Extinction

Definition

In crystallography, extinction refers to the phenomenon where certain reflections in a diffraction pattern become absent or significantly weakened due to specific factors related to the crystal structure. This can occur as a result of systematic absences linked to the symmetry of the crystal, and understanding extinction is crucial for accurately interpreting diffraction data and solving crystal structures.

5 Must Know Facts For Your Next Test

1. Extinction can occur in both single crystals and polycrystalline materials, impacting the visibility of certain diffraction peaks.
2. The extinction effect is closely tied to the arrangement of atoms within the crystal lattice and their interactions with incident X-rays.
3. Certain types of extinction, like dynamic extinction, arise from multiple scattering events where X-rays bounce off more than one atom before being detected.
4. Extinction can be a critical factor in determining the accurate unit cell parameters during structure solution as it can alter intensity measurements.
5. Understanding extinction helps in refining crystallographic models, as it provides insight into how well the proposed structure fits with the observed data.

Review Questions

• How does extinction affect the interpretation of diffraction patterns in crystallography?
  • Extinction impacts the interpretation of diffraction patterns by causing certain reflections to be absent or reduced in intensity, which can lead to incomplete or misleading information about the crystal structure. Recognizing extinction helps crystallographers identify systematic absences linked to symmetry and refine their models accordingly. This understanding is essential for accurate structure determination, as it ensures that all relevant reflections are considered.
• Discuss the role of space groups in understanding extinction phenomena within crystal structures.
  • Space groups play a crucial role in understanding extinction phenomena because they define the symmetry operations that dictate which reflections will be present or systematically absent in a diffraction pattern. By analyzing the space group associated with a crystal, researchers can predict extinction behavior based on its symmetry elements. This prediction assists in identifying which reflections should appear and helps resolve ambiguities when interpreting diffraction data.
• Evaluate how different types of extinction impact the refinement process of crystal structures.
  • Different types of extinction, such as dynamic and kinematic extinction, have significant implications for the refinement process of crystal structures. Dynamic extinction occurs due to multiple scattering effects that complicate intensity measurements, while kinematic extinction is related to single scattering and systematic absences based on symmetry. Understanding these extinctions allows crystallographers to adjust their refinement strategies effectively, ensuring better fits between observed and calculated intensities, ultimately leading to more accurate structural models.

Related terms

systematic absences: These are specific spots in a diffraction pattern that are missing due to the symmetry operations of the crystal lattice, which dictate which reflections should appear.

Laue conditions: Conditions that must be met for constructive interference to occur in the diffraction of X-rays by a crystal, which helps determine the allowed reflections.

space group: A classification of a crystal based on its symmetry elements, including rotations, translations, and reflections that influence which diffraction spots are present or absent.