Large Positive Thermal Expansion and Small Band Gap in Double-ReO₃-Type Compound NaSbF₆.

Area of Science:

• Materials Science
• Solid State Chemistry
• Crystallography

Background:

• Double-ReO3-type fluorides often display unusual thermal expansion properties, including negative thermal expansion (NTE).
• Understanding thermal expansion mechanisms in these materials is crucial for developing advanced functional materials.

Purpose of the Study:

• To investigate the phase transition and thermal expansion behavior of NaSbF6.
• To elucidate the underlying mechanisms responsible for its thermal expansion properties.
• To characterize the electronic band structure of NaSbF6.

Main Methods:

• Temperature-dependent X-ray diffractions were used to study phase transitions.
• Raman spectroscopy was employed to analyze atomic vibrations.
• First-principles calculations were performed to determine the electronic band gap.

Main Results:

• NaSbF6 undergoes a phase transition from rhombohedral to cubic between 303 and 323 K.
• The compound exhibits significant positive thermal expansion (PTE) with a volumetric coefficient of thermal expansion (αv) of 62 ppm/K in its cubic phase.
• Raman spectroscopy indicated stiffened low-frequency transverse vibrations of fluorine atoms, contrasting with NTE compounds.
• A relatively small band gap of 3.76 eV was observed, attributed to Sb 5s-F 2p orbital hybridization.

Conclusions:

• The large volumetric PTE in NaSbF6 is driven by the elongation of Na-F bonds overcoming polyhedral contraction.
• The stiffened fluorine vibrations contribute to its unique positive thermal expansion behavior.
• NaSbF6 possesses a smaller band gap compared to insulating ScF3 and CaZrF6, suggesting potential semiconductor properties.