Crystal structures of two new high-pressure oxynitrides with composition SnGe₄N₄O₄, from single-crystal electron...

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• High-pressure and high-temperature synthesis are crucial for discovering novel materials.
• Characterizing nanocrystalline materials presents significant challenges for traditional methods.
• The Sn-Ge-N-O system is underexplored, particularly under extreme synthesis conditions.

Purpose of the Study:

• To synthesize and characterize a novel phase of SnGe4N4O4.
• To determine the crystal structure of the new phase using advanced techniques.
• To investigate potential polymorphism in SnGe4N4O4 under high-pressure synthesis.

Main Methods:

• Synthesis of SnGe4N4O4 at high pressures (16-20 GPa) and temperatures (1200-1500°C).
• Powder X-ray diffraction (XRD) using synchrotron radiation for initial phase identification.
• Transmission electron microscopy (TEM) for crystal morphology and composition analysis.
• Automated diffraction tomography (ADT) for three-dimensional crystal structure determination of nanocrystals.
• Density functional theory (DFT) calculations and atomic resolution scanning transmission electron microscopy (STEM) for structural confirmation.

Main Results:

• Successful synthesis of SnGe4N4O4 under high-pressure, high-temperature conditions.
• Identification of a previously unknown phase with stoichiometry SnGe4N4O4.
• Determination of the crystal structure of the new phase as nolanite-type in space group P63mc using ADT.
• Discovery and structural solution of a rhombohedral 6R polytype of SnGe4N4O4.

Conclusions:

• Automated diffraction tomography (ADT) is highly effective for solving crystal structures of nanocrystalline materials.
• The nolanite-type structure represents a new structural motif for SnGe4N4O4.
• Polymorphism exists in SnGe4N4O4, with both nolanite-type and a 6R polytype identified.