Order-disorder twinning model and stacking faults in alpha-NTO

Area of Science:

• Crystallography
• Materials Science
• Solid State Chemistry

Background:

• The triclinic structure of 5-nitro-2,4-dihydro-1,2,4-triazol-3-one (alpha-NTO) was recently determined.
• Crystals of alpha-NTO exhibit complex fourfold twinning.
• The asymmetric unit contains Z' = 4 independent molecules.

Purpose of the Study:

• To investigate the origin of the fourfold twinning in alpha-NTO crystals.
• To explain the presence of multiple independent molecules (Z'=4) in the asymmetric unit.
• To apply order-disorder theory to understand the observed crystal structure.

Main Methods:

• Analysis of the published crystal structure of alpha-NTO.
• Application of layer group symmetry concepts.
• Utilizing order-disorder theory and local symmetry operations.
• Identification of ordered structures and common reflections.

Main Results:

• The alpha-NTO structure features layers with p2(1)/b 1 (1) symmetry.
• Stacking of these layers leads to the loss of overall symmetry and explains Z'=4.
• Twinning arises from stacking faults, with layers adopting four equivalent positions.
• Near-neighbor molecular relationships are preserved across twin boundaries.
• Four maximally ordered structures and their common reflections were identified.

Conclusions:

• Stacking faults in layered structures are a primary cause of twinning in alpha-NTO.
• Order-disorder theory provides a framework for understanding the twinning and multiple independent molecules.
• The study elucidates the relationship between local symmetry, stacking, and macroscopic crystal properties.