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The Role of Dynamic Polyhedral Rotations in the Phase Transitions of Rb3AlF6 and Cs3AlF6
Graham King1, Aydar Rakhmatullin2, František Šimko3
1Canadian Light Source, 44 Innovation Boulevard, Saskatoon, SK S7N 2 V3, Canada.
Inorganic Chemistry
|July 8, 2025
Summary
Rubidium and cesium aluminum fluorides exhibit phase transitions linked to the freezing of aluminum fluoride (AlF6) octahedra. Rb3AlF6 shows two transitions, while Cs3AlF6 maintains its structure across a wide temperature range.
Area of Science:
- Solid-state chemistry
- Materials science
- Crystallography
Background:
- Cryolite-related alkali metal aluminum fluorides are important materials.
- Understanding their structural dynamics and phase transitions is key to material properties.
Purpose of the Study:
- Investigate structures and phase transitions in rubidium and cesium aluminum fluoride phases.
- Relate these transitions to the dynamic motions of aluminum fluoride (AlF6) octahedra.
Main Methods:
- Low-temperature synchrotron powder diffraction.
- Magic-angle spinning (MAS) Nuclear Magnetic Resonance (NMR) spectroscopy.
- Reverse Monte Carlo (RMC) modeling of X-ray pair distribution function (PDF) data.
Main Results:
- Rb3AlF6 exhibits two phase transitions around 270 K and 250 K, involving ordered freezing of AlF6 octahedra rotations.
- Cs3AlF6 shows freezing of octahedral rotations near room temperature but maintains its C2/m structure down to 80 K.
- Dynamic motions and local structure of cubic Cs3AlF6 were analyzed.
Conclusions:
- Phase transitions in Rb3AlF6 are driven by the freezing of dynamic AlF6 octahedra rotations.
- Cs3AlF6 displays structural stability across a broad temperature range despite octahedral dynamics.
- Combined diffraction, NMR, and modeling provide comprehensive insights into these fluoride phases.
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