Question

Palladium metal crystallizes with a face-centred cubic structure. Heating palladium in hydrogen causes uptake of hydrogen into the lattice. (a) What sites are being filled in the palladium structure? (b) What type of solid solution is being formed?

Short answer

(a) In the face-centered cubic structure of palladium, hydrogen atoms fill the octahedral and tetrahedral interstitial sites. (b) The type of solid solution formed when palladium absorbs hydrogen is an interstitial solid solution.

Step-by-step solution

Understanding Face-Centred Cubic Structure

In a face-centred cubic (FCC) unit cell, atoms are located at each of the corners and the centres of all the cubic faces. Each of the corner atoms is shared by eight neighbouring unit cells, contributing \( \frac{1}{8} \) of each atom to a given cell. Each of the face-centred atoms is shared between two neighbouring unit cells, i.e., each contributes \( \frac{1}{2} \) of an atom to a given unit cell. This results in an equivalent of four atoms in total per unit cell (\( 8 \times \frac{1}{8} + 6 \times \frac{1}{2} = 4 \)) for a FCC structure.

Identifying the Filled Sites

When a smaller atom such as hydrogen is absorbed into the lattice of a larger atom such as palladium, it usually slots into the interstitial sites. In a FCC lattice, the interstitial sites are octahedral and tetrahedral sites. The octahedral sites are located at the edge-centers and body-center, while tetrahedral sites are located inside the unit cell.

Type of Solid Solution Formed

Solid solutions are formed when some of the host metal atoms are replaced or when interstitial sites are filled by other atoms. Since the absorbed hydrogen atoms do not replace the palladium atoms but fit into the interstitial sites of the lattice, the type of solid solution formed is an interstitial solid solution. So, finally, (a) Hydrogen atoms are filling the octahedral and tetrahedral interstitial sites in the face-centred cubic structure of palladium. (b) An interstitial solid solution is being formed when palladium absorbs hydrogen.

Key concepts

Face-Centered Cubic Structure

When studying the fascinating world of solid solution chemistry, it's essential to understand the spatial arrangement of atoms within a crystal lattice. One such arrangement is the face-centered cubic (FCC) structure, a common pattern for metallic crystals, including the palladium mentioned in our exercise. Imagine a cube where each corner, as well as the center of each face, is occupied by an atom. Now, these aren't solitary atoms belonging exclusively to one cube; they are shared with adjacent unit cells, making the atomic arrangement highly efficient.

In an FCC structure, the atoms at the corners contribute just a fraction - precisely one-eighth - to the overall unit cell, since these corners are shared by eight different cells. Meanwhile, the atoms located at the center of each face are shared with one other unit cell, therefore contributing half of their presence to the unit cell in question. This unique sharing leads to an equivalent of four whole atoms within a single FCC unit cell. This tightly packed structure not only lends itself to a high density but is also crucial for understanding how other elements, such as hydrogen, can be incorporated into metals like palladium.

Interstitial Sites

The crystalline architecture of metals provides not only room for the original atoms but also smaller spaces known as interstitial sites. To visualize these, imagine the gaps in between the larger metal atoms. These are the spots where smaller 'guest' atoms can be squeezed into without altering the primary framework of the host metal. Interstitial sites are akin to the pockets of air in a sponge, often smaller than the atoms at the lattice points themselves.

In the context of FCC structures, two types of interstitial sites are significant: octahedral and tetrahedral. Octahedral sites are shaped like an octahedron and are located at the edge-centers and body-center of the unit cell, while tetrahedral sites, shaped like a tetrahedron, nestle within the cube, forming smaller, more compact spaces. These sites are crucial for processes like the palladium-hydrogen absorption since they determine where the hydrogen atoms will be positioned within the palladium lattice.

Palladium-Hydrogen Absorption

The interaction between palladium and hydrogen is a classic model of metal-hydride chemistry - a field that explores how hydrogen is absorbed by metals - and serves as a fundamental example of an interstitial solid solution. When palladium is exposed to hydrogen gas, it acts like a sponge, absorbing hydrogen atoms into its lattice. However, the hydrogen doesn't displace palladium atoms; instead, it neatly fits into those octahedral and tetrahedral interstitial sites within the palladium's FCC structure we explored earlier.

This kind of combination doesn't form a whole new compound but rather a special mixture called an interstitial solid solution. This means the hydrogen atoms are now part of the palladium crystal structure, albeit interstitially, modifying properties such as ductility, electrical resistance, and the ability to catalyze reactions. The beauty of such absorption is that it can typically be reversed, releasing the hydrogen without permanent changes to the palladium - a feature that's being researched for potential uses in energy storage and fuel cells.