[Encyclios]

Substitutional solid solutions

In metal alloys formed by substitutional solid solution, ions of another metal replace as many ions in the crystal lattice of the main metal; for this to happen, the ions of the two metals must be of similar size; this is the case, for example, of brass, a copper-zinc alloy.

To form a simple substitutional solid solution there are conditions that must be verified:

1. The substituting ions must have the same charge. Otherwise, other structural changes occur, with formation of vacancies and interstitial species to maintain electroneutrality.
2. The substituting ions must be of very similar size. From experimental data on metal alloys, it is believed that the upper limit for substitution of dissimilar metals is around 15% of the metal radius. For non-metallic solid solutions the limiting difference in size appears to be a little over 15% but it is difficult to quantify precisely mainly because of uncertainties about ionic radii and because the formation of solid solutions is very much temperature dependent. In fact, often several solid solutions can be formed at high temperatures while at lower temperatures the formation of the same can be very reduced or practically impossible. In general the formation of solid solutions is thermodynamically favored by high temperatures (as the TDS term increases).
3. An important factor is the crystal structure of the two component salts. For systems that exhibit complete solid solution behavior, it is essential that the two extreme members be isostructural. The reverse is not true, of course, i.e., it is not enough for two phases to be isostructural to form solid solutions (e.g., LiF and CaO both have rock salt structure but do not form solid solutions).