Clathrate III (Bromine Hydrate): Structural Relationship with Clathrate I

Area of Science:

• Solid-state chemistry
• Crystallography
• Materials science

Background:

• Tetragonal structure I (TS-I) clathrates are found in various systems, including bromine hydrate, ionic hydrates, and Zintl clathrates.
• Existing descriptions of TS-I lack a clear unifying structural principle for comparison with other clathrate structures.

Purpose of the Study:

• To propose a new structural description for TS-I clathrates.
• To compare TS-I with clathrate I (CS-I) based on the new description.
• To investigate the relationship and potential phase transitions between TS-I and CS-I.

Main Methods:

• Development of a new structural description for TS-I, centered on the '4-column' unit.
• Comparative analysis of TS-I and CS-I structures using the '4-column' as a common element.
• Analysis of ionic hydrates of tetraalkylammonium salts as a model system for TS-I formation.

Main Results:

• The '4-column', comprising four nearest columns of 14-hedral T-cages and their internal filling, is identified as the main structural unit of TS-I.
• The '4-column' is the maximal common structural element between TS-I and CS-I, suggesting TS-I is a modification of CS-I.
• No structural or reconstructive phase transition exists between TS-I and CS-I due to fundamental structural differences.

Conclusions:

• TS-I can be viewed as a modification of CS-I, potentially arising from interrupted crystal growth.
• The integral nature of '4-columns' in TS-I is confirmed by analyzing tetraalkylammonium salt hydrates.
• A hypothesis is proposed: the formation of TS-I crystals may necessitate special guest molecules or ions that occupy multiple cages or the host lattice.