Structural characterization of HPM-7, a more ordered than expected germanosilicate zeolite

Area of Science:

• Materials Science
• Crystallography
• Nanotechnology

Background:

• Germanosilicates are advanced porous materials with diverse applications.
• Understanding the precise atomic arrangement and potential defects in novel germanosilicates is crucial for optimizing their properties.
• HPM-7 is a newly synthesized germanosilicate using specific organic dication templates.

Purpose of the Study:

• To determine the precise crystal structure and topology of the germanosilicate HPM-7.
• To investigate the potential for structural disorder within the HPM-7 framework.
• To understand the role of organic structure-directing agents in templating specific germanosilicate topologies.

Main Methods:

• Powder X-ray diffraction (PXRD) combined with DIFFaX simulations to model potential intergrowths.
• 3D Electron Diffraction Tomography (3D EDT) for initial structural determination.
• High-Resolution Scanning Transmission Electron Microscopy (HR-STEM) for detailed structural analysis and defect identification.

Main Results:

• HPM-7 primarily exhibits the POS (Pore-Ordered Silicate) topology, consistent with POS-A.
• Simulations suggest that intergrowths of similar polymorphs (POS-A to POS-C) are difficult to distinguish by PXRD.
• HR-STEM revealed rare instances of disorder compatible with the POS-D polymorph, indicating a potential secondary structural motif.
• The synthesis avoids polymorphs with very similar energies and structures, suggesting specific templating by the organic dications.

Conclusions:

• The organic dications effectively direct the formation of the POS topology in HPM-7.
• While predominantly ordered, HPM-7 may contain subtle disorder related to alternative polymorphs.
• The findings highlight the importance of advanced characterization techniques in elucidating complex structures and defects in porous materials.