Binary gem-like compound

Binary diamondoid compounds are diamondoid compounds that are constituted out of just two chemical elements.

Possible motivations for preferring them over single element compounds (allotropes)

• desired biodegradability
• making use of abundant reactive elements like e.g. calcium beside just carbon silicon and maybe boron
• better accessibility of these materials in earlier productive nanosystems
• access of properties that are not emulatable by bond topology and bond strain alone (that is by metamaterial structure) most prominently electronic properties

An exhaustive list of the binary compounds of interest is possible

There aren't that many elements in the periodic table that are available in vast abundance. Systematically combining them to pairs does lead to a manageable amount of possibilities. A pretty exhaustive list can be given. After checking for the stability and suitability of these compounds the list of the ones that turn out to seem suitable as structural building materials become even shorter.

Many ternary compounds can be derived from binary ones by suitable substitution of atoms. For orientation something like pseudo phase diagrams can be used.

Classification by resistance against water

binary compounds that do not react or dissolve in water

One important subclass of the water stable binary compounds are the passivation layer minerals of today's industrial metals.
A big advantage of them is that their effect on human skin (in bulk contact - not nanoparticle form!) is widely known to be safe for most of them.
Other binary water stable compounds are:

• 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 Nierite (Mohs 9), silicon nitride
• BN Qingsongite (Mohs 9-10), cubic boron nitride - very similar to diamond (also cubic and hexagonal "allotropes" - and a graphitic form)
• 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 (Mohs 9-9.5 !!)
• 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)

• MnO₂ Pyrolusite (Mohs 6-6.5)
• ZrO₂ Baddeleyite (Mohs 5.5-6) cubic Zirconia (Mohs 8-8.5)

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 formConstructing Isosurfaces with Sharp Edges and Cornerss chemically and mechanically rather stable compounds with many nonmetals.

• TiC titanium carbide (May form a passivation layer when in contact to moist air / water => sealed use only?)
• 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 (same crystal structure as superhard stishovite SiO₂)

Lead and tin:

• α-PbO₂ Scrutinyite (Mohs ?? | density 9.867 g/cm³ calculated)
• β-PbO₂ Plattnerite (Mohs 5.5 | density ~9.06 g/cm³)
• SnO₂ Cassiterite (Mohs 6-7 | density 6.98 - 7.1 g/cm³)

Misc (rare elements):

• MoO₂ Tugarinovite (de) [3] (Mohs 4.6)
• GeO₂ Argutite (Mohs 6-7)
• CrN Carlsbergite (Mohs 7 | 5.7g/ccm)

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

III - V compounds

Note that nitrogen and phosphor forms four covalent bonds here instead the usual three. This can be pictured as their lone pair of electrons sticking into the electron deficient orbitals of boron or aluminum. The character of this bond is distributed over all four bonds such that perfectly tetrahedral symmetry is reached.

• BN cubic boron nitride - highly stable and similar to diamond
• BP boron phosphide - rather stable thus maybe low toxicity (?)
• AlN aluminium nitride - slowly attacked by water - low toxicity
• AlP aluminium phosphide - highly toxic - releases phosphine when in contact with water

The elements Ga,In,Th & As,Sb,Bi that are also in group III and V respectively are rather scarce and thus not considered here.

Table of III - V compounds: (wikipedia)

Silicon dioxide and related compounds

All compounds reached by full substitution of silicon or oxygen by their groupmembers carbon or sulfur respectively are rather unstable. Partial substitutions should work though. See: pseudo phase diagrams.

• allotropes SiO₂ (e.g. quartz,...)
• structurally equivalent solid CO₂ - probably explosive (similar to room-temperature solid nitrogen) since normally the well known low energy gas
• structurally equivalent solid CS₂ - normally a molecular liquid
• structurally equivalent SiS₂ - normally a soft solid made from polymeric chains

Some interesting oddballs (not necessarily diamondoid)

• CS2, SO3, Osmium oxide, ...

Related

• For binary chemical compounds suitable for advanced APT sorted by the chemical element they contain check out the page: Chemical element