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The structure of orange HgI2. I. Polytypic layer structure
Marc Hostettler1, Henrik Birkedal, Dieter Schwarzenbach
1Institute of Crystallography, BSP Dorigny, University of Lausanne, Switzerland. marc.hostettler@ic.unil.ch
Acta Crystallographica. Section B, Structural Science
|November 29, 2002
Summary
Metastable orange mercury(II) iodide (HgI2) crystals exhibit complex polytypic structures. Analysis reveals nearly equal volumes of two maximum degree of order (MDO) structures with stacking disorder, averaging 5 layers thick.
Area of Science:
- Materials Science
- Crystallography
- Solid State Chemistry
Background:
- Metastable orange HgI2 crystals possess intricate structures based on Hg(4)I(10) supertetrahedra.
- Understanding the polytypic variations and stacking disorder is crucial for materials characterization.
Purpose of the Study:
- To determine the tetragonal polytypic structures of metastable orange HgI2.
- To quantitatively analyze the stacking disorder within these crystal structures.
Main Methods:
- X-ray diffraction data analysis, initially treating the crystal as a conglomerate of MDO structures.
- Refinement of parameters and volume ratios for independent MDO structures.
- Quantitative analysis of stacking disorder using a Markov chain model and intensity profile fitting.
Main Results:
- Two distinct MDO polytypic structures of HgI2 were identified with nearly identical layer geometries and displacement parameters.
- Layer contacts in the two stacking modes were found to be identical.
- The crystal comprises approximately equal volumes of these MDO structures, with an average domain thickness of about 5 layers (30 Å).
Conclusions:
- The metastable orange HgI2 crystal is a fractal complication of the stable red form.
- Stacking disorder significantly influences the observed diffraction patterns.
- The crystal structure is characterized by a mixture of ordered polytypes with limited domain thickness.