Introduction:
Diffusion is the process by which particles move from an area of high concentration to an area of low concentration, resulting in a homogenous mixture. However, solids do not diffuse into one another. This phenomenon can be explained by the following reasons.

Intermolecular forces:
Solids have strong intermolecular forces that hold the molecules in a fixed position, making it difficult for them to move. The particles vibrate in their fixed position, but they do not move from their place. These intermolecular forces are stable enough to keep the particles in a fixed position, which means that the particles cannot move from their place and diffuse into one another.

Particle size:
The size of the particles plays a crucial role in determining whether diffusion can occur or not. In solids, the particles are closely packed together, making it difficult for them to move from one place to another. The particles are held in place by the intermolecular forces, and the size of the particles makes it difficult for them to move through the spaces between the particles of another solid.

State of matter:
Solids have a fixed shape and volume, which means that the particles are closely packed together and cannot move freely. In contrast, liquids and gases have a higher degree of freedom, which allows them to move more freely and diffuse into one another. The state of matter determines the degree of freedom the particles have, which ultimately determines whether diffusion can occur or not.

Conclusion:
In summary, solids do not diffuse into one another due to the strong intermolecular forces that hold the particles in a fixed position, the size of the particles, and the state of matter. These factors make it difficult for solids to move from one place to another and mix homogeneously.