Crystal Structure Simulator — SC / BCC / FCC / HCP / Diamond

The packing fraction (or atomic packing factor) is the volume occupied by atoms in a unit cell divided by the total volume of the unit cell. For a structure with n atoms per cell, each of radius r:

Where a is the lattice constant. The relationship between a and r depends on the structure (e.g., in FCC, atoms touch along the face diagonal, so $a = 2\sqrt{2}r$).

To determine crystal structure experimentally, X-ray diffraction is used. Constructive interference occurs only when Bragg's Law is satisfied, linking the wavelength, plane spacing, and diffraction angle.

For cubic crystals, the interplanar spacing d for planes (hkl) is calculated from the lattice constant a. This is the formula the simulator uses when you input hkl indices.

Metallurgy & Material Selection: The packing fraction directly influences material properties. FCC metals like copper and aluminum are generally more ductile, which is desirable for forming wires and sheets. BCC metals like tungsten and room-temperature iron are often stronger but less ductile.

X-ray Crystallography: This is the primary method for determining the atomic structure of new materials. By measuring the angles (θ) where diffraction peaks occur (satisfying Bragg's Law), scientists can work backwards to find the d-spacings and ultimately solve the entire crystal structure.

Semiconductor Manufacturing: The diamond cubic structure, with its relatively low packing fraction of ~34%, is the crystal structure of silicon and germanium. The specific arrangement of atoms and the resulting voids are critical for how electrons move, forming the basis of all computer chips.

Alloy Design: Understanding where voids are in a crystal structure (like the octahedral and tetrahedral sites in FCC) is key for designing alloys. For instance, carbon atoms in steel fit into the interstitial sites of iron's BCC lattice, dramatically strengthening the material.

First, regarding the relationship between "atomic radius r" and "lattice constant a" when using this tool. When you adjust r in the simulator, a changes accordingly. However, this calculation assumes an ideal state where atoms are rigid spheres and are in close contact without gaps. In real materials, this relationship breaks down depending on the nature of the atomic bonding (metallic, covalent, etc.). For example, the "atomic radius" back-calculated from the actual lattice constant of iron (BCC) does not perfectly match the value obtained from this geometric formula $a = 4r / \sqrt{3}$. Keep in mind that the tool is an "ideal model" for understanding the basic principles.

Next, regarding the angle displayed as the "diffraction angle θ" in the Bragg condition calculation. This is the angle between the incident X-ray and the crystal planes (the Bragg angle). A common misunderstanding is confusing this with the "detector position (2θ)". In an actual X-ray diffractometer, the incident angle relative to the sample is θ, and the detector moves to a position of 2θ for measurement. If you set λ=0.154 nm (CuKα radiation), FCC a=0.3615 nm (Al), and hkl=(111) in the tool, it calculates θ≈19.3°. However, in an experimental protocol, you would set this angle as the sample tilt angle and search for the peak with the detector at approximately 38.6°.

Finally, the reason why the "packing fraction" of HCP (hexagonal close-packed structure) is nearly the same as that of FCC, at about 74%. Looking at the 3D models, the atomic arrangements seem completely different, right? The key point to understand here is that the state of "close packing" inherently has the same spatial packing efficiency. The only difference is the stacking sequence: FCC is ABCABC..., while HCP is ABAB... Global properties like packing fraction and coordination number (both 12) remain the same. Try rotating both structures in the tool and observe how the close-packed planes (the {111} planes for FCC, the basal plane for HCP) are connected; you'll get a tangible sense of this "difference in stacking".