How Lewis acidity of the cationic framework affects KNaNbOF(5) polymorphism.

Area of Science:

• Inorganic Chemistry
• Solid-State Chemistry
• Crystallography

Background:

• The valence matching principle guides the synthesis of inorganic materials with desired structural properties.
• Polymorphism in KNaNbOF5 presents distinct centrosymmetric (CS) and noncentrosymmetric (NCS) structures with differing cation coordination environments.

Purpose of the Study:

• To elucidate the loss of inversion symmetry in the NCS polymorph of KNaNbOF5 compared to its CS counterpart.
• To investigate the role of cation Lewis acidity and coordination environment in dictating structural symmetry.
• To study the heat-induced phase transition between CS and NCS polymorphs.

Main Methods:

• Hydrothermal synthesis utilizing varying K:Na ratios in precursor solutions.
• In situ powder X-ray diffraction to monitor phase transitions under temperature changes.

Main Results:

• The NCS polymorph exhibits five contacts to both potassium and sodium, differing from the CS polymorph's four potassium and six sodium contacts.
• Lower average Lewis acidity in the NCS polymorph, achieved via potassium-rich synthesis, correlates with the loss of inversion symmetry.
• Secondary distortions in the NCS polymorph weaken the Nb═O bond compared to the CS polymorph.
• A heat-induced phase transition from CS to NCS was observed, with cooling proceeding through intermediate phases.

Conclusions:

• Valence matching and cation Lewis acidity are critical factors controlling the symmetry of KNaNbOF5 polymorphs.
• Synthesis conditions, specifically the K:Na ratio, can be tuned to favor the formation of the NCS polymorph.
• The observed phase transition highlights the dynamic structural behavior of KNaNbOF5 under thermal stimuli.