stishovite
• Tectosilicates: [1] [2]
• Al₂O₃ aluminum oxide aka corundum or sapphire
• Fe₃C iron carbide aka cementite
• iron silicides & iron borides ? - unknown properties
• FeS₂ FeS iron sulfides - pyrite and marcasite ...
• Fe₂O₃ hämatite
• Fe₃O₄ magnetite
• FeO wüstite
• Cu₃P copper(I) phosphide (copper is not too abundant)
• Cu_XS_Y copper sulfides CuS covellite, Cu2S chalcocite, many more ...
• B₆O boron suboxide (hardest known oxide)

Theres is a big stable group of B-C-N compounds, a few aluminum (Al2O3,AlB) and few silicon (SiC,SiO2,N4Si3) compounds.

There seem to be no binary iron minerals that have hardness above mohs 6.5

Titanium forms chemically and mechanically rather stable compounds with many nonmetals.

• TiC titanium carbide
• TiSi₂ titanium disilicide (unknown mechanical properties ?)
• TiB₂ titanium diboride
• TiN titanium nitride
• TiP titanium(III) phoshide (metallic conductivity)
• titanium sulphides TiS (goldbrown), TiS₂ (bronze/golden yellow), Ti₂S₃ (black,graphitic), TiS₃, Ti₃S₄, Ti₄S₅, Ti₄S₈, Ti₈S₉
• TiO₂ Ti₂O₃ titanium oxide polymorphs: rutile anatase brookite

binary compounds which very slowly dissolve in water and are thought to be rather nontoxic

Solubility is good for an envirounmental viewpoint (decay time of abandoned scrap material) but bad for engineering materials. Especially in nanosystems the slightes bit of dissolvation completely destroys the outermost layer of nanomachinery. This makes sealing of products and high system reduncancy even more necessary than it is when more stable materials are used.

• Al₄C₃ aluminum carbide - hydrolyses to aluminum hydroxide and methane
• AlN aluminum nitride - oxidizes in air @ room temperature (layer <= 10nm) - hydrolyzes slowly in water to aluminum oxide and ammonia
• S₂N₂ disulfur dinitride shock sensitive - decomposes explosively above 30°
• S₄N₄ tetrasulfur tetranitride explosive decomposition to nitrogen and sulfur 4N₂ + S₈
• (SN)_X polythiazyl - conductive inorganic polyner chain
• P the allotropes of elementar phosphorus
• S the allotropes of elementar sulfur

simplest most water stable compounds of abundant alkaline eart metals

• MgO periclase also magnesium oxide aka magnesia very low but nonzero water solubility
• MgO₂ magnesium peroxide irritant, environmentally persistent
• CaS calcium sulfite decomposes with water to calcium hydroxide and hydrogen sulfide gas - oldhamite end member
• MgB₂ magnesium diboride (high temperature superconductor)
• CaB₂ calcium diboride ??

most water stable solid fluorides from abundant metals

• TiF₃ titanium fluoride
• MgF₂ magnesium fluoride aka sellaide
• CaF₂ calcium fluoride aka fluorite

dangerous compounds to stay away from

• solid nitrogen (except you want to make highly potent explosives)
• AlP extremely toxic
• Al₂S₃ toxic - H₂S generation
• sulphur phosphorus compounds - highly toxic
• Fe₃P highly toxic
• BF₃ BCl₃ PCl₃ all highly toxic (but gasseous anyway)

reactive but useful compounds

Many other highly reactive compounds may be useful when encapsulated and serving a non structural like electronic or other function.

• Mg₃N₂ magnesium nitride

Passivation layer minerals of today's industrial metals

We do have daily skin contact with these minerals without even realizing it.
Often these minerals are naturally present as ores from which the metals are extracted.

• aluminum ...
• titanium ...
• zinc ...
• tin ...
• copper ...
• nickle ...
• chromium ...
• vanadium,niobium ...

Passivation of passivation layer minerals

Here an interesting problem occurs. To prevent two atomically precisely flat blocks from fusing seamlessly together on contact their surfaces must look differently than their insides. Specifically it is often a good idea to cover the whole surface with lone pairs of electrons. But further oxidation of an already oxidized material will probably not work or be rather unstable [to investigate]. What should be doable almost always is hydrogen passivation. (Such passivation may cause higher friction due to high lateral spacing between the small hydrogen atoms sitting atop larger atoms and the low lateral stiffness of the single bonded hydrogen atoms) It may be necessary to find a special solution for each indivitual material - nitrogen phosphorus and sulfur may often be useful for plugging surfaces closed.

Pseudo phase diagrams

For orientation what kind of low level metamaterials can be built with binary compounds one can create something like "pseudo phase diagrams". "pseudo" since the structure of the material at a specific point in the diagram is not defined by the thermodynamic history of the material but by the way it was mechanosynthesized. There are lots of special positions in the diagram that arise due to the specific choosen crystal structure and checkerbord pattern.

An example of such an pseudo phase diagram would be a square with CO2 (upper left) SiO2 (upper right) beta-C3N4 (lower left) Si3N4 (lower right) as their "end members". (solid CO2 is likely to be explosive but with a sufficient number of C atoms substituted with Si atoms it will be stable - it may be possible to draft a frobidden zone around the solid CO2 corner). In this specific diargam from top to bottom from oxides to nitrides the crystal structure must change significantly (due to the changing valence number) making a less continuous transition.

[todo: add existing images of such diagrams]