Superstructure of Mullite-type KAl₉O_14.

Area of Science:

• Solid State Chemistry
• Materials Science
• Crystallography

Background:

• Mullite-type structures are known for their thermal stability and diverse applications.
• Understanding new polymorphs is crucial for expanding the library of advanced materials.

Purpose of the Study:

• To synthesize and characterize a novel polymorph of KAl9O14.
• To elucidate the crystallographic and microstructural properties of the new compound.
• To investigate the high-temperature behavior and phase transitions of KAl9O14.

Main Methods:

• Synthesis of KAl9O14 whiskers.
• Chemical composition analysis using energy-dispersive X-ray spectroscopy (EDS).
• Structural determination via single-crystal X-ray diffraction.
• Microstructural observation using transmission electron microscopy (TEM).
• In situ high-temperature optical microscopy and Raman spectroscopy for phase transition studies.

Main Results:

• Successful synthesis of large KAl9O14 whiskers with a mullite-type structure.
• Detailed crystallographic data: space group P21/n, a = 8.1880(8), b = 7.6760(7), c = 8.7944(9) Å, β = 110.570(8)°.
• Observation of nanosized twin domains and one-dimensional diffuse scattering using TEM.
• The structure features edge-sharing AlO6 octahedral chains linked with AlO4 tetrahedra and AlO5 trigonal bipyramids.
• KAl9O14 was observed to disproportionate into K β-alumina and corundum at high temperatures.

Conclusions:

• A new KAl9O14 polymorph with a mullite-type structure has been successfully synthesized and characterized.
• The detailed structural analysis reveals a complex arrangement of aluminum-oxygen polyhedra.
• The observed high-temperature disproportionation provides insights into the material's thermal stability limits and potential decomposition pathways.