Average structures of the disordered β-phase of Pigment Red 170: a single-crystal X-ray diffraction study

Area of Science:

• Crystallography
• Materials Science
• Solid-State Chemistry

Background:

• Pigment Red 170 (β-P.R. 170) is an industrially significant pigment.
• The β-phase exhibits severe layer stacking disorder, complicating its crystal structure determination.
• X-ray diffraction patterns are a complex superposition of Bragg diffraction and diffuse scattering.

Purpose of the Study:

• To investigate the disordered crystal structure of β-P.R. 170.
• To evaluate two plausible average monoclinic structure models (B2(1)/g and P2(1)/a).
• To assess the applicability of the average structure concept to β-P.R. 170.

Main Methods:

• Single-crystal X-ray diffraction analysis.
• Structure-factor calculations for proposed models.
• Analysis of Bragg diffraction and diffuse scattering patterns.
• Application of order-disorder theory.

Main Results:

• Two average monoclinic structures (B2(1)/g and P2(1)/a) were proposed, differing in disorder extent and space group.
• Both models are valid superposition structures according to order-disorder theory.
• Structure-factor calculations showed similar Bragg reflection patterns for both models, with B2(1)/g being more economical.
• Diffuse scattering analysis is crucial but challenging, questioning the practical applicability of the average structure concept.

Conclusions:

• The severe disorder in β-P.R. 170 challenges conventional crystallographic analysis.
• While the B2(1)/g model offers a more economical representation, the P2(1)/a model cannot be dismissed.
• Further detailed analysis of diffuse scattering is required for a definitive structural understanding and to validate the average structure concept for β-P.R. 170.