Classification dimensions for mechanosynthesized gemstone like compounds
When transitioning between gemstones via a dense connection of covalent bonds,
as is relevant for future mechanosynthesis or atomically precise nanostructures,
there are two qualitatively different classification cases cases:
- Keeping the same structure but changing the composition (isostructural transition / compositional transition)
- Keeping the same composition but changing the structure (isocompositional transition / structural transition)
Isostructural transition / compositional transition
Some example cases collected here: Simple crystal structures of especial interest
In some cases this is likely chemically very stable
- silicon to silicon carbide (moissanite) to diamond
- TiC to TiN to TiO (all simple cubic)
In some other cases this is likely chemically highly incompatible despite the same structure being present
- diamond to sphalerite(aka zincblende) – both zincblende structure
- lonsdaleite to wurzite – both wurzite structure
- NaCl to TiC - both simple cubic but one a salt and the other a conductive ceramic
A bit more general would be "isomotifal"
That would be transitions that also allowing for changes in symmetry
due to things like Jahn Teller distortions.
E.g. cubic becoming tetragonal but the motif stays the same.
Isostructural and isostoichometric
This just excludes transitions like from diamond CC to moissanite SiC.
Isocompositional transition / structural transition
Same elements but different structures
Isocompositional and isostoichometric compounds
Good example are the very many known polymorphs of SiO2. Quartz is only one of many.
Also the polymorphs of TiO2 and the ones of CaCO3.
The same chemical composition makes covalent transitions highly plausible
Albeit the amount of "sewing structure" for transitioning between the structures can wildly vary
depending on the similarity or dissimilarity of the two involved structures to transition between.
E.g. in the case of Si3N4 stoichometry silicon nitride,
a transition between trigonal alpha and hexagonal beta phase
along the common highest rotation symmetry main axis may be much easier and straightforwards
compared to a transition from either of these two phases to the cubic gamma phase.
isostoichometric compounds
There can be both structural or compositional variations/chnanges or both.
Just the ratio between the elements involved needs to stay the same.
Mostly useful in conjunction with and as a refinement to the other two.