Neo-polymorph

(wiki-TODO: There is a significant mixup with neo-isotypes here. Move relevant parts to the new page.)

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:

• different atom types (elements) fall 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.

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

Of course arbitrary many elements/layertypes/... are allowed. A,B,C,D,...

Note: Thermodynamic accessibility refers to all the crude processes available today (2020) that only allow to handle matter in statistical quantities: melting, mixing, cooling, pressurizing, irradiating, (cook mix and stir thermodynamic synthesis) ... This explicitly excludes advanced mechanosynthesis.

(wiki-TODO: ABABAB ABCABC - no same letters following adjacently in case of SiC structure - fix discussion above accordingly eventually)

Examples

See: pseudo phase diagrams for more details on this.

Related

• 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

Examples with likely missing end members

Take gem grade silicon carbide aka moissanite as starting point.
Then increasinvgly substitute the silicon Si atoms with titanium Ti atoms in a checkerboard fashion.
It is known that pure TiC likes to be in simple cubic rock salt structure, so at some degree of substitution.
the structure might become weakly metastable (worst case explosive) or entirely unstable.

Revesely, staring with simple cubic rock salt structure TiC and
substituting Ti with Si (or Ge) in checkerboard fashion seem more limited if possible at all.

Titanium carbide MXenes Ti₂C₁, Ti₃C₂, Ti₄C₃(?)
Might be checkerboard pattern Si/Ge substitutable by some degree.
Due to todays experimental syntehsis route via MAX phases and etching out A,
it is likely unclear in how far titanium carbide MXenescan be extended from
2D materials to a 3D materials that in the limit have Ti₁C₁ stoichometry.
That is: In how far they are metastable to transformation to
the known to be low energy stable simple cubic rock salt structure polymorph of TiC.

Side-note: MXenes have dense fcc==ccp structure akin to many metals.
Unlike moissanite and dioamondoids in general which are sparse,
half the atomic coordination, and voids due to being two interspersed fcc lattices.

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