Neo-polymorph

(wiki-TODO: There was a significant mixup with neo-isotypes here. Do some more cleanup & checks if transition succeeded.)

A neo-polymorphic compound (or neo-isomorphic compound) is a highly (meta)stable non equilibrium polymorph
of a material with a certain fixed stoichiometry that is exclusively accessible through mechanosynthesis.
If just one element is involved the term neo-allotrope can be applied too/instead.

This includes patterns where specifically ordered states are thermodynamically not more attractive
than disordered (or in other undesired form ordered) states but
where a (sufficiently) high activation energy lies between the ordered and unordered states
making the via conventional means inacessible states highly kinetically metastable.

The patterns can be:

• different stacking geometry, specific example:
  A crossover between Diamond (cubic stacking) and lonsdaleite (hexagonal stacking).
  (When pointing up a tetrapod of carbon bonds there are two ways one can orient the up facing three bonds in a six direction hexagon).
• changes between different crystal structures of the same composition (adn stoichometry) so long nigh seamless covalent transitions are possible

Excluded here:

• changes of atom types and stoichometry falls under neo-isotypes instead:
  specific example: A crossover gemstone between Rutile (polymorph of TiO₂) and stishovite (polymorph of SiO₂).
  The pattern making elements are Ti & Si. Oxygen atoms stay at their places.

Stacking order patterns:

• ABABBABBBBAABABAABAA – unwanted unordered state – may be the only one that is thermodynamically accessible
• ABABABABABABABABABAB – unwanted ordered state – may be the only one that is thermodynamically accessible
• ABBAABBAABBAABBAABB – neo-polymorph – wanted peculiarly ordered state – not thermodynamically accessible - but accessible via mechanosynthesis

Note: in the case of SiC (moissanite) there is the constraint of no two same letters following adjacently. Above examples for simplicity.

Also neo-polymorphs and neo-isotypes can intermix here to neo gemstones like compounds or neo-gems for short.
All sorts of II-IV semiconductor compounds have fitting structure stacking order variations in {111} directions.
So structural stacking order can be augmented with material type stacking order.
Material stacking order not being constrained to the by {111} directions but by lattice strain.

Note: Thermodynamic accessibility refers to all the crude processes available today (2025)
that only allow to handle matter in statistical quantities: <br< melting, mixing, cooling, pressurizing, irradiating, (cook mix and stir thermodynamic synthesis) ...
This explicitly excludes advanced mechanosynthesis.

Examples

See: pseudo phase diagrams for more details on this.

• Dialondeite cubic to hexagonal series
• Moissanite cubic to hexagonal series
• genrally: cubic zincblende structure to hexagonal wurzite structure series (for all applicable compounds respectively, mostly III-V semiconductors)

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Other patternd back and forth transitions between small chunks of highly compatible crystal structures:

• withe sapphire to deltalumide (unclear if transitionable)
• silicon nitride trigonal alpha to hexagonal beta & (unclear) to cubic gamma
• SiO₂ low pressure modification transitions - see page: Quartz
• … (wiki-TODO: Find more examples to put here)

Related

• Neo-gems
• Neo-isotypes

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• Classifications for gemstone like compounds

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• Pseudo phase diagram - mapping out neo-polymorphs
• Simple crystal structures of especial interest - browse there as a starting point

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• Diamondoid compound
• Binary diamondoid compound

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• Kaehler brackets

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Examples:

• Dialondeite

External links

Wikipedia pages:

• Polymorphism (materials science)
• Polymorphs of silicon carbide
• isomer, stereoisomer, conformational isomer
• Superstructure (condensed matter)
• Isomorphism_(crystallography)

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• Isotype: (de) Translated citation: "Minerals of the same structural type are called isotypes. They crystallize in the same class of crystals and form similar crystal forms. "
• Isotype: (wikipedia de) Isotyp

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• www.mineralienatlas.de
  lists minerals with equal or similar structures for any given mineral
  so thhis can serve as a possible starting point to find potential neo-polymorphs