Structural Phase Transitions in closo-Dicarbadodecaboranes C₂B₁₀H₁₂

Area of Science:

• Materials Science
• Solid-state Chemistry
• Crystallography

Background:

• Thermal polymorphism in closo-dicarbadodecaboranes (C2B10H12) isomers influences their solid-state properties.
• Understanding the rotational dynamics of these icosahedral clusters is crucial for materials design.

Purpose of the Study:

• To determine the crystal structures of three thermal polymorphs (I, II, III) for ortho-, meta-, and para-isomers of C2B10H12.
• To elucidate the relationship between molecular structure, rotational dynamics, and thermal behavior.
• To explore the potential of these compounds in designing advanced solid electrolytes.

Main Methods:

• Synchrotron radiation X-ray powder diffraction was employed to analyze crystal structures.
• Density functional theory (DFT) calculations complemented experimental diffraction data.
• Calorimetric and spectroscopic studies were used for validation.

Main Results:

• Crystal structures of three thermal polymorphs (I, II, III) were determined for all C2B10H12 isomers.
• Phase I exhibits isotropic rotations, while Phase II shows anisotropic rotations of the clusters.
• Phase III represents ordered, deformed structures of the cubic unit cells from Phases I and II.
• The ortho-isomer's ordering in Phase III creates a superstructure due to its dipole moment.
• Thermal polymorphism is explained by dispersive and anisotropic local interactions, modulated by cluster dynamics.
• C2B10H12 clusters demonstrate good oxidation resistance, comparable to CB11H12- anions.

Conclusions:

• The study provides a comprehensive structural and dynamic understanding of C2B10H12 thermal polymorphism.
• The findings support the design of mixed compounds, such as Na(CB11H12)(C2B10H12), for solid electrolytes.
• Tuning cation content in these electrolytes can lower the transition temperature to disordered conducting phases.