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 dimenstions:
- Keeping the same structure but changing the composition (isostructural transition / compositional transition): neo-isotypes (non statistic solid solutions)
- Keeping the same composition but changing the structure (isocompositional transition / structural transition): neo-polymorphs & neo-allotropes
Isostructural transition / compositional transition
Same structures but different elements.
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 to cubic boron nitride (quingsongite)
- TiC to TiN to TiO to other transition metal monoxides (all simple cubic conductive ceramics)
- NaCl to KCl to KBr (all simple cubic insulating salts)
- MgO to CaO (all salt flavoured oxides, latter being highly reactive with water needing sealing)
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
Cases with one side being a less thermodynamically stable less common gem:
- titappire series as the leukosapphire to tistarite (exotic titanium oxide) to hematite transition (all hematite structure)
- stishotile series as the rutile to stishovite (exotic ultra-hard silicon oxide) transition (all rutile structure)
These still can be highly kinetically metastable.
Note that even diamond is merely metastable.
Bulk internal isostructural transitions for entirely new classes of materials
The here in this wiki defined neo-polymorphs and neo-allotropes
(same but just one element) are istostructural transitions
(including systematic checkerboard compounds and quasistatistic compounds)
that are (/will be as of 2025) only accessible via mechanosynthesis.
The potentially accessible space of these neo-polymorphs
can be mapped out by pseudo phase diagrams.
These pseudo phase diagram transitions are bulk-internal (and rational number gradual)
rather than a transition via an interface. 'Making for new base material classes
where the structure is overall larger scale unstrained.
Isostructural gradient / compositional gradient – transition
Composition can be made to spacially change more or less gradually (but still rational number quantized).
For smooth transitions on slightly larger scales some fancy stuff could eventually be tried
like e.g. gif image format like dithering. On the most discrete (and most easy) end
of the spacial transition spectrum it is just a single plane transition.
Related here is isostructural bending from mismatching lattice constants.
It can be a undesired challenge or be used for intentional curving of structures. <nr>
"Isomotifal" transition
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.
Or e.g. puts constraints on isostructural metal silicates that otherwise
without artificial constraint would be quite flexible on interchange of e.g. Mg & Fe.
Isocompositional transition / structural transition
Same elements but different structures
Bulk internal isocompositional transitions
Stacking order variants are probably good example. See: Dialondeite
Though this stacking works only for in one direction/dimension.
Otherwise there seems to be very limited possibiliy for this.
For one thing the nitinol transition comes to mind,
though this needs permanent stress for a pernmanent phase transition.
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.
Related
- Neo-isotypes & neo-polymorphs classifieable within pseudo phase diagrams