Limits of construction kit analogy

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Construction kit analogy: A defined number of connection points in defined directions.

In general atoms do not behave like building blocks of a construction toy.

In the case of construction toys (think wood ball and stick model):

  • The pieces have a defined number of connection points that are either male or female or androgynous.
  • The connections usually have a clearly defined directions.
  • There's a large variety of directions.

In case of atoms the connectivity behavior is rather context dependent. That is the character of the bonds emerges from the combination of atoms that are put next to each other. In some cases the character of bonds depends even on second to next and further away atoms but this rarely changes the fundamental character of the bond.

Luckily there's a large class of materials that do behave very predictably just based on the location in the periodic table (diamondoid materials in the more narrow sense). With a bit more knowledge the number of compounds with crudely but sufficiently predictable properties can be vastly expanded (gemstone like materials in a broad sense).

Rules to get construction toy like behavior

Maybe avoid ionic bonds (salts)

  • combination of electronegativities

One may want to avoid to put elements with extremely different electronegativities next to each other. If one does so it's likely that one get's strongly ionic bonds. Salts. Those leave no real freedom for the directions of bonds. The crystal structures (one of a very few very simple ones) is given by the radii of the combined elements.

In case one needs a material only for structural elements (no fine tuned geometries and sliding interfaces) salts can be of use.

Due to their polarity salts are often water soluble so its better to use them only in internal sealed spaces not exposed to weather attacked surfaces. There are exceptions that are barely water soluble though. One is periclase MgO another Fluorite CaF2.

Strongly avoid metallic bonds

One usually should avoid to put metals atoms next to each other. While the bonds are quite strong radially they are very weak against rotations. This leads to two problems.

  • Thermal motions at temperatures as low as room temperature can move atoms around in an erratic skitter motion (technical term "surface diffusion")
  • As in the case of salts very few compact crystal structures are available (in contrast to salts some stacking order freedom is left though)

The fist problem (surface diffusion) is highly undesirable or rather unacceptable. It's like trying to build things when they auto-destruct themselves right away.

One may be able to mechanosynthesize metallic compounds in an cryogenic environment where thermal motion is reduced enough such that for all practical purposes surface diffusion is totally prevented.

Either one keeps the system at those temperatures after mechanosynthesis. (Limited application space. Interplanetary space far from sun maybe?) or one tightly seals the block of metallic compound in a non diffusing hull leaving no space for surface (or internal) diffusion.

Long range delocalisation of shell electrons often leads to weird and interesting (potentially useful) quantum effects.
See: non mechanical technology path & color emulation

To make the many metallic elements of the periodic table accessible (especially the more abundant ones) one can look at interspersing non metal atoms between the metal atoms forcing electron localisation and with that a more directed covalent bond character. Look out for unusual coordination numbers of non-metals (that do not match the group in the periodic table) though.

This way one ends up with a very big class of ptoentially useful gemstones for future advanced nanosystems.
Prime examples are leukosapphire Al2O3 and rutile TiO2.

There's an intermediate range of electrically conductive compounds that have covalent character. In this class fall the iron oxides and iron sulfides. Also titanium nitride.

Slightly related here are sp2 carbon structures where the sigma bonds provide the covalent linking part and the pi bonds widely delocalized conductive metallic bonds.

Look out for unusual coordination numbers

  • cubic boron nitride BN (delocalized in boron nitride)
  • electron deficiency bonds
  • different orbital hybridisations

A lone pair (that's usually more of a passivating structure on diamondoid structures) can stick into empty orbitals forming an electron deficiency bond.

To get a feeling for what coordination numbers are possible and likely one can look at the possible oxidation numbers for the elements.

Compounds with tetrahedral coordinated oxygen

  • stishovite SiO2 (Mohs 9.5)
  • zincite ZnO (Mohs 4), Bromellite BeO (Mohs 9)

Look out for bounded instabilities

Sometimes gemstone like compounds in certain structure types can have internal instabilities that influence exterior shape. Instabilities that are present at slightly elevated temperatures or at room temperature or even below.

Examples are:

  • Ions too small for their interstitial position may have two stable positions with a very small energy barrier in-between.
  • Rotational or vibrational degrees of freedom can become activated.
    E.g. cristiobalite (a polymorph of SiO2 with wurtzite like structure akin to the hexagonal diamond form called lonsdaleite) has its α-β transition from low-temp-tetragonal to high-temp-cubic. The oxygen atoms that connect the silicon atoms indirectly are not straight but kinked connections (due to oxygens remaining lone pair of electrons). When they oxygen connections start to rotate at about 110°C the effect of the kinks start to get canceled out. Normal quartz has a similar α-β transition albeit at much higher temperature (at 573°C from trigonal to hexagonal).
  • A lifting of quantum mechanical degeneracy by breaking symmetry lowers energy can occur. (Jahn–Teller effect)
  • More exotic compounds can even have sub-lattices melt.
    E.g. iodides (AgI, CuI, ...) - the cation metal sub-lattice melts - see fast ion conductors

While in most cases likely to avoid in construction materials for specific applications (temperature sensors?) these effects should be pretty useful. Note that for crystolecules there is no risk of fracturing due to sudden local inhomogeneous volume change since they are too small and usually faultless.

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