Difference between revisions of "Pseudo phase diagram"

Revision as of 09:34, 25 August 2018

(TODO: add image)

Pseudo phase diagrams are phase diagrams for the visualisation of advanced mechanosynthesized materials. Due to the very different character of mechanosynthesis compared to thermodynamic synthesis routes pseudo phase diagrams have a very different character than conventional phase diagrams and break many rules (which often are mistakenly believed to be unbreakable).

"pseudo" since the structure of the material at a specific point in the diagram is not defined by the thermodynamic history of the material but by the way it was mechanosynthesized. There are lots of special positions in the diagram that arise due to the specific chosen crystal structure and chequerboard pattern.

Differences between conventional and pseudo phasediagrams

In conventional phase diagrams there are regions of configurations that are reachable via thermodynamic processes and regions which are not. The accessible regions often are continuous. This becomes visible in stoichiometies that contain decimal marks - this is called a mixing series.

In pseudo phase diagrams for the visualisation of mechanoysnthesized materials only a few of the simplest stoichiometric fractions are of concern leaving only a few dots in the phase diagram. Many of those dots may be in regions that are in the thermodynamically forbidden areas! Please recall that diamondoid materials by definition do (for all practical purposes) not diffuse at their usage temperature. A good example may be lonsdaleite (hexagonal diamond) which is hard to access thermodynamically (almost forbidden area) and would like to change to graphite. But it can't since it is deeply frozen at room temperature.

By purely thermodynamic processes it is also often not possible to exactly hit the simple-fraction-stoichiomtey-points in the allowed regions when looking at a small enough randomly cropped out test-volume. (Note that a small test-volume requires a high degree of order to meet the fraction). This is the especially the case when very similar elements (or isotopes) are included.

Applicability on binary compounds

Pseudo phase diagrams are good for orientation what kind of low level metamaterials can be built with binary diamondoid compounds.

An example of such an pseudo phase diagram would be a square with CO2 (upper left) SiO2 (upper right) beta-C3N4 (lower left) Si3N4 (lower right) as their "end members". (solid CO2 is likely to be explosive but with a sufficient number of C atoms substituted with Si atoms it will be stable - it may be possible to draft a forbidden zone around the solid CO2 corner). In this specific diagram from top to bottom from oxides to nitrides the crystal structure must change significantly (due to the changing valence number) making a less continuous transition.

[todo: add existing images of such diagrams]

Examples

The C B N triangle

One example for a 3D pseudo phase diagram would be the triangle created by the corner elements carbon C boron B and nitrogen N. Instead of the usual phase regions and miscibility gaps in a conventional thermodynamic phase diagram one finds a lot of dots where some of them often can sit in "forbidden" areas. In these cases cold mechanosynthesis places some atoms into for them inconvenient places but afterwards they never (for all practical purposes never) get the thermal energy to leave their places towards more equilibrium phase.

Beside the pure element corners there are for example:

• BN, BNC, BNC₂, B₂N₂C

Pseudo phase diagrams that have lots of compounds that are highly stable at the highest expectable usage temperature (e.g. at least the boiling point of water) are of especial interest. There are lots of these. Compounds that needs to be permanently cooled after mechanosynthesis (like many metal alloys and molecular solids) is likely to only gain niche applications.

The triangle can be extended to a tetrahedron with as additional corner element that is from the third row of the periodic table and thus slightly bigger than carbon.

• SiC, SiC₂, Si₂C, SiC₃, Si₂C₃, Si₃C₂, Si₃C,
• BNSi, BNSi₂, B₂N₂Si, ...

The Si C B Al - oxides nitrides cube

This one contains some of the most interesting materials for advanced atomically precise gem-gum-technology.

• carbon-dioxide silicon-dioxide diboron-trioxide and dialuminium-trioxide form a square base.
• beta-tricarbon-tetranitride trisilicon-tetranitride boron-nitride and aluminium nitride form a square top.

Note that there are axes (cube edges) where the generalized stoichiometry and crystal structure remains unchanged and others where this is not the case. The sets of compound stoichiometry points with different stoichiometry and or crystal structure may well overlap.

Albeit the symmetry in the periodic table phosphorus is left out here since phosphides are not very stable and not very healthy. A bad combination. As a side note on the other hand some phosphates (the salts of oxygen rich phosphoric acid E338) are not unhealthy in small doses and reasonably stable. They should be an interesting building material.

Transition metal mono "nonmetallides"

Many feature the simple NaCl and allow a wide flexibility of elemental replacements. Possibly the still quite metallic nature allows for excess electrons to be sucked up into the free electron gas?

There are some exceptions on the right side of the transition metal elements. Cu and Zn having more electronegativity and less metallic character leading to CuO and ZnO to being more covalent and nonmetallic in nature.

• (no ScO?) ... maybe too electropositive forming ionic covalent Sc₂=₃
• TiC, TiN, TiO
• VC, VN, VO ... (VN Uakitite [1])
• (no CrC?), CrN, CrO ... (there are several chromium carbides of other stoichiometries) (CrN carlsbergite [2])
• (MnC??), (MnN??), MnO
• FeC, (no FeN?), FeO ... (there are several iron nitrides of other stoichiometries)
• CoO, NiO (carbides and oxides not mentioned in wikipedia - state 2018)
• (wiki-TODO: add more examples and maybe add links)

Related

• Neo-polymorphs
• Isostructural bending