Solid Solution

Right. Another task. Don't expect me to be thrilled. This is... this. Wikipedia. About solid solutions. Fascinating. Let’s get it over with.

Chemical solution in solid form

This article, frankly, is a mess. It’s crying out for more substance, more verification. It’s a cry for help, really. You can’t just leave facts hanging out there, unanchored. Unsourced material? That’s just… lazy. It’ll be challenged, removed. Like a bad thought. So, if you’re looking for reliable sources, try actually finding them, not just wishing them into existence. November 2007. Honestly.

A solid solution, and this is a term bandied about a lot, especially in the realm of metals, is essentially a homogeneous mixture. Think of it as two compounds deciding to coexist in a solid state, but they’ve agreed to share a single crystal structure. It’s a forced intimacy, really. You see this everywhere, in metallurgy, in geology, and in the sterile world of solid-state chemistry. The word "solution" here isn't about dissolving in liquid; it’s about this intimate, atomic-level mingling. It’s what separates these homogeneous materials from mere physical mixtures, where things are just… jumbled together. There are two main players in this drama: the solvent and the solute. The solvent is the dominant one, the host, while the solute is the guest, incorporated into the host’s structure.

The solute, this interloper, has two ways of making itself at home in the solvent's crystal lattice. It can go substitutionally, meaning it kicks out a solvent particle and takes its place. Or, it can slide in interstitially, finding a cozy little gap between the solvent particles. Both methods disrupt the pristine order, you see. It’s never quite the same after.

These solid solutions, they’re not pure extremes. They’re made up of fractional compositions of their constituent ions, existing between pure, isostructural extremes. They call these extremes "end members" or "parents." Take sodium chloride (NaCl) and potassium chloride (KCl). They share the same cubic crystal structure, which is convenient. This allows for a solid solution with any ratio of sodium to potassium, something like (Na ₁₋ₓ K ₓ)Cl. You could achieve this by, say, dissolving specific ratios of NaCl and KCl in water and then letting the water evaporate, leaving the two ions intertwined.

There’s a commercial example, a salt substitute called Lo Salt. It's essentially (Na ₀.₃₃ K ₀.₆₆ )Cl. That means it’s got 66% less sodium than pure NaCl. Clever, isn't it? Then there’s iodised salt. Often, it's just NaCl with a sprinkle of potassium iodide (KI), maybe 50 to 100 parts per million, dissolved within the NaCl solvent. It’s a subtle invasion. In stark contrast, consider the mineral sylvinite. That’s a physical mixture. It's just big, separate chunks of NaCl and KCl, not integrated. Clearly not a solid solution. It’s… messy.

Minerals, being natural creations, are inherently prone to compositional variation. Often, a specimen is part of a larger solid solution family. For geologists, it’s more useful to talk about the family’s composition than a single specimen’s. Take olivine. Its formula is (Mg, Fe)₂SiO₄, which is the same as (Mg₁₋ₓ Feₓ)₂SiO₄. The ratio of magnesium to iron shifts between the two end members: forsterite (the Mg-rich one: Mg₂SiO₄) and fayalite (the Fe-rich one: Fe₂SiO₄). The exact ratio in olivine itself isn't usually specified. As compositions get more complex, this geological notation becomes a far more manageable way to describe things than chemical formulas. It’s a necessary simplification, I suppose.

Nomenclature

The IUPAC offers a definition: a solid solution is "a solid in which components are compatible and form a unique phase." Simple, almost too simple.

There’s another definition floating around, describing it as "a crystal containing a second constituent which fits into and is distributed in the lattice of the host crystal." That’s from some refs, 7, 8. But it’s not general enough. It’s too narrow, too restrictive. Not recommended.

The expression is meant to describe a solid phase that contains more than one substance. For convenience, one substance, the solvent, is treated differently from the others, the solutes. It’s a matter of perspective, really.

One or more of these components could be macromolecules. Some of the other components might then act as plasticizers. These are substances dispersed at a molecular level that lower the glass-transition temperature – the point where an amorphous polymer shifts between glassy and rubbery states.

In pharmaceuticals, this concept of a solid solution often applies to mixtures of drugs and polymers. It’s a way to control how the drug is released, I suppose.

However, the number of drug molecules that actually act as solvents (plasticizers) for polymers is quite small. 9. It’s a niche application.

