Crystal Structure Types to Know for Crystallography

Understanding crystal structure types is key in crystallography. These structures, like cubic close-packed and hexagonal close-packed, reveal how atoms arrange themselves, affecting properties like density and stability in various materials, from metals to semiconductors.

1. Cubic close-packed (face-centered cubic)

   • Atoms are arranged in a repeating pattern with one atom at each corner and one in the center of each face of the cube.
   • Coordination number is 12, meaning each atom is in contact with 12 others.
   • High packing efficiency of approximately 74%, making it one of the densest structures.
   • Common metals with this structure include aluminum, copper, and gold.

2. Hexagonal close-packed

   • Atoms are arranged in a hexagonal lattice with two layers of atoms per unit cell.
   • Coordination number is also 12, similar to cubic close-packed.
   • Packing efficiency is about 74%, comparable to face-centered cubic.
   • Common in metals like magnesium, titanium, and zinc.

3. Body-centered cubic

   • Contains one atom at each corner of the cube and one atom in the center of the cube.
   • Coordination number is 8, indicating each atom is in contact with 8 others.
   • Lower packing efficiency of about 68% compared to close-packed structures.
   • Found in metals such as iron, chromium, and tungsten.

4. Simple cubic

   • Atoms are located only at the corners of the cube, with one atom per unit cell.
   • Coordination number is 6, meaning each atom is in contact with 6 others.
   • Packing efficiency is low at about 52%, making it less common.
   • Example includes polonium and some alkali metals.

5. Diamond cubic

   • A variation of the face-centered cubic structure with a unique arrangement of carbon atoms.
   • Each carbon atom is tetrahedrally coordinated to four other carbon atoms.
   • High packing efficiency and strong covalent bonding contribute to its hardness.
   • Found in diamond and silicon crystals.

6. Wurtzite (hexagonal zinc sulfide)

   • Hexagonal structure with a tetrahedral coordination of atoms.
   • Each zinc atom is surrounded by four sulfur atoms and vice versa.
   • Exhibits anisotropic properties, meaning its physical properties vary with direction.
   • Commonly found in minerals and semiconductor materials.

7. Rock salt (sodium chloride)

   • Face-centered cubic structure where sodium and chloride ions alternate in a 1:1 ratio.
   • Coordination number is 6 for both sodium and chloride ions.
   • High ionic bonding strength contributes to its stability and high melting point.
   • Commonly known as table salt.

8. Cesium chloride

   • Simple cubic structure with cesium ions at the corners and chloride ions in the center.
   • Coordination number is 8, with each cesium ion surrounded by 8 chloride ions.
   • Unique due to its large ionic size, leading to a distinct crystal geometry.
   • Used in various applications, including in the production of cesium clocks.

9. Fluorite

   • Face-centered cubic structure with calcium ions at the corners and fluoride ions in the center.
   • Coordination number is 4 for fluoride ions and 8 for calcium ions.
   • Exhibits a high degree of ionic character due to the difference in electronegativity.
   • Commonly found in mineral deposits and used in the production of fluorine.

10. Perovskite

    • Characterized by a general formula of ABX3, where A and B are cations and X is an anion.
    • Exhibits a cubic structure with a unique arrangement that allows for various ionic sizes.
    • High versatility in properties, making it important in materials science and electronics.
    • Found in minerals and used in applications like solar cells and superconductors.