Effects of merohedric twinning on the diffraction pattern

Area of Science:

• Crystallography
• Materials Science
• Solid State Physics

Background:

• Merohedric twinning involves perfectly overlapped lattices, making detection challenging, particularly for class I inversion twinning.
• Distinguishing between untwinned, twinned, and related space group models is crucial for accurate crystal structure determination.

Purpose of the Study:

• To develop a systematic approach for identifying merohedric twinning using crystallographic reflection conditions.
• To differentiate between three potential structural models: untwinned (H), twinned (t-H), and related space group (G).

Main Methods:

• Analysis of crystallographic reflection conditions to evaluate proposed structural models.
• Comparison of predicted reflection conditions for H, t-H, and G models against experimental data.

Main Results:

• In 71 cases, reflection conditions directly exclude the G model.
• In seven cases, reflection conditions suggest a space group inconsistent with the H model extended by the twin operation, leading to failed structure solution or refinement.
• Class IIB twinning, involving different crystal families, can be identified by its unique impact on reflection conditions.

Conclusions:

• Reflection conditions provide a powerful tool for detecting and characterizing merohedric twinning.
• The proposed method aids in resolving ambiguities in crystal structure analysis arising from twinning.