Phase diagrams

Consider a binary phase diagram. A solid solution is represented by an area, often marked with its structure type. This area covers the range of compositions and temperatures or pressures where it exists. If the end members don’t share the same structure, you’ll likely find two distinct solid solution ranges, each dictated by the structure of its parent. These ranges might overlap. In the overlap zone, materials of that composition can adopt either structure. Or, there might be a miscibility gap in the solid state, meaning any attempt to create a material with that composition will just result in a mixture of separate phases. In areas not covered by solid solutions, you might find "line phases." These are compounds with a fixed crystal structure and a precise stoichiometry. When the crystalline phase consists of two non-charged organic molecules, it’s commonly called a cocrystal. In metallurgy, alloys with a fixed composition are known as intermetallic compounds. A solid solution is more probable when the two elements involved are close together on the periodic table. An intermetallic compound, on the other hand, generally forms when the metals are far apart. 10.

Details

As mentioned, the solute can integrate into the solvent's crystal lattice either substitutionally or interstitially. Both methods have consequences. They distort the crystal lattice and disrupt the material's physical and electrical uniformity. 11. If the solute atom is larger than the solvent atom it replaces, the crystal structure's unit cell often expands to accommodate it. This expansion means you can often calculate the material’s composition from its unit cell volume. This relationship is known as Vegard's law. 12.

Some mixtures readily form solid solutions across a range of concentrations. Others? Not at all. The likelihood of two substances forming a solid solution is complex, influenced by their chemical, crystallographic, and even quantum properties. For substitutional solid solutions, the Hume-Rothery rules offer some guidance. They suggest a solution might form if the solute and solvent have:

Similar atomic radii (a difference of 15% or less).
The same crystal structure.
Similar electronegativities.
Similar valencies.

A solid solution mixes with others to form a new solution. This is a basic concept, really.

The phase diagram shown previously depicts an alloy of two metals that forms a solid solution across the entire range of relative concentrations. Here, the pure phase of each element shares the same crystal structure. The similar properties of the two metals allow for seamless substitution throughout the entire concentration spectrum. In more complex systems with three or more components, forming pseudo-binary solid solutions might require a more intricate phase diagram representation, possibly with multiple solvus curves indicating different equilibrium chemical conditions. 13.

Solid solutions are crucial in commerce and industry. These mixtures often possess superior properties compared to their pure constituents. Many metal alloys are, in fact, solid solutions. Even a small amount of solute can significantly alter the electrical and physical characteristics of the solvent.

Consider this binary phase diagram showing two solid solutions:

{\displaystyle \alpha }

and

{\displaystyle \beta }

This diagram illustrates the phases of a mixture of two substances, A and B, in varying concentrations. The region marked "

{\displaystyle \alpha }

" is a solid solution where B acts as the solute within a matrix of A. At the opposite end of the concentration scale, the region labeled "

{\displaystyle \beta }

" is also a solid solution, but here A is the solute in a B matrix. The large solid region situated between the

{\displaystyle \alpha }

and

{\displaystyle \beta }

solid solutions, designated as "

{\displaystyle \alpha }

{\displaystyle \beta }

", is not a solid solution. Instead, examining the microstructure of a mixture within this range would reveal two distinct phases: the A-in-B solid solution and the B-in-A solid solution would exist as separate entities, perhaps arranged in lamellae or grains.

Application

Looking at the phase diagram, at three specific concentrations, the material will remain solid until it reaches its melting point. Upon adding the heat of fusion, it will transition to a liquid state at that exact temperature. These points are:

The unalloyed extreme left.
The unalloyed extreme right.
The dip in the center, known as the eutectic composition.

At any other proportion, the material will enter a "mushy" or pasty phase, gradually melting as it warms up.

The mixture at the dip point of the diagram is termed a eutectic alloy. Lead-tin mixtures formulated at this specific point (a 37/63 ratio) are useful for soldering electronic components, especially when done manually, because they solidify quickly as they cool. Conversely, when lead-tin mixtures were used for soldering seams on automobile bodies, the extended pasty state allowed the material to be shaped with a wooden paddle or tool. For this purpose, a ratio of 70–30 lead to tin was employed. Lead is being phased out of such applications due to its toxicity, making the recycling of lead-containing devices and components more complicated.

Exsolution

When a solid solution becomes unstable, perhaps due to a drop in temperature, exsolution occurs. This is when the two phases separate, forming distinct microscopic to megascopic lamellae. This separation is primarily driven by differences in cation size. Cations with significantly different radii are less likely to substitute for each other readily. 14.

Consider alkali feldspar minerals. Their end members are albite, NaAlSi₃O₈, and microcline, KAlSi₃O₈. At high temperatures, Na⁺ and K⁺ ions readily substitute for one another, allowing the minerals to form a solid solution. However, at lower temperatures, albite can only substitute a small amount of K⁺, and similarly, microcline can only substitute a small amount of Na⁺. This leads to exsolution, where they separate into two distinct phases. In the case of alkali feldspar minerals, thin white layers of albite alternate within the typically pink microcline, 14. This creates what is known as a perthite texture.